RFC 7600
IPv4 Residual Deployment via IPv6 - A Stateless Solution (4rd)
Internet Engineering Task Force (IETF) R. Despres Request for Comments: 7600 RD-IPtech Category: Experimental S. Jiang, Ed. ISSN: 2070-1721 Huawei Technologies Co., Ltd R. Penno Cisco Systems, Inc. Y. Lee Comcast G. Chen China Mobile M. Chen BBIX, Inc. July 2015 IPv4 Residual Deployment via IPv6 - A Stateless Solution (4rd)Abstract
This document specifies a stateless solution for service providers to progressively deploy IPv6-only network domains while still offering IPv4 service to customers. The solution's distinctive properties are that TCP/UDP IPv4 packets are valid TCP/UDP IPv6 packets during domain traversal and that IPv4 fragmentation rules are fully preserved end to end. Each customer can be assigned one public IPv4 address, several public IPv4 addresses, or a shared address with a restricted port set. Status of This Memo This document is not an Internet Standards Track specification; it is published for examination, experimental implementation, and evaluation. This document defines an Experimental Protocol for the Internet community. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7600.
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Table of Contents
1. Introduction ....................................................4 2. Terminology .....................................................5 3. The 4rd Model ...................................................7 4. Protocol Specifications .........................................9 4.1. NAT44 on CE ................................................9 4.2. Mapping Rules and Other Domain Parameters .................10 4.3. Reversible Packet Translations at Domain Entries and Exits .................................................11 4.4. Address Mapping from CE IPv6 Prefixes to 4rd IPv4 Prefixes ..................................................17 4.5. Address Mapping from 4rd IPv4 Addresses to 4rd IPv6 Addresses ............................................19 4.6. Fragmentation Processing ..................................23 4.6.1. Fragmentation at Domain Entry ......................23 4.6.2. Ports of Fragments Addressed to Shared-Address CEs .................................24 4.6.3. Packet Identifications from Shared-Address CEs .....26 4.7. TOS and Traffic Class Processing ..........................26 4.8. Tunnel-Generated ICMPv6 Error Messages ....................27 4.9. Provisioning 4rd Parameters to CEs ........................27 5. Security Considerations ........................................30 6. IANA Considerations ............................................31 7. Relationship with Previous Works ...............................31 8. References .....................................................33 8.1. Normative References ......................................33 8.2. Informative References ....................................34 Appendix A. Textual Representation of Mapping Rules ...............37 Appendix B. Configuring Multiple Mapping Rules ....................37 Appendix C. Adding Shared IPv4 Addresses to an IPv6 Network .......39 C.1. With CEs within CPEs .......................................39 C.2. With Some CEs behind Third-Party Router CPEs ..............41 Appendix D. Replacing Dual-Stack Routing with IPv6-Only Routing ...42 Appendix E. Adding IPv6 and 4rd Service to a Net-10 Network .......43 Acknowledgements ..................................................44 Authors' Addresses ................................................44
1. Introduction
For service providers to progressively deploy IPv6-only network domains while still offering IPv4 service to customers, the need for a stateless solution, i.e., one where no per-customer state is needed in IPv4-IPv6 gateway nodes of the provider, has been discussed in [Solutions-4v6]. This document specifies one such solution, named "4rd" for IPv4 Residual Deployment. Its distinctive properties are that TCP/UDP IPv4 packets are valid TCP/UDP IPv6 packets during domain traversal and that IPv4 fragmentation rules are fully preserved end to end. Using this solution, IPv4 packets are transparently tunneled across IPv6 networks (the reverse of IPv6 Rapid Deployment on IPv4 Infrastructures (6rd) [RFC5969], in which IPv6 packets are statelessly tunneled across IPv4 networks). While IPv6 headers are too long to be mapped into IPv4 headers (which is why 6rd requires encapsulation of full IPv6 packets in IPv4 packets), IPv4 headers can be reversibly translated into IPv6 headers in such a way that, during IPv6 domain traversal, UDP packets having checksums and TCP packets are valid IPv6 packets. IPv6-only middleboxes that perform deep packet inspection can operate on them, in particular for port inspection and web caches. In order to deal with the IPv4 address shortage, customers can be assigned shared public IPv4 addresses with statically assigned restricted port sets. As such, it is a particular application of the Address plus Port (A+P) approach [RFC6346]. Deploying 4rd in networks that have enough public IPv4 addresses, customer sites can also be assigned full public IPv4 addresses. 