Pros and Cons of IPv6 Transition Technologies for IPv4aaS
draft-lmhp-v6ops-transition-comparison-03

Abstract

Several IPv6 transition technologies have been developed to provide customers with IPv4-as-a-Service (IPv4aaS) for ISPs with an IPv6-only access and/or core network. All these technologies have their advantages and disadvantages, and depending on existing topology, skills, strategy and other preferences, one of these technologies may be the most appropriate solution for a network operator.

This document examines the five most prominent IPv4aaS technologies considering a number of different aspects to provide network operators with an easy to use reference to assist in selecting the technology that best suits their needs.

Status of This Memo

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This Internet-Draft will expire on January 7, 2020.

Copyright Notice

Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Requirements Language
• 2. Overview of the Technologies
• 2.1. 464XLAT
• 2.2. Dual-Stack Lite
• 2.3. Lightweight 4over6
• 2.4. MAP-E
• 2.5. MAP-T
• 3. High-level Architectures and their Consequences
• 3.1. Service Provider Network Traversal
• 3.2. Network Address Translation
• 3.3. IPv4 Address Sharing
• 3.4. CE Provisioning Considerations
• 3.5. Support for Multicast
• 4. Detailed Analysis
• 4.1. Architectural Differences
• 4.1.1. Basic Comparison
• 4.2. Tradeoff between Port Number Efficiency and Stateless Operation
• 4.3. Support for Public Server Operation
• 4.4. Support and Implementations
• 4.4.1. OS Support
• 4.4.2. Support in Cellular and Broadband Networks
• 4.4.3. Implementation Code Sizes
• 4.5. Typical Deployment and Traffic Volume Considerations
• 4.5.1. Deployment Possibilities
• 4.5.2. Cellular Networks with 464XLAT
• 4.6. Load Sharing
• 4.7. Logging
• 4.8. Optimization for IPv4-only devices/applications
• 5. Acknowledgements
• 6. IANA Considerations
• 7. Security Considerations
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Appendix A. Change Log
• A.1. 01 - 02
• A.2. 02 - 03
• Authors' Addresses

1. Introduction

As the deployment of IPv6 becomes more prevalent, it follows that network operators will move to building single-stack IPv6 core and access networks to simplify network planning and operations. However, providing customers with IPv4 services continues to be a requirement for the foreseeable future. To meet this need, the IETF has standardized a number of different IPv4aaS technologies for this [LEN2017] based on differing requirements and deployment scenarios.

The number of technologies that have been developed makes it time consuming for a network operator to identify the most appropriate mechanism for their specific deployment. This document provides a comparative analysis of the most commonly used mechanisms to assist operators with this problem.

Five different IPv4aaS solutions are considered. The following IPv6 transition technologies are covered:

1. 464XLAT [RFC6877]
2. Dual Stack Lite [RFC6333]
3. lw4o6 (Lightweight 4over6) [RFC7596]
4. MAP-E [RFC7597]
5. MAP-T [RFC7599]

We note that [RFC6180] gives guidelines for using IPv6 transition mechanisms during IPv6 deployment addressing a much broader topic, whereas this document focuses on a small part of it.

1.1. Requirements Language

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 BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

2. Overview of the Technologies

The following sections introduce the different technologies analyzed in this document, describing some of their most important characteristics.

2.1. 464XLAT

464XLAT is a single/dual translation model, which uses a customer-side translator (CLAT) located in the customer's device to perform stateless NAT64 translation [RFC7915] (more precisely, stateless NAT46, a stateless IP/ICMP translation from IPv4 to IPv6). IPv4-embedded IPv6 addresses [RFC6052] are used for both source and destination addresses. Commonly, a /96 prefix (either the 64:ff9b::/96 Well-Known Prefix, or a Network-Specific Prefix) is used as the IPv6 destination for the IPv4-embedded client traffic.

In the operator's network, the provider-side translator (PLAT) performs stateful NAT64 [RFC6146] to translate the traffic. The destination IPv4 address is extracted from the IPv4-embedded IPv6 packet destination address and the source address is from a pool of public IPv4 addresses.

Alternatively, when a dedicated /64 is not available for translation, the CLAT device uses a stateful NAT44 translation before the stateless NAT46 translation.

Note that we generally do not see state close to the end-user as equally problematic as state in the middle of the network.

In typical deployments, 464XLAT is used together with DNS64, see Section 3.1.2 of [I-D.ietf-v6ops-nat64-deployment]. When an IPv6-only client or application communicates with an IPv4-only server, the DNS64 server returns the IPv4-embedded IPv6 address of the IPv4-only server. In this case, the IPv6-only client sends out IPv6 packets, thus CLAT functions as an IPv6 router and the PLAT performs a stateful NAT64 for these packets. In this case, there is a single translation.

Alternatively, one can say that the DNS64 + stateful NAT64 is used to carry the traffic of the IPv6-only client and the IPv4-only server, and the CLAT is used only for the IPv4 traffic from applications or devices that use literal IPv4 addresses or non-IPv6 compliant APIs.

          Private +----------+ Translated  +----------+     _______
  +------+  IPv4  |   CLAT   |    4-6-4    | Stateful |    ( IPv4  )
  | IPv4 |------->| Stateless|------------>|  PLAT    +--( Internet )
  |Device|<-------|   NAT46  |<------------|  NAT64   |   (________)
  +------+        +----------+      ^      +----------+ 
                                    |                    
                              Operator IPv6
                                network
            

Figure 1: Overview of the 464XLAT architecture

Note: in mobile networks, CLAT is commonly implemented in the user's equipment (UE or smartphone).

2.2. Dual-Stack Lite

Dual-Stack Lite (DS-Lite) [RFC6333] was the first of the considered transition mechanisms to be developed. DS-Lite uses a 'Basic Broadband Bridging' (B4) function in the customer's CE router that encapsulates IPv4 in IPv6 traffic and sends it over the IPv6 native service-provider network to a centralized 'Address Family Transition Router' (AFTR). The AFTR performs encapsulation/decapsulation of the 4in6 traffic and translates the IPv4 payload to public IPv4 source address using a stateful NAPT44 function.

                                         +-------------+
        Private +----------+ IPv4-in-IPv6|Stateful AFTR|
+------+  IPv4  |    B4    |    tunnel   |------+------+     _______
| IPv4 |------->| Encap./  |------------>|Encap.|      |    ( IPv4  )
|Device|<-------|  decap.  |<------------|  /   | NAPT +--( Internet )
+------+        +----------+      ^      |Decap.|  44  |   (________)
                                  |      +------+------+
                            Operator IPv6
                              network
          

Figure 2: Overview of the DS-Lite architecture

2.3. Lightweight 4over6

Lightweight 4over6 (lw4o6) is a variant of DS-Lite. The main difference is that the stateful NAPT44 function is relocated from the centralized AFTR to the customer's B4 element (called a lwB4). The AFTR (called a lwAFTR) function therefore only performs A+P routing and 4in6 encapsulation/decapsulation.

Routing to the correct client and IPv4 address sharing is achieved using the Address + Port (A+P) model [RFC6346] of provisioning each lwB4 with a unique tuple of IPv4 address unique range of layer-4 ports. The client uses these for NAPT44.

The lwAFTR implements a binding table, which has a per-client entry linking the customer's source IPv4 address and allocated range of layer-4 ports to their IPv6 tunnel endpoint address. The binding table allows egress traffic from customers to be validated (to prevent spoofing) and ingress traffic to be correctly encapsulated and forwarded. As there needs to be a per-client entry, an lwAFTR implementation needs to be optimized for performing a per-packet lookup on the binding table.

Direct communication between two lwB4s is performed by hair-pinning traffic through the lwAFTR.

                +-------------+             +----------+
        Private |    lwB4     | IPv4-in-IPv6| Stateless|
+------+  IPv4  |------+------|    tunnel   |  lwAFTR  |     _______
| IPv4 |------->|      |Encap.|------------>|(encap/A+P|    ( IPv4  )
|Device|<-------| NAPT |  /   |<------------|bind. tab +--( Internet )
+------+        |  44  |Decap.|      ^      | routing) |   (________)
                +------+------+      |      +----------+
                              Operator IPv6
                                  network
          

Figure 3: Overview of the lw4o6 architecture

2.4. MAP-E

MAP-E uses a stateless algorithm to embed portions of the customer's allocated IPv4 address (or part of an address with A+P routing) into the IPv6 prefix delegated to the client. This allows for large numbers of clients to be provisioned using a single MAP rule (called a MAP domain). The algorithm also allows for direct IPv4 peer-to-peer communication between hosts provisioned with common MAP rules.

The CE (Customer-Edge) router typically performs stateful NAPT44 [RFC2663] to translate the private IPv4 source addresses and source ports into an address and port range defined by applying the MAP rule applied to the delegated IPv6 prefix. The client address/port allocation size is a design parameter. The CE router then encapsulates the IPv4 packet in an IPv6 packet [RFC2473] and sends it directly to another host in the MAP domain (for peer-to-peer) or to a Border Router (BR) if the IPv4 destination is not covered in one of the CE's MAP rules.

The MAP BR is provisioned with the set of MAP rules for the MAP domains it serves. These rules determine how the MAP BR is to decapsulate traffic that it receives from client, validating the source IPv4 address and layer 4 ports assigned, as well as how to calculate the destination IPv6 address for ingress IPv4 traffic.

                +-------------+             +----------+
        Private |   MAP CE    | IPv4-in-IPv6| Stateless|
+------+  IPv4  |------+------|    tunnel   |  MAP BR  |     _______
| IPv4 |------->|      |Encap.|------------>|(encap/A+P|    ( IPv4  )
|Device|<-------| NAPT |  /   |<------------|algorithm +--( Internet )
+------+        |  44  |Decap.|      ^      | routing) |   (________)
                +------+------+      |      +----------+
                              Operator IPv6
                                  network
          

Figure 4: Overview of the MAP-E architecture

2.5. MAP-T

MAP-T uses the same mapping algorithm as MAP-E. The major difference is that double stateless translation (NAT46 in the CE and NAT64 in the BR) is used to traverse the ISP's IPv6 single-stack network. MAP-T can also be compared to 464XLAT when there is a double translation.

A MAP CE typically performs stateful NAPT44 to translate traffic to a public IPv4 address and port-range calculated by applying the provisioned Basic MAP Rule (BMR - a set of inputs to the algorithm) to the delegated IPv6 prefix. The CE then performs stateless translation from IPv4 to IPv6 [RFC7915]. The MAP BR is provisioned with the same BMR as the client, enabling the received IPv6 traffic to be statelessly NAT64 translated back to the public IPv4 source address used by the client.

Using translation instead of encapsulation also allows IPv4-only nodes to correspond directly with IPv6 nodes in the MAP-T domain that have IPv4-embedded IPv6 addresses.

                +-------------+             +----------+
        Private |   MAP CE    |  Translated | Stateless|
+------+  IPv4  |------+------|    4-6-4    |  MAP BR  |     _______
| IPv4 |------->|      |State-|------------>|(NAT64/A+P|    ( IPv4  )
|Device|<-------| NAPT | less |<------------|algorithm +--( Internet )
+------+        |  44  |NAT46 |      ^      | routing) |   (________)
                +------+------+      |      +----------+
                              Operator IPv6
                                  network
          

Figure 5: Overview of the MAP-T architecture

3. High-level Architectures and their Consequences

3.1. Service Provider Network Traversal

For the data-plane, there are two approaches for traversing the IPv6 provider network:

• 4-6-4 translation
• 4-in-6 encapsulation

In the scope of this document, all of the encapsulation based mechanisms use IP-in-IP tunnelling [RFC2473]. This is a stateless tunneling mechanism which does not require any additional tunnel headers.

It should be noted that both of these approaches result in an increase in the size of the packet that needs to be transported across the operator's network when compared to native IPv4. 4-6-4 translation adds a 20-bytes overhead (the 20-byte IPv4 header is replaced with a 40-byte IPv6 header). Encapsulation has a 40-byte overhead (an IPv6 header is prepended to the IPv4 header).

The increase in packet size can become a significant problem if there is a link with a smaller MTU in the traffic path. This may result in traffic needing to be fragmented at the ingress point to the IPv6 only domain (i.e., the NAT46 or 4in6 encapsulation endpoint). It may also result in the need to implement buffering and fragment re-assembly in the BR node.

The advice given in [RFC7597] Section 8.3.1 is applicable to all of these mechanisms: It is strongly recommended that the MTU in the IPv6-only domain be well managed and that the IPv6 MTU on the CE WAN-side interface be set so that no fragmentation occurs within the boundary of the IPv6-only domain.

3.2. Network Address Translation

For the high-level solution of IPv6 service provider network traversal, MAP-T uses double stateless translation. First at the CE from IPv4 to IPv6 (NAT46), and then from IPv6 to IPv4 (NAT64), at the service provider network.

464XLAT may use double translation (stateless NAT46 + stateful NAT64) or single translation (stateful NAT64), depending on different factors, such as the use of DNS by the applications and the availability of a DNS64 function (in the host or in the service provider network). For deployment guidelines, please refer to [I-D.ietf-v6ops-nat64-deployment].

The first step for the double translation mechanisms is a stateless NAT from IPv4 to IPv6 implemented as SIIT (Stateless IP/ICMP Translation Algorithm) [RFC7915], which does not translate IPv4 header options and/or multicast IP/ICMP packets. With encapsulation-based technologies the header is transported intact and multicast can also be carried.

Single and double translation results in native IPv6 traffic with a layer-4 next-header. The fields in these headers can be used for functions such as hashing across equal-cost multipaths or ACLs. For encapsulation, there is an IPv6 header followed by an IPv4 header. This results in less entropy for hashing algorithms, and may mean that devices in the traffic path that perform header inspection (e.g. router ACLs or firewalls) require the functionality to look into the payload header.

Solutions using double translation can only carry port-aware IP protocols (e.g. TCP, UDP) and ICMP when they are used with IPv4 address sharing (please refer to Section 4.3 for more details). Encapsulation based solutions can carry any other protocols over IP, too.

An in-depth analysis of stateful NAT64 can be found in [RFC6889].

3.3. IPv4 Address Sharing

As public IPv4 address exhaustion is a common motivation for deploying IPv6, transition technologies need to provide a solution for allowing public IPv4 address sharing.

In order to fulfill this requirement, a stateful NAPT function is a necessary function in all of the mechanisms. The major differentiator is where in the architecture this function is located.

The solutions compared by this document fall into two categories:

• CGN-based approaches (DS-Lite, 464XLAT)
• A+P-based approaches (lw4o6, MAP-E, MAP-T)

In the CGN-based model, a device such as a CGN/AFTR or NAT64 performs the NAPT44 function and maintains per-session state for all of the active client's traffic. The customer's device does not require per-session state for NAPT.

In the A+P-based model, a device (usually a CE) performs stateful NAPT44 and maintains per-session state only co-located devices, e.g. in the customer's home network. Here, the centralized network function (lwAFTR or BR) only needs to perform stateless encapsulation/decapsulation or NAT64.

Issues related to IPv4 address sharing mechanisms are described in [RFC6269] and should also be considered.

The address sharing efficiency of the five technologies is significantly different, it is discussed in Section 4.2

lw4o6, MAP-E and MAP-T can also be configured without IPv4 address sharing, see the details in Section 4.3. However, in that case, there is no advantage in terms of public IPv4 address saving. In the case of 464XLAT, this can be achieved as well through EAMT [RFC7757].

Conversely, both MAP-E and MAP-T may be configured to provide more than one public IPv4 address (i.e., an IPv4 prefix shorter than a /32) to customers.

Dynamic DNS issues in address-sharing contexts and their possible solutions using PCP (Port Control Protocol) are discussed in detail in [RFC7393].

3.4. CE Provisioning Considerations

All of the technologies require some provisioning of customer devices. The table below shows which methods currently have extensions for provisioning the different mechanisms.

3.5. Support for Multicast

The solutions covered in this document are all intended for unicast traffic. [RFC8114] describes a method for carrying encapsulated IPv4 multicast traffic over an IPv6 multicast network. This could be deployed in parallel to any of the operator's chosen IPv4aaS mechanism.

4. Detailed Analysis

4.1. Architectural Differences

4.1.1. Basic Comparison

The five IPv4aaS technologies can be classified into 2x2=4 categories on the basis of two aspects:

• Technology used for service provider network traversal. It can be single/double translation or encapsulation.
• Presence or absence of NAPT44 per-flow state in the operator network.

4.2. Tradeoff between Port Number Efficiency and Stateless Operation

464XLAT and DS-Lite use stateful NAPT at the PLAT/AFTR devices, respectively. This may cause scalability issues for the number of clients or volume of traffic, but does not impose a limitation on the number of ports per user, as they can be allocated dynamically on-demand and the allocation policy can be centrally managed/adjusted.

A+P based mechanisms (Lw4o6, MAP-E, and MAP-T) avoid using NAPT in the service provider network. However, this means that the number of ports provided to each user (and hence the effective IPv4 address sharing ratio) must be pre-provisioned to the client.

Changing the allocated port ranges with A+P based technologies, requires more planning and is likely to involve re-provisioning both hosts and operator side equipment. It should be noted that due to the per-customer binding table entry used by lw4o6, a single customer can be re-provisioned (e.g., if they request a full IPv4 address) without needing to change parameters for a number of customers as in a MAP domain.

It is also worth noting that there is a direct relationship between the efficiency of customer public port-allocations and the corresponding logging overhead that may be necessary to meet data-retention requirements. This is considered in Section 4.7 below.

Determining the optimal number of ports for a fixed port set is not an easy task, and may also be impacted by local regulatory law, which may define a maximum number of users per IP address, and consequently a minimum number of ports per user.

On the one hand, the "lack of ports" situation may cause serious problems in the operation of certain applications. For example, Miyakawa has demonstrated the consequences of the session number limitation due to port number shortage on the example of Google Maps [MIY2010]. When the limit was 15, several blocks of the map were missing, and the map was unusable. This study also provided several examples for the session numbers of different applications (the highest one was Apple's iTunes: 230-270 ports).

The port number consumption of different applications is highly varying and e.g. in the case of web browsing it depends on several factors, including the choice of the web page, the web browser, and sometimes even the operating system [REP2014]. For example, under certain conditions, 120-160 ports were used (URL: sohu.com, browser: Firefox under Ubuntu Linux), and in some other cases it was only 3-12 ports (URL: twitter.com, browser: Iceweasel under Debian Linux).

There may be several users behind a CE router, especially in the broadband case (e.g. Internet is used by different members of a family simultaneously), so sufficient ports must be allocated to avoid impacting user experience.

Furthermore, assigning too many ports per CE router will result in waste of public IPv4 addresses, which is a scarce and expensive resource. Clearly this is a big advantage in the case of 464XLAT where they are dynamically managed, so that the number of IPv4 addresses for the sharing-pool is smaller while the availability of ports per user don't need to be pre-defined and is not a limitation for them.

There is a direct tradeoff between the optimization of client port allocations and the associated logging overhead. Section 4.7 discusses this in more depth.

We note that common CE router NAT44 implementations utilizing Netfilter, multiplexes active sessions using a 3-tuple (source address, destination address, and destination port). This means that external source ports can be reused for unique internal source and destination address and port sessions. It is also noted, that Netfilter cannot currently make use of multiple source port ranges (i.e. several blocks of ports distributed across the total port space as is common in MAP deployments), this may influence the design when using stateless technologies.

Stateful technologies, 464XLAT and DS-Lite (and also NAT444) can therefore be much more efficient in terms of port allocation and thus public IP address saving. The price is the stateful operation in the service provider network, which allegedly does not scale up well. It should be noticed that in many cases, all those factors may depend on how it is actually implemented.

XXX MEASUREMENTS ARE PLANNED TO TEST IF THE ABOVE IS TRUE. XXX

We note that some CGN-type solutions can allocate ports dynamically "on the fly". Depending on configuration, this can result in the same customer being allocated ports from different source addresses. This can cause operational issues for protocols and applications that expect multiple flows to be sourced from the same address. E.g., ECMP hashing, STUN, gaming, content delivery networks. However, it should be noticed that this is the same problem when a network has a NAT44 with multiple public IPv4 addresses, or even when applications in a dual-stack case, behave wrongly if happy eyeballs is flapping the flow address between IPv4 and IPv6.

The consequences of IPv4 address sharing [RFC6269] may impact all five technologies. However, when ports are allocated statically, more customers may get ports from the same public IPv4 address, which may results in negative consequences with higher probability, e.g. many applications and service providers (Sony PlayStation Network, OpenDNS, etc.) permanently black-list IPv4 ranges if they detect that they are used for address sharing.

Both cases are, again, implementation dependent.

We note that although it is not of typical use, one can do deterministic, stateful NAT and reserve a fixed set of ports for each customer, as well.

4.3. Support for Public Server Operation

Mechanisms that rely on operator side per-flow state do not, by themselves, offer a way for customers to present services on publicly accessible layer-4 ports.

Port Control Protocol (PCP) [RFC6877] provides a mechanism for a client to request an external public port from a CGN device. For server operation, it is required with NAT64/464XLAT, and it is supported in some DS-Lite AFTR implementations.

A+P based mechanisms distribute a public IPv4 address and restricted range of layer-4 ports to the client. In this case, it is possible for the user to configure their device to offer a publicly accessible server on one of their allocated ports. It should be noted that commonly operators do not assign the Well-Known-Ports to users (unless they are allocating a full IPv4 address), so the user will need to run the service on an allocated port, or configure port translation.

Lw4o6, MAP-E and MAP-T may be configured to allocated clients with a full IPv4 address, allowing exclusive use of all ports, and non-port-based layer 4 protocols. Thus, they may also be used to support server/services operation on their default ports. However, when public IPv4 addresses are assigned to the CE router without address sharing, obviously there is no advantage in terms of IPv4 public addresses saving.

It is also possible to configure specific ports mapping in 464XLAT/NAT64 using EAMT [RFC7757], which means that only those ports are "lost" from the pool of addresses, so there is a higher maximization of the total usage of IPv4/port resources.

4.4. Support and Implementations

4.4.1. OS Support

A 464XLAT client (CLAT) is implemented in Windows 10, Linux (including Android), Windows Mobile, Chrome OS and iOS, but at the time of writing is not available in MacOS.

The remaining four solutions are commonly deployed as functions in the CE device only, however in general, except DS-Lite, the vendors support is poor.

The OpenWRT Linux based open-source OS designed for CE devices offers a number of different 'opkg' packages as part of the distribution:

• '464xlat' enables support for 464XLAT CLAT functionality
• 'ds-lite' enables support for DSLite B4 functionality
• 'map' enables support for MAP-E and lw4o6 CE functionality
• 'map-t' enables support for MAP-T CE functionality

For the operator side functionality, some free open-source implementations exist:

CLAT, NAT64, EAMT:

http://www.jool.mx

MAP-BR, lwAFTR, CGN, CLAT, NAT64:

VPP/fd.io https://gerrit.fd.io/r/#/admin/projects/

lwAFTR:

https://github.com/Igalia/snabb

DSLite AFTR:

https://www.isc.org/downloads/

4.4.2. Support in Cellular and Broadband Networks

Several cellular networks use 464XLAT, whereas we are not aware of any deployment of the four other technologies in cellular networks, as they are not implemented in UE devices.

In broadband networks, there are some deployments of 464XLAT, MAP-E and MAP-T. lw4o6 and DS-Lite have more deployments, with DS-Lite being the most common, but lw4o6 taking over in the last years.

4.4.3. Implementation Code Sizes

As hint to the relative complexity of the mechanisms, the following code sizes are reported from the OpenWRT implementations of each technology are 17kB, 35kB, 15kB, 35kB, and 48kB for 464XLAT, lw4o6, DS-Lite, MAP-E, MAP-T, and lw4o6, respectively (https://openwrt.org/packages/start).

We note that the support for all five technologies requires much less code size than the total sum of the above quantities, because they contain a lot of common functions (data plane is shared among several of them).

4.5. Typical Deployment and Traffic Volume Considerations

4.5.1. Deployment Possibilities

Theoretically, all five IPv4aaS technologies could be used together with DNS64 + stateful NAT64, as it is done in 464XLAT. In this case the CE router would treat the traffic between an IPv6-only client and IPv4-only server as normal IPv6 traffic, and the stateful NAT64 gateway would do a single translation, thus offloading this kind of traffic from the IPv4aaS technology. The cost of this solution would be the need for deploying also DNS64 + stateful NAT64.

However, this has not been implemented in clients or actual deployments, so only 464XLAT always uses this optimization and the other four solutions do not use it at all.

4.5.2. Cellular Networks with 464XLAT

Actual figures from existing deployments, show that the typical traffic volumes in an IPv6-only cellular network, when 464XLAT technology is used together with DNS64, are:

• 75% of traffic is IPv6 end-to-end (no translation)
• 24% of traffic uses DNS64 + NAT64 (1 translation)
• Less than 1% of traffic uses the CLAT in addition to NAT64 (2 translations), due to an IPv4 socket and/or IPv4 literal.

Without using DNS64, 25% of the traffic would undergo double translation.

4.6. Load Sharing

If multiple network-side devices are needed as PLAT/AFTR/BR for capacity, then there is a need for a load sharing mechanism. ECMP (Equal-Cost Multi-Path) load sharing can be used for all technologies, however stateful technologies will be impacted by changes in network topology or device failure.

Technologies utilizing DNS64 can also distribute load across PLAT/AFTR devices, evenly or unevenly, by using different prefixes. Different network specific prefixes can be distributed for subscribers in appropriately sized segments (like split-horizon DNS, also called DNS views).

Stateless technologies, due to the lack of per-flow state, can make use of anycast routing for load sharing and resiliency across network-devices, both ingress and egress; flows can take asymmetric paths through the network, i.e., in through one lwAFTR/BR and out via another.

Mechanisms with centralized NAPT44 state have a number of challenges specifically related to scaling and resilience. As the total amount of client traffic exceeds the capacity of a single CGN instance, additional nodes are required to handle the load. As each CGN maintains a stateful table of active client sessions, this table may need to be syncronized between CGN instances. This is necessary for two reasons:

• To prevent all active customer sessions being dropped in event of a CGN node failure.
• To ensure a matching state table entry for an active session in the event of asymmetric routing through different egress and ingress CGN nodes.

4.7. Logging

In the case of 464XLAT and DS-Lite, the user of any given public IPv4 address and port combination will vary over time, therefore, logging is necessary to meet data retention laws. Each entry in the PLAT/AFTR's generates a logging entry. As discussed in Section 4.2, a client may open hundreds of sessions during common tasks such as web-browsing, each of which needs to be logged so the overall logging burden on the network operator is significant. In some countries, this level of logging is required to comply with data retention legislation.

One common optimization available to reduce the logging overhead is the allocation of a block of ports to a client for the duration of their session. This means that logging entry only needs to be made when the client's port block is released, which dramatically reducing the logging overhead. This comes as the cost of less efficient public address sharing as clients need to be allocated a port block of a fixed size regardless of the actual number of ports that they are using.

Stateless technologies that pre-allocate the IPv4 addresses and ports only require that copies of the active MAP rules (for MAP-E and MAP-T), or binding-table (for lw4o6) are retained along with timestamp information of when they have been active. Support tools (e.g., those used to serve data retention requests) may need to be updated to be aware of the mechanism in use (e.g., implementing the MAP algorithm so that IPv4 information can be linked to the IPv6 prefix delegated to a client). As stateless technologies do not have a centralized stateful element which customer traffic needs to pass through, so if data retention laws mandate per-session logging, there is no simple way of meeting this requirement with a stateless technology alone. Thus a centralized NAPT44 model may be the only way to meet this requirement.

Deterministic CGN [RFC7422] was proposed as a solution to reduce the resource consumption of logging.

4.8. Optimization for IPv4-only devices/applications

When IPv4-only devices or applications are behind a CE connected with IPv6-only and IPv4aaS, the IPv4-only traffic flows will necessarily, be encapsulated/decapsulated (in the case of DS-Lite, lw4o6 and MAP-E) and will reach the IPv4 address of the destination, even if that service supports dual-stack. This means that the traffic flow will cross thru the AFTR, lwAFTR or BR, depending on the specific transition mechanism being used.

Even if those services are directly connected to the operator network (for example, CDNs, caches), or located internally (such as VoIP, etc.), it is not possible to avoid that overhead.

However, in the case of those mechanism that use a NAT46 function, in the CE (464XLAT and MAP-T), it is possible to take advantage of optimization functionalities, such as the ones described in [I-D.palet-v6ops-464xlat-opt-cdn-caches].

Using those optimizations, because the NAT46 has already translated the IPv4-only flow to IPv6, and the services are dual-stack, they can be reached without the need to translate them back to IPv4.

5. Acknowledgements

The authors would like to thank Ole Troan for his thorough review of this draft and acknowledge the inputs of Mark Andrews, Edwin Cordeiro, Fred Baker, Alexandre Petrescu, Cameron Byrne, Tore Anderson, Mikael Abrahamsson, Gert Doering, Satoru Matsushima, Mohamed Boucadair, Tom Petch, Yannis Nikolopoulos, and TBD ...

6. IANA Considerations

This document does not make any request to IANA.

7. Security Considerations

According to the simplest model, the number of bugs is proportional to the number of code lines. Please refer to Section 4.4.3 for code sizes of CE implementations.

For all five technologies, the CE device should contain a DNS proxy. However, the user may change DNS settings. If it happens and lw4o6, MAP-E and MAP-T are used with significantly restricted port set, which is required for an efficient public IPv4 address sharing, the entropy of the source ports is significantly lowered (e.g. from 16 bits to 10 bits, when 1024 port numbers are assigned to each subscriber) and thus these technologies are theoretically less resilient against cache poisoning, see [RFC5452]. However, an efficient cache poisoning attack requires that the subscriber operates an own caching DNS server and the attack is performed in the service provider network. Thus, we consider the chance of the successful exploitation of this vulnerability as low.

An in-depth security analysis of all five IPv6 transition technologies and their most prominent free software implementations according to the methodology defined in [LEN2018] is planned.

8. References

8.1. Normative References

8.2. Informative References

Appendix A. Change Log

A.1. 01 - 02

• Ian Farrer has joined us as an author.
• Restructuring: the description of the five IPv4aaS technologies was moved to a separate section.
• More details and figures were added to the description of the five IPv4aaS technologies.
• Section titled "High-level Architectures and their Consequences" has been completely rewritten.
• Several additions/clarification throughout Section titled "Detailed Analysis".
• Section titled "Performance Analysis" was dropped due to lack of results yet.
• Word based text ported to XML.
• Further text cleanups, added text on state sync and load balancing. Additional comments inline that should be considered for future updates.

A.2. 02 - 03

• The suggestions of Mohamed Boucadair are incorporated.
• New considerations regarding possible optimizations.

Authors' Addresses