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General IPv6 Operations This document describes an extension of the Stateless IP/ICMP Translation for IPv6 Internet Data Centre Environments architecture (SIIT-DC), which allows applications, protocols, or nodes that are incompatible with IPv6, and/or Network Address Translation to operate correctly in an SIIT-DC environment. This is accomplished by introducing a new component called an SIIT-DC Edge Relay, which reverses the translations made by an SIIT-DC Border Relay. The application and/or node is thus provided with seemingly native IPv4 connectivity that provides end-to-end address transparency. The reader is expected to be familiar with the SIIT-DC architecture described in I-D.ietf-v6ops-siit-dc.

SIIT-DC describes an architecture where IPv4-only users can access IPv6-only services through a stateless translator called an SIIT-DC Border Relay (BR). This approach has certain limitations, however. In particular, the following cases will work poorly or not at all: Application protocols that do not support NAT (i.e., the lack of end-to-end transparency of IP addresses). Nodes that cannot connect to IPv6 networks at all, or that can only connect such networks if they also provide IPv4 connectivity (i.e., dual-stacked networks). Application software which makes use of legacy IPv4-only APIs, or otherwise makes assumptions that IPv4 connectivity is available. By extending the SIIT-DC architecture with a new component called an Edge Relay (ER), all of the above can be made to work correctly in an otherwise IPv6-only network environment using SIIT-DC. The purpose of the ER is to reverse the IPv4-to-IPv6 packet translations previously done by the BR for traffic arriving from IPv4 clients and forward this as "native" IPv4 to the node or application. In the reverse direction, IPv4 packets transmitted by the node or application are intercepted by the ER, which translates them to IPv6 before they are forwarded to the BR, which in turn will reverse the translations and forward them to the IPv4 client. The node or application is thus provided with "virtual" IPv4 Internet connectivity that retains end-to-end transparency for the IPv4 addresses.

This document makes use of the following terms: A device or a logical function that translates traffic between IPv4 clients and IPv6 services. See . A device or logical function that provides "native" IPv4 connectivity to IPv4-only nodes or applications. It functions in the same way as a BR, but is located close to the IPv4-only nodes/applications it is supporting rather than on the network border. An IPv4 address representing an IPv6 service, with which IPv4-only clients communicates. It is coupled with an IPv6 Service Address using an EAM. Packets sent to this address is translated to IPv6 by the BR and possibly back to IPv4 again by the ER, and vice versa in the opposite direction. An IPv6 address assigned to an application, node, or service; either directly or indirectly (through an ER). It is coupled with an IPv4 Service Address using an EAM. IPv4-only clients communicates with the IPv6 Service Address through SIIT-DC. A bi-directional coupling between an IPv4 Service Address and an IPv6 Service Address configured in an BR/ER. When translating between IPv4 and IPv6, the BR/ER changes the address fields in the translated packet's IP header according to any matching EAM. The EAM algorithm is specified in . An IPv6 prefix into which the entire IPv4 address space is mapped, according to the algorithm in . The Translation Prefix is routed to the BR's IPv6 interface. When translating between IPv4 and IPv6, an BR/ER will insert/remove the Translation Prefix into/from the address fields in the translated packet's IP header, unless an EAM exists for the IP address that is being translated. As defined in Section 1.3 of . Short for "Internet Data Centre"; a data centre whose main purpose is to deliver services to the public Internet, the use case SIIT-DC is primarily targeted at. IDCs are typically operated by Internet Content Providers or Managed Services Providers. The Stateless IP/ICMP Translation algorithm, as specified in . Short for "Translation". Used in figures to indicate where a BR/ER uses SIIT to translate IPv4 packets to IPv6 and vice versa. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

An Edge Relay (ER) is at its core an implementation of the Stateless IP/ICMP Translation algorithm that supports Explicit Address Mappings. It provides virtual IPv4 connectivity for nodes or applications which require this to operate correctly in an SIIT-DC environment. Packets from the IPv4 Internet destined for an IPv4 Service Address is first translated to IPv6 by a BR. The resulting IPv6 packets are subsequently forwarded to the ER that owns the IPv6 Service Address the translated packets are addressed to. The ER then translates them back to IPv4 before forwarding them to the IPv4 application or node. In the other direction, the exact same translations happen, only in reverse. This process provides end-to-end transparency of IPv4 addresses. An ER may handle an arbitrary number of IPv4/IPv6 Service Addresses. All the EAMs configured in the BR that involve the IPv4/IPv6 Service Addresses handled by an ER MUST also be present in the ER's configuration. An ER may be implemented in two distinct ways; as a software-based service residing inside an otherwise IPv6-only node, or as a network-based service that provides an isolated IPv4 network segment to which nodes that require IPv4 can connect. In both cases native IPv6 connectivity may be provided simultaneously with the virtual IPv4 connectivity. Thus, dual-stack connectivity is facilitated in case the node or application support it. The choice between a node- or network-based ER is made on a per-service or per-node basis. An arbitrary number of each type of ER may co-exist in an SIIT-DC architecture. This section describes the different approaches and discusses which approach fits best for the various use cases.

A Node-based Edge Relay ----------+ [IPv6-only IDC network] | +----------------+| | | /--(ER/XLAT)--AF_INET Dual-stack || \-------------------------+ | Application || | \------------AF_INET6 Software || | +----------------+| +--------------------------------------+ ]]>

A node-based ER is typically implemented as a logical software function that runs inside the operating system of an IPv6 node. It provides applications running on the same node with IPv4 connectivity. Its IPv4 Service Address SHOULD be considered a regular local address that allows application running on the same node to use it with IPv4-only API calls, e.g., to create AF_INET sockets that listen for and accept incoming connections to its IPv4 Service Address. An ER may accomplish this by creating a virtual network adapter to which it assigns the IPv4 Service Address and points a default IPv4 route. This approach is similar to the "Bump-in-the-Stack" approach discussed in , however it does not include an Extension Name Resolver. As shown in , if the application supports dual-stack operation, IPv6 clients will be able to communicate with it directly using native IPv6. Neither the BR nor the ER will intercept this communication. Support for IPv6 in the application is however not a requirement; the application may opt not to establish any IPv6 sockets. Foregoing IPv6 in this manner will simply preclude connectivity to the service from IPv6-only clients; connectivity to the service from IPv4 clients (through the BR) will continue work in the same way. The ER requires a dedicated IPv6 Service Address for each IPv4 Service Address it has configured. The IPv6 network MUST forward traffic to these IPv6 Service Addresses to the node, whose operating system MUST in turn forward them to the ER. This document does not attempt to fully explore the multitude of ways this could be accomplished, however considering that the IPv6 protocol is designed for having multiple addresses assigned to a single node, one particularly straight-forward way would be to assign the ER's IPv6 Service Addresses as secondary IPv6 addresses on the node itself so that it the upstream router learns of their location using the IPv6 Neighbor Discovery Protocol.

A Basic Network-based Edge Relay --+ | | +----------------+| +-----|-----+ [v4-only] | | IPv4-only || | (ER/XLAT)-----[network]--------AF_INET Application || +-----------+ [segment] | | Software || | +----------------+| +---------------------------+ ]]>

A network-based ER performs the exact same as a node-based ER does, only that instead of assigning the IPv4 Service Addresses to an internal-only virtual network adapter, traffic destined for them are forwarded onto a network segment to which nodes that require IPv4 connectivity connect to. The ER also functions as the default IPv4 router for the nodes on this network segment. Each node on the IPv4 network segment MUST acquire and assign an IPv4 Service Address to a local network interface. While this document does not attempt to explore all the various methods by which this could be accomplished, some examples are provided in . The basic ER illustrated in establishes an IPv4-only network segment between itself and the IPv4-only nodes it serves. This is fine if the nodes it provides IPv4 access have no support for IPv6 whatsoever; however if they are dual-stack capable, it is would not be ideal to take away their IPv6 connectivity in this manner. While it is RECOMMENDED to use a node-based ER in this case, appropriate implementations of a node-based ER might not be available for every node. If the application protocol in question does not work correctly in a NAT environment, standard SIIT-DC cannot be used either, which leaves a network-based ER is the only remaining solution. The following subsections contains examples on how the ER could be implemented in a way that provides IPv6 connectivity for dual-stack capable nodes.

A Network-based Edge Relay "On A Stick" --+ | +-------------+ | +----------------+| | | /---AF_INET Dual-stack || [Dual-stack network segment]----< | Application || | \--AF_INET6 Software || | +----------------+| +----------------------------+ ]]>

The ER "On A Stick" approach illustrated in ensures that the dual-stack capable node retains native IPv6 connectivity by connecting the ER's IPv4 and IPv6 interfaces to the same network segment, alternatively by using a single dual-stacked interface. Native IPv6 traffic between the IDC network and the node bypasses the ER entirely, while IPv4 traffic from the node will be routed directly to the ER (because it acts as its default IPv4 router), where it is translated to IPv6 before being transmitted to the upstream default IPv6 router. The ER could attract inbound traffic to the IPv6 Service Addresses by responding to the upstream router's IPv6 Neighbor Discovery messages for them.

A Network-based Edge Relay containing an IPv6 Bridge --+ +-----------|--------------+ | +----------------+| | | /---AF_INET Dual-stack || [Dual-stack network segment]----< | Application || | \--AF_INET6 Software || | +----------------+| +----------------------------+ ]]>

The ER illustrated in will transparently bridge IPv6 frames between its upstream and downstream interfaces. IPv6 packets addressed the ER's own IPv6 Service Addresses from the upstream IDC network are intercepted (e.g., by responding to IPv6 Neighbor Discovery messages for them) and routed through the translation function before being forwarded out its downstream interface as IPv4 packets. The downstream network segment thus becomes dual-stacked.

The IPv6 Path MTU between the ER and the BR will typically be larger than the default value defined in Section 4 of (1280), as it will typically contained within a single administrative domain. Therefore, it is RECOMMENDED that the IPv6 Path MTU configured in the ER is raised accordingly. It is RECOMMENDED that the ER and the BR use identical configured IPv6 Path MTU values.

In order to avoid IPv6 fragmentation, an ER SHOULD ensure that the IPv4 MTU used by applications or nodes is equal to the configured IPv6 Path MTU - 20, so that an maximum-sized IPv4 packet can fit in an unfragmented IPv6 packet. This ensures that the application may do its part in avoiding IP-level fragmentation from occurring, e.g., by segmenting/fragmenting outbound packets at the application layer, and advertising the maximum size its peer may use for inbound packets (e.g., through the use of the TCP MSS option). A node-based ER could accomplish this by configuring this MTU value on the virtual network adapter, while a network-based ER could do so by advertising the MTU to its downstream nodes using the DHCPv4 Interface MTU Option.

If the generation of IPv6 Atomic Fragments is disabled, the value of the IPv4 Identification header will be lost during the translation. Conversely, enabling the generation of IPv6 Atomic Fragments will ensure that the IPv4 Identification Header will carried end-to-end. Note that for this to work bi-directionally, IPv6 Atomic Fragment generation MUST be enabled on both the BR and the ER. Apart from certain diagnostic tools, there are few (if any) application protocols that make use of the IPv4 Identification header. Therefore, the loss of the IPv4 Identification value will therefore generally not cause any problems. IPv6 Atomic Fragments and their impact on the IPv4 Identification header is further discussed in Section 4.9.2 of .

Although SIIT-DC is primarily intended to facilitate communication between IPv4-only nodes on the Internet and services located in an IPv6-only IDC network, an IPv4-only node or application located behind an ER might need to communicate with other nodes or services in the IDC. The IPv4-only node or application will need to so through the ER, as it will typically be incapable to contact IPv6 destinations directly. The following subsections discusses various methods on how to facilitate such communication.

If the BR supports hairpinning as described in Section 4.2 of I-D.ietf-v6ops-siit-eam, the easiest solution is to make the target service available through SIIT-DC in the normal way, that is, by provisioning an EAM to the BR that assigns an IPv4 Service Address with the target service's IPv6 Service Address. This allows the IPv4-only node or application to transmit packets destined for the target service's IPv4 Service Address, which the ER will then translate to a corresponding IPv4-converted IPv6 address by inserting the Translation Prefix . When this IPv6 packet reaches the BR, it will be hairpinned and transmitted back to the target service's IPv6 Service Address (where it could possibly pass through another ER before reaching the target service). Return traffic from the target service will be hairpinned in the same fashion.

Hairpinned IPv4-IPv4 packet flow | DST 2001:db8:46::192.0.2.2 |---\ +---------------+ +----------------------------+ | (XLAT#2) +-[Pkt#4: IPv4]-+ +--[Pkt#3: IPv6]-------------+ ( @ BR ) | SRC 192.0.2.1 | (XLAT#3) | SRC 2001:db8:46::192.0.2.1 | | | DST 192.0.2.2 |<--(@ ER B)--| DST 2001:db8:b:: |<--/ +---------------+ +----------------------------+ ]]>

illustrates the flow of a hairpinned packet sent from the IPv4-only node/app behind ER A towards an IPv6-only node/app behind ER B. ER A is configured with the EAM {192.0.2.1,2001:db8:a::}, ER B with {192.0.2.2,2001:db8:b::}. The BR is configured with both EAMs, and supports hairpinning. Note that if the target service had not been located behind an ER, the third and final translation (XLAT#3) would not have happened, i.e., the target service/node would have received and responded to packet #3 directly. If the IPv4-only nodes/services do not need connectivity with the public IPv4 Internet, private IPv4 addresses could be used as their IPv4 Service Addresses in order to conserve the IDC operator's pool of public IPv4 addresses.

If the BR does not support hairpinning, or if the hairpinning solution is not desired for some other reason, intra-IDC IPv4 traffic may be facilitated by configuring additional EAMs on the ER for each service the IPv4-only node or application needs to communicate with. This makes the IPv6 traffic between the ER and the target service's IPv6 Service Address follow the direct path through the IPv6 network. The traffic does not pass the BR, which means that this solution might yield better latency than the hairpinning approach. The additional EAM configured in the ER consists of the target's IPv6 Service Address and an IPv4 Service Address. The IPv4-only node or application will contact the target's assigned IPv4 Service Address using its own IPv4 Service Address as the source. The ER will then proceed to translate this to an IPv6 packet with the local application/node's own IPv6 Service Address as source and the target service's IPv6 Service Address as the destination, and forward this to the IPv6 network. Replies from the target service will undergo these translations in reverse. If the target service is also located behind another ER, that other ER MUST also be provisioned with an additional EAM that contains the origin IPv4-only application/node's IPv4 and IPv6 Service Addresses. Otherwise, the target service's ER will be unable to translate the source address of the incoming packets.

Non-hairpinned IPv4-IPv4 packet flow | DST 2001:db8:b:: | +---------------+ +------------------+ | +-[Pkt#3: IPv4]-+ | | SRC 192.0.2.1 | (XLAT#2) | | DST 192.0.2.2 |<-------(@ ER B)------/ +---------------+ ]]>

illustrates the flow of a packet carrying intra-IDC IPv4 traffic between two IPv4-only nodes/applications that are both located behind ERs. Both ER A and ER B are configured with two EAMs: {192.0.2.1,2001:db8:a::} and {192.0.2.2,2001:db8:b::}. The packet will follow the regular routing path through the IPv6 IDC network; the BR is not involved and the packet will not be hairpinned. The above approach is not mutually exclusive with the hairpinning approach described in : If both EAMs above are also configured on the BR, both 192.0.2.1 and 192.0.2.2 would be reachable from other IPv4-only services/nodes using the hairpinning approach. They would also be reachable from the IPv4 Internet. Note that if the target service in this example was not located behind an ER, but instead was a native IPv6 service listening on 2001:db8:b::, the second translation step in would not occur; the target service would receive and respond to packet #2 directly. As with the hairpinning approach, if the IPv4-only nodes/services do not need connectivity to/from the public IPv4 Internet, private IPv4 addresses could be used as their IPv4 Service Addresses. Alternatively, in the case where the target service is on native IPv6, the target's assigned IPv4 Service Address has only local significance behind the ER. It could therefore be assigned from the IPv4 Service Continuity Prefix.

The author would like to especially thank the authors of 464XLAT: Masataka Mawatari, Masanobu Kawashima, and Cameron Byrne. The architecture described by this document is merely an adaptation of their work to a data centre environment, and could not have happened without them. The author would like also to thank the following individuals for their contributions, suggestions, corrections, and criticisms: Fred Baker, Tobias Brox, Ray Hunter, Shucheng LIU (Will), Andrew Yourtchenko.

This draft makes no request of the IANA. The RFC Editor may remove this section prior to publication.

This section discusses security considerations specific to the use of an ER. See the Security Considerations section in for additional security considerations applicable to the SIIT-DC architecture in general.

If the ER receives an IPv4 packet from the application/node from a source address it does not have an EAM for, both the source and destination addresses will be rewritten according to . After undergoing the reverse translation in the BR, the resulting IPv4 packet routed to the IPv4 network will have a spoofed IPv4 source address. The ER SHOULD therefore ensure that ingress filtering is used on the ER's IPv4 interface, so that such packets are immediately discarded. If the ER receives an IPv6 packet with both the source and destination address equal to one of its local IPv6 Service Addresses, the resulting packet would appear to the IPv4-only application/node as locally generated, as both the source address and the destination address will be the same address. This could trick the application into believing the packet came from a trusted source (itself). To prevent this, the ER SHOULD discard any received IPv6 packets that have a source address that is either 1) equal to any of its local IPv6 Service Addresses, or 2) after translation from IPv6 to IPv4, equal to any of its local IPv4 Service Addresses.

One relatively straight-forward way to provide IPv4 connectivity between the ER and the IPv4 node(s) it serves is to ensure the IPv4 Service Address(es) can be enclosed within a larger IPv4 prefix. The ER may then claim one address in this prefix for itself, and use it to provide an IPv4 default router address. The ER may then proceed to assign the IPv4 Service Address(es) to its downstream node(s) using DHCPv4. For example, if the IPv4 Service Addresses are 192.0.2.26 and 192.0.2.27, the ER would configure the address 192.0.2.25/29 on its IPv4-facing interface and would add the two IPv4 Service Addresses to its DHCPv4 pool. One disadvantage of this method is that IPv4 communication between the IPv4 node(s) behind the ER and other services made available through SIIT-DC becomes impossible, if those other services are assigned IPv4 Service Addresses that also are covered by the same IPv4 prefix (e.g., 192.0.2.28). This happens because the IPv4 nodes will mistakenly believe they have an on-link route to the entire prefix, and attempt to resolve the addresses using ARP, instead of sending them to the ER for translation to IPv6. This problem could however be overcome by avoiding assigning IPv4 Service Addresses which overlaps with an IPv4 prefix handled by an ER (at the expense of wasting some potential IPv4 Service Addresses), or by ensuring that the overlapping IPv6 Service Addresses are only assigned to services which do not need to communicate with the IPv4 node(s) behind the ER. A third way to avoid this problem is discussed in .

In order to avoid the problem discussed in , a private unrouted IPv4 network that does not encompass the IPv4 Service Address(es) could be used to provide connectivity between the ER and the IPv4-only node(s) it serves. An IPv4-only node must then assign its IPv4 Service Address as secondary local address, while the ER routes each of the IPv4 Service Addresses to its assigned node using that node's private on-link IPv4 address as the next-hop. This approach would ensure there are no overlaps with IPv4 Service addresses elsewhere in the infrastructure, but on the other hand it would preclude the use of DHCPv4 for assigning the IPv4 Service Addresses. This approach creates a need to ensure that the IPv4 application is selecting the IPv4 Service Address (as opposed to its private on-link IPv4 address) as its source address when initiating outbound connections. This could be accomplished by altering the Default Address Selection Policy Table on the IPv4 node.