4rd also supports scenarios where a set of public IPv4 addresses are assigned to customer sites. The design of 4rd builds on a number of previous proposals made for IPv4-via-IPv6 transition technologies (Section 7). In some use cases, IPv4-only applications of 4rd-capable customer nodes can also work with stateful NAT64s [RFC6146], provided these are upgraded to support 4rd tunnels in addition to their IP/ICMP translation [RFC6145]. The advantage is then a more complete IPv4 transparency than with double translation. How the 4rd model fits in the Internet architecture is summarized in Section 3. The protocol specifications are detailed in Section 4. Sections 5 and 6 deal with security considerations and IANA considerations, respectively. Previous proposals that influenced
this specification are listed in Section 7. A few typical 4rd use cases are presented in Appendices A, B, C, D, and E.2. Terminology
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 [RFC2119]. ISP: Internet Service Provider. In this document, the service it offers can be DSL, fiber-optics, cable, or mobile. The ISP can also be a private-network operator. 4rd (IPv4 Residual Deployment): An extension of the IPv4 service where public IPv4 addresses can be statically shared among several customer sites, each one being assigned an exclusive port set. This service is supported across IPv6-routing domains. 4rd domain (or Domain): An ISP-operated IPv6 network across which 4rd is supported according to the present specification. Tunnel packet: An IPv6 packet that transparently conveys an IPv4 packet across a 4rd domain. Its header has enough information to reconstitute the IPv4 header at Domain exit. Its payload is the original IPv4 payload. CE (Customer Edge): A customer-side tunnel endpoint. It can be in a node that is a host, a router, or both. BR (Border Relay): An ISP-side tunnel endpoint. Because its operation is stateless (neither per CE nor per session state), it can be replicated in as many nodes as needed for scalability. 4rd IPv6 address: IPv6 address used as the destination of a Tunnel packet sent to a CE or a BR. NAT64+: An ISP NAT64 [RFC6146] that is upgraded to support 4rd tunneling when IPv6 addresses it deals with are 4rd IPv6 addresses. 4rd IPv4 address: A public IPv4 address or, in the case of a shared public IPv4 address, a public transport address (public IPv4 address plus port number). PSID (Port-Set Identifier): A flexible-length field that algorithmically identifies a port set.
4rd IPv4 prefix: A flexible-length prefix that may be a public IPv4
prefix, a public IPv4 address, or a public IPv4 address followed
by a PSID.
Mapping rule: A set of parameters that are used by BRs and CEs to
derive 4rd IPv6 addresses from 4rd IPv4 addresses. Mapping
rules are also used by each CE to derive a 4rd IPv4 prefix from
an IPv6 prefix that has been delegated to it.
EA bits (Embedded Address bits): Bits that are the same in a 4rd
IPv4 address and in the 4rd IPv6 address derived from it.
BR Mapping rule: The Mapping rule that is applicable to off-domain
IPv4 addresses (addresses reachable via BRs). It can also apply
to some or all CE-assigned IPv4 addresses.
CE Mapping rule: A Mapping rule that is applicable only to
CE-assigned IPv4 addresses (shared or not).
NAT64+ Mapping rule: The Mapping rule that is applicable to IPv4
addresses reachable via a NAT64+.
CNP (Checksum Neutrality Preserver): A field of 4rd IPv6 addresses
that ensures that TCP-like checksums do not change when IPv4
addresses are replaced with 4rd IPv6 addresses.
4rd Tag: A 16-bit tag whose value allows 4rd CEs, BRs, and NAT64+s
to distinguish 4rd IPv6 addresses from other IPv6 addresses.
3. The 4rd Model
4rd Domain +-----------------------------+ | IPv6 routing | | Enforced ingress filtering | +---------- ... | | | | +------+ Customer site | |BR(s) | IPv4 +------------+ | BR IPv6 prefix --> |and/or| Internet | dual-stack | | |N4T64+| | +--+ | +------+ | |CE+-+ <-- a CE IPv6 prefix | | | +--+ | | +---------- | | | | +------------+ | <--IPv4 tunnels--> +------------ => Derived | (Mesh or hub-and-spoke | 4rd IPv4 prefix| topologies) | IPv6 | | Internet ... | | | +------------ +-----------------------------+ <== one or several Mapping rules (e.g., announced to CEs in stateless DHCPv6) Figure 1: The 4rd Model in the Internet Architecture How the 4rd model fits in the Internet architecture is represented in Figure 1. A 4rd domain is an IPv6 network that includes one or several 4rd BRs or NAT64+s at its border with the public IPv4 Internet and that can advertise its IPv4-IPv6 Mapping rule(s) to CEs according to Section 4.9. BRs of a 4rd Domain are all identical as far as 4rd is concerned. In a 4rd CE, the IPv4 packets that need to reach a BR will be transformed (as detailed in Section 4.3) into IPv6 packets that have the same anycast IPv6 prefix, which is the 80-bit BR prefix, in their destination addresses. They are then routed to any of the BRs. The 80-bit BR IPv6 prefix is an arbitrarily chosen /64 prefix from the IPv6 address space of the network operator and appended with 0x0300 (16-bit 4rd Tag; see R-9 in Section 4.5).
