The Mapping of Address and Port Using Translation feature provides connectivity to IPv4 hosts across IPv6 domains. Mapping of address and port using translation (MAP-T) builds on the existing stateless IPv4 and IPv6 address translation techniques that are specified in RFCs 6052, 6144, and 6145.

MAP-T is a mechanism that performs double translation (IPv4 to IPv6 and vice versa) on customer edge (CE) devices and border routers. The Mapping of Address and Port Using Translation feature supports only the MAP-T border router functionality. This feature does not support the MAP-T CE functionality.

The Mapping of Address and Port Using Translation feature leverages the Network Address Translation 64 (NAT64) translation engine and adds the MAP-T border router function to the NAT64 stateless function. MAP-T is enabled on IPv4 and IPv6 interfaces. MAP-T uses IPv4 and IPv6 forwarding, IPv4 and IPv6 fragmentation functions, and NAT64 translation functions. A MAP-T domain is one or more MAP CE devices and a border router, all connected to the same IPv6 network.

A MAP-T CE device connects a user’s private IPv4 address and the native IPv6 network to the IPv6-only MAP-T domain. The MAP-T border router uses the stateless IPv4/IPv6 translation to connect external IPv4 networks to all devices available in the one or more MAP-T domains. MAP-T requires only one IPv6 prefix per network and supports the regular IPv6 prefix/address assignment mechanisms. The MAP-T domain contains regular IPv6-only hosts or servers that have an IPv4-translatable IPv6 address. MAP-T does not require the operation of an IPv4 overlay network or the introduction of a non-native-IPv6 network device or server functionality.

Mapping rules define the mapping between an IPv4 prefix and an IPv4 address or between a shared IPv4 address and an IPv6 prefix/address. Each mapping of address and port using translation (MAP-T) domain uses a different mapping rule.

A MAP-T configuration has one basic mapping rule (BMR), one default mapping rule (DMR), and one or more forwarding mapping rules (FMRs) for each MAP-T domain. You must configure the DMR before configuring the BMR for a MAP-T domain.

The three types of mapping rules are described below:

• A BMR configures the MAP IPv6 address or prefix. The basic mapping rule is configured for the source address prefix. You can configure only one basic mapping rule per IPv6 prefix. The basic mapping rule is used by the MAP-T CE to configure itself with an IPv4 address, an IPv4 prefix, or a shared IPv4 address from an IPv6 prefix. The basic mapping rule can also be used for forwarding packets, where an IPv4 destination address and a destination port are mapped into an IPv6 address/prefix. Every MAP-T node (a CE device is a MAP-T node) must be provisioned with a basic mapping rule. You can use the port-parameters command to configure port parameters for the MAP-T BMR.

• A DMR is a mandatory rule that is used for mapping IPv4 information to IPv6 addresses for destinations outside a MAP-T domain. A 0.0.0.0/0 entry is automatically configured in the MAP rule table (MRT) for this rule.

• An FMR is used for forwarding packets. Each FMR results in an entry in the MRT for the rule IPv4 prefix. FMR is an optional rule for mapping IPv4 and IPv6 destinations within a MAP-T domain.

  Note	FMR is not supported by the Mapping of Address and Port Using Translation feature.

The mapping of address and port using translation (MAP-T) customer edge (CE) device address format is defined by the IETF draft Mapping of Address and Port (MAP) . Address formats are used during mapping rule operations to construct the source and destination IPv6 addresses.

Note	Forwarding mapping rule (FMR) is not supported by the Mapping of Address and Port Using Translation feature.

The figure below shows the mapped CE address format as defined in MAP-T configuration. This address format is used in basic mapping rule (BMR) and FMR operations.

The figure below shows the address format used by the MAP-T default mapping rule (DMR), an IPv4-translated address that is specific to MAP-T configuration.

Mapping of address and port using translation (MAP-T) customer edge (CE) devices and border routers use the ICMP/ICMPv6 translation for address sharing of port ranges.

Unlike TCP and UDP, which provide two port fields to represent source and destination addresses, the Internet Control Message Protocol (ICMP) and ICMP Version 6 (ICMPv6) query message headers have only one ID field.

When an ICMP query message originates from an IPv4 host that exists beyond a MAP-T CE device, the ICMP ID field is exclusively used to identify the IPv4 host. The MAP-T CE device rewrites the ID field to a port-set value that is obtained through the basic mapping rule (BMR) during the IPv4-to-IPv6 translation, and the border router translates ICMPv6 packets to ICMP.

When a MAP-T border router receives an ICMP packet that contains an ID field that is bound for a shared address in the MAP-T domain, the MAP-T border router uses the ID field as a substitute for the destination port to determine the IPv6 destination address. The border router derives the destination IPv6 address by mapping the destination IPv4 address without the port information for packets that do not contain the ID field, and the corresponding CE device translates the ICMPv6 packets to ICMP.

Mapping of address and port using translation (MAP-T) uses path maximum transmission unit (MTU) discovery and fragmentation for IPv4-to-IPv6 translation because the size of IPv4 (more than 20 octets) and IPv6 (40 octets) headers is different. The MTU defines the largest size of a packet that an interface can transmit without the need to fragment the packet. IP packets larger than the MTU must go through IP fragmentation procedures.

When an IPv4 node performs path MTU discovery by setting the Don't Fragment (DF) bit in the packet header, path MTU discovery operates end-to-end across the MAP-T border router and customer edge (CE) translators. During IPv4 path MTU discovery, either the IPv4 device or the IPv6 device can send ICMP “Packet Too Big” messages to the sender. When IPv6 devices send these messages as Internet Control Message Protocol Version 6 (ICMPv6) errors, the packets that follow the message pass through the translator and result in an appropriate ICMP error message sent to the IPv4 sender.

When the IPv4 sender does not set the DF bit, the translator fragments the IPv4 packet and includes the packet with fragment headers to fit the packet in the minimum MTU 1280-byte IPv6 packets. When packets are fragmented, either by the sender or by IPv4 devices, the low-order 16 bits of the fragment identification are carried end-to-end across the MAP-T domain to ensure that packets are reassembled correctly.

EA bits —Embedded address bits. The IPv4 EA bits in the IPv6 address identify an IPv4 prefix/address (or part thereof) or a shared IPv4 address (or part thereof) and a port-set identifier.

IP fragmentation —The process of breaking a datagram into a number of pieces that can be reassembled later. The IP source, destination, identification, total length, and fragment offset fields, along with the More fragments and Don't Fragment (DF) flags in the IP header, are used for IP fragmentation and reassembly. A DF bit is a bit within the IP header that determines whether a device is allowed to fragment a packet.

IPv4-translatable address —IPv6 addresses that are used to represent IPv4 hosts. These addresses have an explicit mapping relationship to IPv6 addresses. This relationship is self-described by mapping the IPv4 address in the IPv6 address. Both stateless and stateful translators use IPv4-translatable (also called IPv4-converted) IPv6 addresses to represent IPv4 hosts.

IPv6-translatable address —IPv6 addresses that are assigned to IPv6 hosts for stateless translation. These IPv6-translatable addresses (also called IPv6-converted addresses) have an explicit mapping relationship to IPv4 addresses. This relationship is self-described by mapping the IPv4 address in the IPv6 address. The stateless translator uses corresponding IPv4 addresses to represent IPv6 hosts. The stateful translator does not use IPv6-translatable addresses because IPv6 hosts are represented by the IPv4 address pool in the translator via dynamic states.

MAP rule —A set of parameters that define the mapping between an IPv4 prefix, an IPv4 address or a shared IPv4 address, and an IPv6 prefix or address. Each MAP domain uses a different mapping rule set.

MAP-T border router —A mapping of address and port using translation (MAP-T)-enabled router or translator at the edge of a MAP domain that provides connectivity to the MAP-T domain. A border relay router has at least one IPv6-enabled interface and one IPv4 interface connected to the native IPv4 network, and this router can serve multiple MAP-T domains.

MAP-T CE —A device that functions as a customer edge (CE) router in a MAP-T deployment. A typical MAP-T CE device that adopts MAP rules serves a residential site with one WAN-side interface and one or more LAN-side interfaces. A MAP-T CE device can also be referred to as a “CE” within the context of a MAP-T domain.

MAP-T domain —Mapping of address and port using translation (MAP-T) domain. One or more customer edge (CE) devices and a border router, all connected to the same IPv6 network. A service provider may deploy a single MAP-T domain or use multiple MAP domains.

MRT —MAP rule table. Address and port-aware data structure that supports the longest match lookups. The MRT is used by the MAP-T forwarding function.

path MTU —Path maximum transmission unit (MTU) discovery prevents fragmentation in the path between endpoints. Path MTU discovery is used to dynamically determine the lowest MTU along the path from a packet’s source to its destination. Path MTU discovery is supported only by TCP and UDP. Path MTU discovery is mandatory in IPv6, but it is optional in IPv4. IPv6 devices never fragment a packet—only the sender can fragment packets.

stateful translation —Creates a per-flow state when the first packet in a flow is received. A translation algorithm is said to be stateful if the transmission or reception of a packet creates or modifies a data structure in the relevant network element. Stateful translation allows the use of multiple translators interchangeably and also some level of scalability. Stateful translation enables IPv6 clients and peers without mapped IPv4 addresses to connect to IPv4-only servers and peers.

stateless translation —A translation algorithm that is not stateful. A stateless translation requires configuring a static translation table or may derive information algorithmically from the messages that it is translating. Stateless translation requires less computational overhead than stateful translation. It also requires less memory to maintain the state because the translation tables and the associated methods and processes exist in a stateful algorithm and do not exist in a stateless one. Stateless translation enables IPv4-only clients and peers to initiate connections to IPv6-only servers or peers that are equipped with IPv4-embedded IPv6 addresses. It also enables scalable coordination of IPv4-only stub networks or ISP IPv6-only networks. Because the source port in an IPv6-to-IPv4 translation may have to be changed to provide adequate flow identification, the source port in the IPv4-to-IPv6 direction need not be changed.