An Internet Protocol Version 6 (IPv6) Profile for 3GPP Mobile Devices
draft-ietf-v6ops-mobile-device-profile-24

Abstract

This document defines a profile that is a superset of that of the connection to IPv6 cellular networks defined in the IPv6 for Third Generation Partnership Project (3GPP) Cellular Hosts document. This document defines an IPv6 profile that a number of operators recommend in order to connect 3GPP mobile devices to an IPv6-only or dual-stack wireless network (including 3GPP cellular network) with a special focus on IPv4 service continuity features.

Both mobile hosts and mobile devices with capability to share their 3GPP mobile connectivity are in scope.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on June 19, 2016.

Copyright Notice

Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Introduction
• 1.1. Terminology
• 1.2. Scope
• 2. Connectivity Recommendations
• 3. Recommendations for Cellular Devices with LAN Capabilities
• 4. Advanced Recommendations
• 5. Security Considerations
• 6. IANA Considerations
• 7. Acknowledgements
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Authors' Addresses

1. Introduction

IPv6 deployment in Third Generation Partnership Project (3GPP) mobile networks is the only viable solution to the exhaustion of IPv4 addresses in those networks. Several mobile operators have already deployed IPv6 [RFC2460] or are in the pre-deployment phase. One of the major hurdles as perceived by some mobile operators is the lack of availability of working IPv6 implementation in mobile devices (e.g., Section 3.3 of [OECD]).

[RFC7066] lists a set of features to be supported by cellular hosts to connect to 3GPP mobile networks. In the light of recent IPv6 production deployments, additional features to facilitate IPv6-only deployments while accessing IPv4-only services should be considered. This document fills this void. Concretely, this document lists means to ensure IPv4 service over an IPv6-only connectivity given the adoption rate of this model by mobile operators. Those operators require that no service degradation is experienced by customers serviced with an IPv6-only model compared to the level of service of customers with legacy IPv4-only devices.

This document defines an IPv6 profile for mobile devices listing specifications produced by various Standards Developing Organizations (including 3GPP, IETF, and GSMA). The objectives of this effort are:

1. List in one single document a comprehensive list of IPv6 features for a mobile device, including both IPv6-only and dual-stack mobile deployment contexts. These features cover various packet core architectures such as GPRS (General Packet Radio Service) or EPC (Evolved Packet Core).
2. Help Operators with the detailed device requirement list preparation (to be exchanged with device suppliers). This is also a contribution to harmonize Operators’ requirements towards device vendors.
3. Vendors to be aware of a set of features to allow for IPv6 connectivity and IPv4 service continuity (over an IPv6-only transport).

The recommendations do not include 3GPP release details. For more information on the 3GPP releases detail, the reader may refer to Section 6.2 of [RFC6459]. More details can be found at [R3GPP].

Some of the features listed in this profile document could require to activate dedicated functions at the network side. It is out of scope of this document to list these network-side functions.

A detailed overview of IPv6 support in 3GPP architectures is provided in [RFC6459]. IPv6-only considerations in mobile networks are further discussed in [RFC6342].

This document is organized as follows:

• Section 2 lists generic recommendations including functionalities to provide IPv4 service over an IPv6-only connectivity.
• Section 3 enumerates a set of recommendations for cellular devices with Local Area Network (LAN) capabilities (e.g., CE routers (Customer Edge routers) with cellular access link, dongles with tethering features).
• Section 4 identifies a set of advanced recommendations to fulfill requirements of critical services such as VoLTE (Voice over Long Term Evolution (LTE)).

1.1. Terminology

This document makes use of the terms defined in [RFC6459]. In addition, the following terms are used:

• 3GPP cellular host (or cellular host for short): denotes a 3GPP device which can be connected to 3GPP mobile networks.
• 3GPP cellular device (or cellular device for short): refers to a cellular host which supports the capability to share its 3GPP mobile connectivity.
• IPv4 service continuity: denotes the features used to provide access to IPv4-only services to customers serviced with an IPv6-only connectivity. A typical example of IPv4 service continuity technique is NAT64 (Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers, [RFC6146]).

PREFIX64 denotes an IPv6 prefix used to build IPv4-converted IPv6 addresses [RFC6052].

1.2. Scope

A 3GPP mobile network can be used to connect various user equipments such as a mobile telephone or a CE router. Because of this diversity of terminals, it is necessary to define a set of IPv6 functionalities valid for any node directly connecting to a 3GPP mobile network. This document describes these functionalities.

Machine-to-machine (M2M) devices profile is out of scope.

This document is structured to provide the generic IPv6 recommendations which are valid for all nodes, whatever their function (e.g., host or CE router) or service (e.g., Session Initiation Protocol (SIP, [RFC3261])) capability. The document also contains sections covering specific functionalities for devices providing some LAN functions (e.g., mobile CE router or broadband dongles).

The recommendations listed below are valid for both 3GPP GPRS and 3GPP EPS (Evolved Packet System). For EPS, PDN-Connection term is used instead of PDP-Context. Other non-3GPP accesses [TS.23402] are out of scope of this document.

This profile is a superset of that of the IPv6 profile for 3GPP Cellular Hosts [RFC7066], which is in turn a superset of IPv6 Node Requirements [RFC6434]. It targets cellular nodes, including GPRS and EPC (Evolved Packet Core), that require features to ensure IPv4 service delivery over an IPv6-only transport in addition to the base IPv6 service. Moreover, this profile also covers cellular CE routers that are used in various mobile broadband deployments. Recommendations inspired from real deployment experiences (e.g., roaming) are included in this profile. Also, this profile sketches recommendations for the sake of deterministic behaviors of cellular devices when the same configuration information is received over several channels.

For conflicting recommendations in [RFC7066] and [RFC6434] (e.g., Neighbor Discovery Protocol), this profile adheres to [RFC7066]. Indeed, the support of Neighbor Discovery Protocol is mandatory in 3GPP cellular environment as it is the only way to convey IPv6 prefix towards the 3GPP cellular device. In particular, MTU (Maximum Transmission Unit) communication via Router Advertisement must be supported since many 3GPP networks do not have a standard MTU setting.

This profile uses a stronger language for the support of Prefix Delegation compared to [RFC7066]. The main motivation is that cellular networks are more and more perceived as an alternative to fixed networks for home IP-based services delivery; especially with the advent of smartphones and 3GPP data dongles. There is a need for an efficient mechanism to assign larger prefixes to cellular hosts so that each LAN segment can get its own /64 prefix and multi-link subnet issues to be avoided. The support of this functionality in both cellular and fixed networks is key for fixed-mobile convergence.

The use of address family dependent Application Programming Interfaces (APIs) or hard-coded IPv4 address literals may lead to broken applications when IPv6 connectivity is in use. As such, means to minimize broken applications when the cellular host is attached to an IPv6-only network should be encouraged. Particularly, (1) name resolution libraries (e.g., [RFC3596]) must support both IPv4 and IPv6; (2) applications must be independent of the underlying IP address family; (3) and applications relying upon Uniform Resource Identifiers (URIs) must follow [RFC3986] and its updates. Note, some IETF specifications (e.g., SIP [RFC3261]) contains broken IPv6 Augmented Backus-Naur Form (ABNF) and rules to compare URIs with embedded IPv6 addresses; fixes (e.g., [RFC5954]) must be used instead.

The recommendations included in each section are listed in a priority order.

This document is not a standard, and conformance with it is not required in order to claim conformance with IETF standards for IPv6. Compliance with this profile does not require the support of all enclosed items. Obviously, the support of the full set of features may not be required in some deployment contexts. However, the authors believe that not supporting relevant features included in this profile (e.g., Customer Side Translator (CLAT, [RFC6877])) may lead to a degraded level of service.

2. Connectivity Recommendations

This section identifies the main connectivity recommendations to be followed by a cellular host to attach to a network using IPv6 in addition to what is defined in [RFC6434] and [RFC7066]. Both dual-stack and IPv6-only deployment models are considered. IPv4 service continuity features are listed in this section because these are critical for Operators with an IPv6-only deployment model. These recommendations apply also for cellular devices (see Section 3).

C_REC#1:

In order to allow each operator to select their own strategy regarding IPv6 introduction, the cellular host must support both IPv6 and IPv4v6 PDP-Contexts [TS.23060].

IPv4, IPv6 or IPv4v6 PDP-Context request acceptance depends on the cellular network configuration.

C_REC#2:

The cellular host must comply with the behavior defined in [TS.23060] [TS.23401] [TS.24008] for requesting a PDP-Context type.

In particular, the cellular host must request by default an IPv6 PDP-Context if the cellular host is IPv6-only and request an IPv4v6 PDP-Context if the cellular host is dual-stack or when the cellular host is not aware of connectivity types requested by devices connected to it (e.g., cellular host with LAN capabilities as discussed in Section 3):

• If the requested IPv4v6 PDP-Context is not supported by the network, but IPv4 and IPv6 PDP types are allowed, then the cellular host will be configured with an IPv4 address or an IPv6 prefix by the network. It must initiate another PDP-Context activation of the other address family in addition to the one already activated for a given APN (Access Point Name). The purpose of initiating a second PDP-Context is to achieve dual-stack connectivity by means of two PDP-Contexts.
• If the subscription data or network configuration allows only one IP address family (IPv4 or IPv6), the cellular host must not request a second PDP-Context to the same APN for the other IP address family.

The network informs the cellular host about allowed PDP types by means of Session Management (SM) cause codes. In particular, the following cause codes can be returned:

• cause #50 "PDP type IPv4 only allowed". This cause code is used by the network to indicate that only PDP type IPv4 is allowed for the requested PDN connectivity.
• cause #51 "PDP type IPv6 only allowed". This cause code is used by the network to indicate that only PDP type IPv6 is allowed for the requested PDN connectivity.
• cause #52 "single address bearers only allowed". This cause code is used by the network to indicate that the requested PDN connectivity is accepted with the restriction that only single IP version bearers are allowed.

The text above focuses on the specification (excerpt from [TS.23060] [TS.23401] [TS.24008]) which explains the behavior for requesting IPv6-related PDP-Context(s).

C_REC#3:

The cellular host must support the PCO (Protocol Configuration Options) [TS.24008] to retrieve the IPv6 address(es) of the Recursive DNS server(s).

• The 3GPP network communicates parameters by means of the protocol configuration options information element when activating, modifying or deactivating a PDP-Context. PCO is a convenient method to inform the cellular host about various services, including DNS server information. It does not require additional protocol to be supported by the cellular host and it is already deployed in IPv4 cellular networks to convey such DNS information.

C_REC#4:

The cellular host must support IPv6 aware Traffic Flow Templates (TFT) [TS.24008].

• Traffic Flow Templates are employing a packet filter to couple an IP traffic with a PDP-Context. Thus a dedicated PDP-Context and radio resources can be provided by the cellular network for certain IP traffic.

C_REC#5:

If the cellular host receives the DNS information in several channels for the same interface, the following preference order must be followed:

• 1. PCO
• 2. RA
• 3. DHCPv6

The purpose of this recommendation is to guarantee for a deterministic behavior to be followed by all cellular hosts when the DNS information is received in various channels.

C_REC#6:

Because of potential operational deficiencies to be experienced in some roaming situations, the cellular host must be able to be configured with a home PDP-Context type(s) and a roaming PDP-Context type(s). The purpose of the roaming profile is to limit the PDP type(s) requested by the cellular host when out of the home network. Note that distinct PDP type(s) and APN(s) can be configured for home and roaming cases.

• A detailed analysis of roaming failure cases is included in [RFC7445].
• The configuration can be either local to the device or be managed dynamically using, for example, Open Mobile Alliance (OMA) management. The support of dynamic means is encouraged.

C_REC#7:

In order to ensure IPv4 service continuity in an IPv6-only deployment context, the cellular host should support a method to learn PREFIX64(s).

• In the context of NAT64, IPv6-enabled applications relying on address referrals will fail because an IPv6-only client will not be able to make use of an IPv4 address received in a referral. This feature allows to solve the referral problem (because an IPv6-enabled application can construct IPv4-embedded IPv6 addresses [RFC6052]) and, also, to distinguish between IPv4-converted IPv6 addresses and native IPv6 addresses. In other words, this feature contributes to offload both CLAT module and NAT64 devices. Refer to Section 3 of [RFC7051] for an inventory of the issues related to the discovery of PREFIX64(s).
• In PCP-based environments, cellular hosts should follow [RFC7225] to learn the IPv6 Prefix used by an upstream PCP-controlled NAT64 device. If PCP is not enabled, the cellular host should implement the method specified in [RFC7050] to retrieve the PREFIX64.

C_REC#8:

In order to ensure IPv4 service continuity in an IPv6-only deployment context, the cellular host should implement the Customer Side Translator (CLAT, [RFC6877]) function in compliance with [RFC6052][RFC6145][RFC6146].

• CLAT function in the cellular host allows for IPv4-only application and IPv4-referals to work on an IPv6-only connectivity. The more applications are address family independent, the less CLAT function is solicited. CLAT function requires a NAT64 capability [RFC6146] in the network.
• The cellular host should only invoke the CLAT in the absence of the IPv4 connectivity on the cellular side, i.e., when the network does not assign an IPv4 address on the cellular interface. Note, NAT64 assumes an IPv6-only mode [RFC6146].
• The IPv4 Service Continuity Prefix used by CLAT is defined in [RFC7335].
• CLAT and/or NAT64 do not interfere with native IPv6 communications.
• CLAT may not be required in some contexts, e.g., if other solutions such as Bump-in-the-Host (BIH, [RFC6535]) are supported.
• The cellular device can act as a CE router connecting various IP hosts on a LAN segment; it is also the case with the use of WLAN (Wireless LAN) tethering or WLAN hotspot from the cellular device. Some of these IP hosts can be dual-stack, others are IPv6-only or IPv4-only. IPv6-only connectivity on the cellular device does not allow IPv4-only sessions to be established for hosts connected on the LAN segment of the cellular device. IPv4 session establishment initiated from hosts located on LAN segment side and destined for IPv4 nodes must be maintained. A solution is to integrate the CLAT function to the LAN segment in the cellular device.

C_REC#9:

The cellular host may be able to be configured to limit PDP type(s) for a given APN. The default mode is to allow all supported PDP types. Note, C_REC#2 discusses the default behavior for requesting PDP-Context type(s).

• This feature is useful to drive the behavior of the UE to be aligned with: (1) service-specific constraints such as the use of IPv6-only for VoLTE (Voice over LTE), (2) network conditions with regards to the support of specific PDP types (e.g., IPv4v6 PDP-Context is not supported), (3) IPv4 sunset objectives, (4) subscription data, etc.
• Note, a cellular host changing its connection between an IPv6-specific APN and an IPv4-specific APN will interrupt related network connections. This may be considered as a brokenness situation by some applications.
• The configuration can be either local to the device or be managed dynamically using, for example, Open Mobile Alliance (OMA) management. The support of dynamic means is encouraged.

3. Recommendations for Cellular Devices with LAN Capabilities

This section focuses on cellular devices (e.g., CE router, smartphones or dongles with tethering features) which provide IP connectivity to other devices connected to them. In such case, all connected devices are sharing the same 2G, 3G or LTE connection. In addition to the generic recommendations listed in Section 2, these cellular devices have to meet the recommendations listed below.

L_REC#1:

For deployments requiring to share the same /64 prefix, the cellular device should support [RFC7278] to enable sharing a /64 prefix between the 3GPP interface towards the GGSN/PGW (WAN interface) and the LAN interfaces.

• Prefix Delegation (refer to L_REC#2) is the target solution for distributing prefixes in the LAN side but, because the device may attach to earlier 3GPP release networks, a mean to share a /64 prefix is also recommended [RFC7278].
• [RFC7278] must be invoked only if Prefix Delegation is not in use.

L_REC#2:

The cellular device must support Prefix Delegation capabilities [RFC3633] and must support Prefix Exclude Option for DHCPv6-based Prefix Delegation as defined in [RFC6603]. Particularly, it must behave as a Requesting Router.

• Cellular networks are more and more perceived as an alternative to fixed broadband networks for home IP-based services delivery; especially with the advent of smartphones and 3GPP data dongles. There is a need for an efficient mechanism to assign larger prefixes (other than /64s) to cellular hosts so that each LAN segment can get its own /64 prefix and multi-link subnet issues to be avoided.
• In case a prefix is delegated to a cellular host using DHCPv6, the cellular device will be configured with two prefixes:
  • (1) one for 3GPP link allocated using stateless address autoconfiguration (SLAAC) mechanism and
  • (2) another one delegated for LANs acquired during Prefix Delegation operation.

  Note that the 3GPP network architecture requires both the WAN (Wide Area Network) and the delegated prefix to be aggregatable, so the subscriber can be identified using a single prefix.

• Without the Prefix Exclude Option, the delegating router (GGSN/PGW) will have to ensure [RFC3633] compliancy (e.g., halving the delegated prefix and assigning the WAN prefix out of the 1st half and the prefix to be delegated to the terminal from the 2nd half).
• Because Prefix Delegation capabilities may not be available in some attached networks, L_REC#1 is strongly recommended to accommodate early deployments.

L_REC#3:

The cellular CE router must be compliant with the requirements specified in [RFC7084].

• There are several deployments, particularly in emerging countries, that relies on mobile networks to provide broadband services (e.g., customers are provided with mobile CE routers).
• Note, this profile does not require IPv4 service continuity techniques listed in Section 4.4 of [RFC7084] because those are specific to fixed networks. IPv4 service continuity techniques specific to the mobile networks are included in this profile.
• This recommendation does not apply to handsets with tethering capabilities; it is specific to cellular CE routers in order to ensure the same IPv6 functional parity for both fixed and cellular CE routers. Note, modern CE routers are designed with advanced functions such as link aggregation that consists in optimizing the network usage by aggregating the connectivity resources offered via various interfaces (e.g., Digital Subscriber Line (DSL), LTE, WLAN, etc.) or offloading the traffic via a subset of interfaces. Ensuring IPv6 features parity among these interface types is important for the sake of specification efficiency, service design simplification and validation effort optimization.

L_REC#4:

If a RA MTU is advertised from the 3GPP network, the cellular device should send RAs to the downstream attached LAN devices with the same MTU as seen on the mobile interface.

• Receiving and relaying RA MTU values facilitates a more harmonious functioning of the mobile core network where end nodes transmit packets that do not exceed the MTU size of the mobile network's GTP (GPRS Tunnelling Protocol) tunnels.
• [TS.23060] indicates providing a link MTU value of 1358 octets to the 3GPP cellular device will prevent the IP layer fragmentation within the transport network between the cellular device and the GGSN/PGW. More details about link MTU considerations can be found in Annex C of [TS.23060].

4. Advanced Recommendations

This section identifies a set of advanced recommendations to fulfill requirements of critical services such as VoLTE. These recommendations apply for mobile hosts, including mobile devices.

A_REC#1:

The cellular host must support ROHC RTP Profile (0x0001) and ROHC UDP Profile (0x0002) for IPv6 ([RFC5795]). Other ROHC profiles may be supported.

• Bandwidth in cellular networks must be optimized as much as possible. ROHC provides a solution to reduce bandwidth consumption and to reduce the impact of having bigger packet headers in IPv6 compared to IPv4.
• "RTP/UDP/IP" ROHC profile (0x0001) to compress RTP packets and "UDP/IP" ROHC profile (0x0002) to compress RTCP packets are required for Voice over LTE (VoLTE) by IR.92.4.0 section 4.1 [IR92]. Note, [IR92] indicates that the host must be able to apply the compression to packets that are carried over the voice media dedicated radio bearer.

A_REC#2:

The cellular host should support PCP [RFC6887].

• The support of PCP is seen as a driver to save battery consumption exacerbated by keepalive messages. PCP also gives the possibility of enabling incoming connections to the cellular device. Indeed, because several stateful devices may be deployed in wireless networks (e.g., NAT64 and/or IPv6 Firewalls), PCP can be used by the cellular host to control network-based NAT64 and IPv6 Firewall functions which will reduce per-application signaling and save battery consumption.
• According to [Power], the consumption of a cellular device with a keep-alive interval equal to 20 seconds (that is the default value in [RFC3948] for example) is 29 mA (2G)/34 mA (3G). This consumption is reduced to 16 mA (2G)/24 mA (3G) when the interval is increased to 40 seconds, to 9.1 mA (2G)/16 mA (3G) if the interval is equal to 150 seconds, and to 7.3 mA (2G)/14 mA (3G) if the interval is equal to 180 seconds. When no keep-alive is issued, the consumption would be 5.2 mA (2G)/6.1 mA (3G). The impact of keepalive messages would be more severe if multiple applications are issuing those messages (e.g., SIP, IPsec, etc.).
• PCP allows to avoid embedding ALGs (Application Level Gateways) at the network side (e.g., NAT64) to manage protocols which convey IP addresses and/or port numbers (see Section 2.2 of [RFC6889]). Avoiding soliciting ALGs allows for more easiness to make evolve a service independently of the underlying transport network.

A_REC#3:

In order for host-based validation of DNS Security Extensions (DNSSEC) to continue to function in an IPv6-only connectivity with NAT64 deployment context, the cellular host should embed a DNS64 function ([RFC6147]).

• This is called "DNS64 in stub-resolver mode" in [RFC6147].
• As discussed in Section 5.5 of [RFC6147], a security-aware and validating host has to perform the DNS64 function locally.
• Because synthetic AAAA records cannot be successfully validated in a host, learning the PREFIX64 used to construct IPv4-converted IPv6 addresses allows the use of DNSSEC [RFC4033] [RFC4034], [RFC4035]. Means to configure or discover a PREFIX64 are required on the cellular device as discussed in C_REC#7.
• [RFC7051] discusses why a security-aware and validating host has to perform the DNS64 function locally and why it has to be able to learn the proper PREFIX64(s).

A_REC#4:

When the cellular host is dual-stack connected (i.e., configured with an IPv4 address and IPv6 prefix), it should support means to prefer native IPv6 connection over connection established through translation devices (e.g., NAT44 and NAT64).

• When both IPv4 and IPv6 DNS servers are configured, a dual-stack host must contact first its IPv6 DNS server. This preference allows to offload IPv4-only DNS servers.
• Cellular hosts should follow the procedure specified in [RFC6724] for source address selection.

5. Security Considerations

The security considerations identified in [RFC7066] and [RFC6459] are to be taken into account.

In the case of cellular CE routers, compliance with L_REC#3 entails compliance with [RFC7084], which in turn recommends compliance with Recommended Simple Security Capabilities in Customer Premises Equipment (CPE) for Providing Residential IPv6 Internet Service [RFC6092]. Therefore, the security considerations in Section 6 of [RFC6092] are relevant. In particular, it bears repeating here that the true impact of stateful filtering may be a reduction in security, and that IETF make no statement, expressed or implied, as to whether using the capabilities described in any of these documents ultimately improves security for any individual users or for the Internet community as a whole.

The cellular host must be able to generate IPv6 addresses which preserve privacy. The activation of privacy extension (e.g., using [RFC7217]) makes it more difficult to track a host over time when compared to using a permanent Interface Identifier. Tracking a host is still possible based on the first 64 bits of the IPv6 address. Means to prevent against such tracking issues may be enabled in the network side. Note, privacy extensions are required by regulatory bodies in some countries.

Host-based validation of DNSSEC is discussed in A_REC#3 (see Section 4).

6. IANA Considerations

This document does not require any action from IANA.

7. Acknowledgements

Many thanks to C. Byrne, H. Soliman, H. Singh, L. Colliti, T. Lemon, B. Sarikaya, M. Mawatari, M. Abrahamsson, P. Vickers, V. Kuarsingh, E. Kline, S. Josefsson, A. Baryun, J. Woodyatt, T. Kossut, B. Stark, and A. Petrescu for the discussion in the v6ops mailing list and for the comments.

Thanks to A. Farrel, B. Haberman, and K. Moriarty for the comments during the IESG review.

Special thanks to T. Savolainen, J. Korhonen, J. Jaeggli, F. Baker, L.M. Contreras Murillo, and M. Abrahamsson for their detailed reviews and comments.

8. References

8.1. Normative References

8.2. Informative References

Authors' Addresses