Considerations of Gradual IPv6-only Deployment in 5G Mobile Networks

Currently, IPv6 has been widely in mobile networks of operators worldwide, and it has even gained the dominant position from the perspective of traffic. However, IPv4 applications still exist in the network, and the support for IPv4 services must still be considered to guarantee the users’ experience. Furthermore, operators have begun experimenting with deploying IPv6-only approach in their mobile networks. The 5G system is defined in the 3GPP standards organization. In the 5G system architecture, the session related to the endpoint's access to the internet is called the Packet Data Unit (PDU) session, and its type determines the IP protocol used for user access. In the 5G standards, the PDU session supports both IPv6 and IPv4 protocols, and it also provides policies to ensure that user equipment (UE) can access the internet. When a UE only supports the IPv4 protocol while the network supports dual-stack (IPv4 and IPv6), the network will provide an IPv4 protocol stack configuration for the UE. Accordingly, for UE only supporting IPv6,the network will provide an IPv6 protocol stack configuration for the UE. Additionally, there are policy configuration schemes related to static addresses and other aspects, but It does not specify the requirements related to IPv6-only technology. There are several IPv6-only transition technologies described in . Most existing deployments utilize 464XLAT technology in cellular network. This document describes the architecture for deploying 464XLAT based IPv6-only technology on user plane in 3GPP 5G system. Based on the field trail, this document also discusses the major issues encountered and potential solutions to address them.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

The following terms are defined in this document: 464XLAT: IPv6-Only Transition Mechanism (IPv6-to-IPv4 Translation) 5GC: 5G Core 5GS: 5G System AMF: Access and Mobility Management Function CLAT: CLAT is customer-side translator (XLAT) that compiles with PLAT: PLAT is provider-side translator (XLAT) that compiles with PDU：Protocol Data Unit IMEI: International Mobile Equipment Identity SMF：Session Management Function UE: User Equipment, e.g., mobile phone. LBO：Local Break Out

Examples of 5GS IPv6-only architectures on user plane are shown in the figures in the following sections. In production 5GS network, there is roaming behavior which specifies where the PDU session anchor and its controlling SMF are located in. That decides whether UE’s PDU sessions get IP configuration and access the Internet from home 5GS network or visited 5GS network. Regarding roaming, 5GS contains three scenarios including non-roaming, roaming with home routed, and roaming with local break out.

Based on wireless 3GPP network architecture defined in [RFC6877], the non-roaming network architecture is depicted as figure 1. When a mobile network operator run only a 5GC, there is just non-roaming network scenario. In this csae, the CLAT function is deployed on the UE, while the PLAT/stateful NAT64 function and DNS64 function are deployed on the network side.

Generally, large mobile operators run multiple 5GCs divided by administrative divisions or other geographical methods. The roaming network scenario with home routed is shown in figure 2. In this case, UEs acquire IP network configuration and access the Internet in their home 5GS network. The IP address allocation strategy and traffic exit interface are decided by their home 5GC. The CLAT function is deployed on the UE, while the PLAT/stateful NAT64 function and DNS64 function are deployed on the home network.

The roaming network scenario with LBO is shown in figure 3. In this case, UEs get IP network configuration and access the Internet in the visited network. The CLAT function is deployed on the UE, while the PLAT/stateful NAT64 function and DNS64 function are deployed on the visited network. Home network also need to support NAT64 and DNS64 when UE is in the non-roaming case. The following table 1 summarizes 5GC's network capabilities where the mobile network shall have to provide IPv6-only connectivity service.

Based on our practices, for large-size mobile network operators, it’s very difficult for operators to deploy IPv6-only capabilities across the whole network at once. There is a transition period that the IPv6-only capability is deployed gradually. This section identifies the major challenges when applying 464XLAT in a production network.

In the scenario where 5GC A supports IPv6-only capability while 5GC B doesn’t. When UE A from 5GC A roams to 5GC B, it only obtains IPv6 configuration and accesses Internet according to the local breakout roaming policy. In this case, the access to IPv4 Internet may fail due to 5GC B lacks NAT64 and DNS64 capabilities.

Regarding UE challenge, a significant number of terminals have not enabled CLAT functionality. The vast majority of Android terminals support and have enabled the CLAT functionality. Most new terminals like smart watch do not support this feature. Moreover, Apple's iOS in China does not enable CLAT functionality.

When IPv6-only users access IPv4 sites, the actual address they reach is generated by the DNS64 server, which combines a special IPv6 prefix with the IPv4 site address to form an IPv6 address. Existing layer 3&layer 4 content billing rules based on IPv4 addresses will no longer be effective and will need to be adjusted to accommodate the IPv6 addresses formed by DNS64.

After enabling the DNS64 functionality, there is an increase in the processing load due to the additional handling of IPv6 queries and the DNS64 conversion, which consumes some device performance. The extent of this performance demand increase depends on the scale of IPv6 queries. In a 5G network, once the DNS64 functionality is enabled, DNS resolution requests from both IPv6-only and dual-stack users will be processed by the DNS64 server. Even IPv4 resolution requests from dual-stack users will be treated as if they were from IPv6-only users, which place a significant load pressure on the DNS server. This may futher impact the service logic of the existing dual-stack users.

One solution is to enhance the core network's capability to configure IP addresses and DNS server addresses based on IMEI number ranges. The IMEI is a unique sequence of 14 to 15 digits assigned to each mobile phone. It serves as a device identification number, enabling service providers to recognize the phone within the network. The primary purpose of the IMEI is to uniquely identify each device, allowing the network to determine whether the UE supports CLAT functionality. By configuring a whitelist of IMEIs that support CLAT functionality in the network, the network can assign an IPv6-only environment to UEs listed in the whitelist.

One possible solution is to use the option108 [RFC 8925] method, allowing UE to choose whether to join the IPv6-only environment. Additionally, a method for configuring DNS64 server addresses needs to be considered. However, in practical deployments, core network support for DHCPv4 functionality is optional and may not be applicable to all networks. Another possible solution is to transmit Pref64 and DNS64 address information through the RA option. In 5G systems, RA is used to advertise IPv6 prefixes in the SLAAC, which is a mandatory functionality. Transmitting this information through RA is also a favorable option. Currently, the IETF has produced two RFCs, namely [RFC 8106] and [RFC 8781]. In practical implementations, the mobile core network supporting IPv6-only environments (IPv6-only mode) should include these two options in the RA messages. Upon receiving this message, the UE can abandon the IPv4 interface and operate in IPv6-only mode. In this solution, the core network does not need to be aware of the protocol stack ultimately used by UE.

TBD.

The comments and suggestions of the following are gratefully acknowledged: