464XLAT/MAT-T Optimization

Because the IPv4-only devices will not be able to query for AAAA records, the NAT46/CLAT/CE will translate the IPv4 addresses from the A record for the CDN/cache destination, using the WKP or NSP, as configured by the operator. If the CDN/cache provider is able to configure, in the relevant interfaces of the CDN/caches, the same IPv6 addresses that will naturally result as the translated destination addresses for the queried A records, preceded by the WKP or NSP, then having more specific routing prefixes, will result in traffic to those destinations being directly forwarded towards those interfaces, instead of needing to traverse the NAT64. For example, let's suppose a provider using the WKP (64:ff9b::/96) and a SmartTV querying for www.example.com:

Note: Examples using text representation as per Section 2.3 of and IPv4 documentation addresses following . It should be remarked that this approach requires that the path to the destination is configured in such way (i.e., more specific routing prefixes), that doesn't traverse the NAT64 devices. Because the WKP is non-routable, this solution will only be possible if the CDN/cache is in the same ASN as the provider network, or somehow interconnected without routing thru Internet. This solution has the additional drawback of the operational complexity/issues added to the operation of the CDN/cache, and the need to synchronize any IPv4 interface address changes with the relevant IPv6 ones, and possibly with routing.

If the NAT46/CLAT/CE, as commonly is the case, is also a DNS proxy/stub resolver, it is possible to modify the behavior and create an "internal" interaction among both of them. This approach uses the existing IPv4 and IPv6 addresses in the A and AAAA records, respectively, so no additional complexity/issues added to the CDN/caches operations. The following sub-sections detail this approach and provide a step-by-step example case.

This optimization MUST be enabled by default, but only when the WAN link is IPv6-only and the NAT46/CLAT is being used. Note that the NAT64/CLAT can only be used if the NAT64 prefix has been discovered. This allows the users to get CEs from the retail and take advantage of the optimization, without requiring any configuration. The CE MUST support a way to completely disable the optimization, in order to allow the operator to turn it off in case it is required. It is expected as well, that the NAT64/CLAT is only enabled if the WAN link is IPv6-only.

The goal is to ensure that only IPv4-only hosts are optimized. Towards that, the CE MUST use ARP and ND (and their relevant caches, if the information of a hosts has been already learnt) each time a new host starts a connection. If it is possible to bind the same MAC address to both an IPv4 and IPv6 address, then the host is not IPv4-only (it may be IPv6-only or dual-stack), and MUST NOT be optimized. The CE MUST maintain a table of IPv4-only hosts and ensure that if any IPv4-only hosts become IPv6-enabled, it is properly handled. This mechanism to detect IPv4-only hosts has two drawbacks: If a subscriber has intermediate dual-stack routers in between the IPv4-only host and the NAT46/CLAT, the IPv4-only hosts will be detected as dual-stack, so no optimization will be performed. This is not the most common scenario, as typically the devices that are more relevant to the optimization (in terms of those that generate more IPv4-only traffic) are directly attached to the CE, or in bridged interfaces. If a host is dual-stack, but has some IPv4-only applications, because the host will be detected as dual-stack, none of the applications will be optimized. This is a good trade-off, considering the most important traffic to optimize is typically coming from real IPv4-only devices such as old SmartTVs/STBs. Furthermore, this avoid breaking other mechanisms present only in dual-stack hosts, such as Happy Eyeballs and simplifies troubleshooting. It needs to be remarked that, if the detection of the IPv4-only host is done incorrectly (either not detecting it or by a false detection), the goal is that no harm is caused. In the worst case, optimization MUST NOT be performed.

In the case of an IPv4-only detected host, the DNS proxy/stub resolver MUST actually perform an additional AAAA query, unless the information is already present in the Additional Section, as per Section 3 of . Note that the NAT46/CLAT MUST already know the WKP or NSP being used in that network. If the response contains at least one IPv6 address not using the WKP/NSP, it means that the destination is IPv6-enabled (because at least one of the IPv6 addresses is not synthesized). This means that it is possible for the NAT46/CLAT, to create an Explicit Address Mapping ().

In the case of an IPv4-only detected host, the CE DNS proxy MUST only return the answer to an A query once any of the following happens: An answer to the AAAA query has been received. A SERVFAIL has been received. The "Resolution Delay" has passed. The Resolution Delay MUST be set to the same value (50 milliseconds) as indicated by Happy Eyeballs .

An EAM Table (EAMT used for short, across the rest of this document) MUST be created/maintained automatically by the NAT46/CLAT, which is responsible to prioritize any available entries in the EAMT, versus the use of any synthetic AAAA. The EAMT entry MUST only be created if all the following conditions are met: There is not already EAMT entry for the same pair of MAC/destination IPv4 address/prefix. The source host is IPv4-only. The DNS proxy is ready to return the A answer (according to ). There is at least one non-synthesized AAAA response. If DNSSEC is available, the response has been locally validated or the AD bit has been set by a trusted resolver, as per . This avoids a slight NAT64 overload and flapping between destination addresses (IPv4/IPv6), which may impact some applications, at the cost of a small extra delay for the initial communication setup, when the EAMT entry doesn't yet exist. Each EAMT entry MUST contain, the fields already described in and a few new extensions (as per section 3.1 of ): ID: EAMT Entry Index (optional). MAC address: Identify the host to which this EAMT entry belongs. Destination IPv4 address/prefix: By default, the prefix length is 32 bits. Destination IPv6 address/prefix: By default, the prefix length is 128 bits. Only non-synthesized addresses are allowed. TTL: It MUST be set to the minimum value from the AAAA/A RR pair. In normal conditions the TTL for both A and AAAA records, of a given FQDN, should be the same, so this ensures a proper behavior if there is any DNS mismatch. FQDN: The one that originated the A query for this EAMT entry. Required in order to ensure a correct detection of cases such as the use of reverse-proxy with a single IPv4 address to multiple IPv6 addresses. Valid/Stale/Invalid: When set to Stale, means that this EAMT entry MUST NOT be used for new connections. When set to Invalid, means that this EAMT entry can be deleted, unless the Auto/Static bit is also set. Auto/Static: When set to 1, means that this EAMT entry has been manually/statically configured, for example by means of an explicit configuration (GUI, CLI, provisioning system, etc.). Note that allowing destination IPv4 and IPv6 prefixes, together with the Valid/Stale/Invalid and Auto/Static flags, allows manual explicit optimization and non-optimization configuration for specific parts of Internet. When a new EAMT entry is first automatically created, it is flagged as "Valid" and "Auto". If a subsequent A query, with a different FQDN, results in an IPv4 address that has already an EAMT entry and a different IPv6 address, it means that some reverse-proxy or similar functionality is being used by the IPv6-enabled service. In this case, the existing EAMT entry will be marked as "Stale". No new EAMT entry is created for that IPv4 address. Otherwise, the optimization will only allow to access the first set of IPv4/IPv6/FQDN, which may break the access to other FQDN that share the same IPv4 address and different IPv6 addresses. In this case the EAMT entry will still become "Invalid" according the TTL, which allows to re-enable optimization if a new query for the A record has changed the situation. For example, maybe the reverse-proxy has been removed, or there is now only a single app using it, so at the time being, the optimization is again possible without creating troubles to other hosts. An EAMT entry marked as "Stale" or "Invalid", only affects the relevant host. Other hosts may have their own EAMT entries or they are using the regular NAT46/CLAT+NAT64 path (without the optimization).

Following this approach, if there is a valid EAMT entry, for a given pair of source-MAC-address/IPv4-destination, the IPv6-native path pointed by the IPv6 address of that EAMT entry, will take precedence versus the NAT64 path, so the traffic will not be forwarded to the NAT64. However, this is not sufficient to ensure that individual applications are able to keep existing connections. In many cases, audio and video streaming may use a single TCP connection lasting from minutes to hours. Instead, the CDN TTLs may be configured in the range from 10 to 300 seconds in order to allow new resolutions to switch quickly and to handle large recursive resolvers (with hundreds of thousands of clients behind them). Consequently, the EAMT entries MUST NOT be used directly to establish a forwarding path, but instead, MUST be used to create a stateful NAT entry for the 4-tuple for the duration of the session/connection. This means that to implement the optimization the NAT46 MUST be stateful. Typically, stateful NAT46 are implemented by means of a stateful NAT44 (which often maybe hardware off-loaded), followed by a stateless NAT46. If the SoC/code is able to do stateless NAT46, this still could be used when the optimization is disabled.

An EATM entry with the Auto/Static bit set, MUST NOT be automatically modified. An EAMT entry with the Auto/Static bit clear, MUST be set to Stale in case of: TTL time-out. Or the conditions for creation of the EAMT entry () aren't longer met. Entries in Stale state MUST be set to Invalid once existing connections time-out. The Invalid EAMT entries MUST be deleted, unless the Auto/Static bit is set. This allows users/operators to set explicit rules for diagnostics or resolution of issues in special situations.

Using the same example as in the previous approach:

The following is an example of the CE behavior after the previous case has already created an EAMT entry and a reverse-proxy is detected: A query for www.example.net A RR is received www.example.net A 192.0.2.1 www.example.net AAAA 2001:db8::e:e:f:f A conflict has been detected The existing EAMT entry for 192.0.2.1 is set to Stale

If multiple A/AAAA RRs are available, any of them could be chosen and the optimization will not present any different result to the hosts compared with a situation where the optimization is not used. Existing DNS proxy/stub resolvers already implement mechanisms for DNS Load Balancing (). This don't need to be modified to implement the optimization if some degree of randomness is already secured. To secure sufficient randomness, a possible algorithm shall ensure that different EAMT entries (for different hosts) are permuted randomly among different A/AAAA records on the A/AAAA RR set.

464XLAT can be deployed/used with and without a DNS64. However, as indicated in , the EAMT entry is only created when the service is IPv6-enabled, because the optimization is only relevant for destinations which already have AAAA records. In those cases DNS64 is not relevant.

Because the EAMT entries are only created when the NAT46/CLAT/CE proxy/stub DNS is being used, any hosts or applications that don't use DNS, will not create the relevant entries. They will not be optimized unless EAMT entries are statically configured.

Hosts or applications may use DNS servers from other networks. For a complete description of reasons for that, refer to Section 4.4 of . In the case the DNS is modified, or some hosts or applications use other DNS servers, the possible scenarios and the implications are: Devices configured to use a DNS proxy/resolver which is not the CE/NAT46/CLAT (as it is the case for some consumer electronics). In this case this optimization will not work, because the EAMT entry will not be created based on their own flows. Nevertheless, the EAMT entry may be manually created. However, the lack of EAMT entry, will not impact negatively in the user’s hosts/applications (the optimization is not performed). It should be noticed that users commonly, don't change the configuration of devices such as SmartTVs or STBs (if they do, some other functionalities, such as CDN/caches optimizations may not work as well), so this only happens typically if the vendor is doing it on-purpose and for good well-known reasons. DNS privacy/encryption. Hosts or applications that use mechanisms for DNS privacy/encryption, such as DoT (, ), DoH () or DoQ (), will not make use of the stub/proxy resolver, so the same considerations as for the previous case applies. Users that modify the DNS in their Operating Systems. This is quite frequent, however commonly Operating Systems are dual-stack, so aren't part of the problem statement described by this document and will not be adversely affected. Users that modify the DNS in the CE. This is less common. In this case, this optimization is not adversely affected, because it doesn't depend on the operator DNS, it works only based on the internal CE interaction between the NAT46/CLAT and the stub/proxy resolver. Note that it may be affected if the operator offers different "DNS views" or "split DNS", however this is not related to this optimization and will anyway impact in the other possible operator optimizations (i.e. CDN/cache features). Combinations of the above ones. No further impact is observed, beyond the ones already described.

If a dual-stack host is being detected as IPv4-only, is because it is not responding to the CE ND messages, so by all means, should be considered, at the time being, as IPv4-only, and consequently EAMT entries will be created and traffic will be optimized for IPv4 flows. However, if this hosts suddenly become IPv6-enabled (or dual-stack), the relevant EAMT entries must be flagged by the CE as "Stale". The host will be able to complete the connections and the entries will be marked as "Invalid" and deleted. Anyway, those EAMT entries, while "Valid", may not be actually used by the dual-stack hosts, because those hosts or applications should prefer IPv6, so most probably was either a temporary failure or done on-purpose (user, troubleshooting). If the host is preferring IPv4 for connecting to the CDN/cache or IPv6-enabled service, it will be actually using the NAT46/CLAT, including the EAMT entry and consequently IPv6, so this mechanism will be correcting an undesirable behavior. This may be also a special case, which actually seems to be an incoherent host or application implementation.

Happy Eyeballs is only enabled in dual-stack hosts. Consequently, it is not affected by this optimization because both, as the host should not be detected as IPv4-only, following . If Happy Eyeballs triggers a fallback to IPv4 for a given host, it will be actually using IPv4 without the optimization, which in turn, uses the IPv6-only WAN link of the NAT46/CLAT/CE. So even if Happy Eyeballs is present, IPv4 is expected to be slower than native IPv6 itself due to delays added by the NAT46/CLAT+NAT64 translations. This optimization reduces those delays by eliminating the second translation (NAT64) for IPv4-only detected hosts. However, there may be cases where this may be understood as problematic. The possible reasons why Happy Eyeballs may trigger an IPv4 fallback, in the case of IPv6-only access networks with IPv4aaS, in general, can be classified as: Failure at the CE or customer LANs. It may happen that the CE or other devices in the customer LANs are showing erratic behaviors or malfunctions. It is difficult to believe that this happens only with IPv6, and if that's the case Happy Eyeballs will not resolve the issue, because IPv4 is provided as a service on top of IPv6. Complete failure of the IPv6-only link or IPv6-only operator's infrastructure (up to the NAT64). In this case, IPv4 will not work for that subscriber. Happy Eyeballs will not resolve the issue, and instead will only be adding some extra delay (the attempt to fallback to IPv4 before timing-out). Complete failure of both IPv4 and IPv6 links behind the operator's NAT64 towards the destination. In this case, typically both, IPv4 and IPv6 will fail (in many cases, they are dual-stack links, not different links). Again, Happy Eyeballs will also fail to resolve the issue. Complete failure only in the IPv6 links behind the operator's NAT64 towards the destination. This is less frequent, bus still miss-configured AAAA RRs, or diverse paths for IPv4 and IPv6 together with outages or IPv6-only routing issues, could generate this problem. In this case, Happy Eyeballs could resolve the issue, however. It will work because the optimization is not enabled for dual-stack hosts. Partial failure: Slower IPv6 vs IPv4 path end-to-end. In general, the added delay of the IPv4 translations and NATs across the path, increases the chances that IPv4 is slower than IPv6. However, it may happen that there is some IPv6 specific link congestion or packet dropping, that generates the reverse situation, so IPv4 become faster than IPv6. Because the optimization is not being used for dual-stack hosts, Happy Eyeballs will be resolving the issue. In summary, the optimization will not change the Happy Eyeballs behaviour. Furthermore, it should be observed that IPv6 failures will also impact other operators (even if not using the optimization), and especially those using only NAT64+DNS64 instead of 464XLAT, or even more, any IPv6-only hosts and applications in any operator network across the entire Internet. It looks like it is very important to make sure that, as IPv6 is more prevalent, there is a better monitoring and failures are detected ASAP, instead of being hidden by Happy Eyeballs, specially in IPv6-only networks. It should be noticed also that in IPv6-only with IPv4aaS, the chances of troubles in the IPv4 paths seem to be higher than in the IPv6, as there are more translations, more devices, more delays, while the optimization will precisely reduce them.

When there is a need to troubleshoot IPv4 from the CE LANs, it may happen that there is an EAMT entry forcing the flow to a given destination(s) to use IPv6, which will distort the results, unless the host being used is dual-stack (which is the most common situation). This can be avoided, using a CLI/GUI or provisioning procedure, to either completely disable the optimization during the troubleshooting, or create specific static EAMT entries, using the Valid/Stale/Invalid and Auto/Static flags, as described in . Consequently, the CE MUST allow both, disabling the optimization and the setup of manual/static EAMT entries.

Instead of using the DNS proxy/stub resolver to create the EAMT entries, the operator may push this table (or parts of it) into the CE/NAT46/CLAT, by using configuration/management mechanisms. This solution has the advantage of not being affected by any DNS changes from the user (the EAMT is created by the operator) and ensures a complete control from the operator. However, it may impact the cases of devices with a DNS configured by the vendor. In general, most of the considerations from the previous approach will apply. One more advantage of this solution is that the EAMT pairs doesn't need to match the "real" IPv4/IPv6 addresses available in the A/AAAA records, as shown in the next example.

EAMT may contain TTLs which probably are derived from DNS ones, or alternatively, a global TTL for the full table. An alternative way to configure the table, is that the CE is actually pulling the table (or parts of it) from the operator infrastructure. In this case it will be mandatory that the entries have individual TTLs, again probably derived from the DNS ones. This approach has three major drawbacks: CDNs are used to do frequent changes at the DNS level, so unless the CDNs offer an API or equivalent convenient solution to keep updated the EAMT, the operator will need to cache the most frequent FQDNs being resolved in their own DNS and based on the TTLs, update the EAMT. It requires a new protocol, or an extension to existing ones, in order to push or pull the EAMT updates. It may generate additional bandwidth utilization in the WAN links for every CE when the EAMT needs to be update, even when a CE reboots.