Using the Mapping rule that applies, each CE derives its 4rd IPv4 prefix from its delegated IPv6 prefix, or one of them if it has several; see Section 4.4 for details. If the obtained IPv4 prefix has more than 32 bits, the assigned IPv4 address is shared among several CEs. Bits beyond the first 32 specify a set of ports whose use is reserved for the CE. IPv4 traffic is automatically tunneled across the Domain, in either mesh topology or hub-and-spoke topology [RFC4925]. By default, IPv4 traffic between two CEs follows a direct IPv6 route between them (mesh topology). If the ISP configures the hub-and-spoke option, each IPv4 packet from one CE to another is routed via a BR. During Domain traversal, each tunneled TCP/UDP IPv4 packet looks like a valid TCP/UDP IPv6 packet. Thus, TCP/UDP access control lists that apply to IPv6, and possibly some other functions using deep packet inspection, also apply to IPv4. In order for IPv4 anti-spoofing protection in CEs and BRs to remain effective when combined with 4rd tunneling, ingress filtering [RFC3704] has to be in effect in IPv6 (see R-12 and Section 5). If an ISP wishes to support dynamic IPv4 address sharing in addition to or in place of 4rd stateless address sharing, it can do so by means of a stateful NAT64. By upgrading this NAT to add support for 4rd tunnels, which makes it a NAT64+, CEs that are assigned no static IPv4 space can benefit from complete IPv4 transparency between the CE and the NAT64. (Without this NAT64 upgrade, IPv4 traffic is translated to IPv6 and back to IPv4, during which time the DF = MF = 1 combination for IPv4, as recommended for host fragmentation in Section 8 of [RFC4821], is lost.) IPv4 packets are kept unchanged by Domain traversal, except that: o The IPv4 Time To Live (TTL), unless it is 1 or 255 at Domain entry, decreases during Domain traversal by the number of traversed routers. This is acceptable because it is undetectable end to end and also because TTL values that can be used with some protocols to test the adjacency of communicating routers are preserved [RFC4271] [RFC5082]. The effect on the traceroute utility, which uses TTL expiry to discover routers of end-to-end paths, is noted in Section 4.3.
o IPv4 packets whose lengths are <= 68 octets always have their
"Don't Fragment" (DF) flags set to 1 at Domain exit even if they
had DF = 0 at Domain entry. This is acceptable because these
packets are too short to be fragmented [RFC791] and so their DF
bits have no meaning. Besides, both [RFC1191] and [RFC4821]
recommend that sources always set DF to 1.
o Unless the Tunnel Traffic Class option applies to a Domain
(Section 4.2), IPv4 packets may have their Type of Service (TOS)
fields modified after Domain traversal (Section 4.7).
4. Protocol Specifications
This section describes detailed 4rd protocol specifications. They
are mainly organized by functions. As a brief summary:
o A 4rd CE MUST follow R-1, R-2, R-3, R-4, R-6, R-7, R-8, R-9, R-10,
R-11, R-12, R-13, R-14, R-16, R-17, R-18, R-19, R-20, R-21, R-22,
R-23, R-24, R-25, R-26, and R-27.
o A 4rd BR MUST follow R-2, R-3, R-4, R-5, R-6, R-9, R-12, R-13,
R-14, R-15, R-19, R-20, R-21, R-22, and R-24.
4.1. NAT44 on CE
R-1: A CE node that is assigned a shared public IPv4 address MUST
include a NAT44 [RFC3022]. This NAT44 MUST only use external
ports that are in the CE-assigned port set.
NOTE: This specification only concerns IPv4 communication between
IPv4-capable endpoints. For communication between IPv4-only
endpoints and IPv6-only remote endpoints, the "Bump-in-the-Host"
(BIH) specification [RFC6535] can be used. It can coexist in a node
with the CE function, including scenarios where the IPv4-only
function is a NAT44 [RFC3022].
4.2. Mapping Rules and Other Domain Parameters
R-2: CEs and BRs MUST be configured with the following Domain parameters: A. One or several Mapping rules, each one comprising the following: 1. Rule IPv4 prefix 2. EA-bits length 3. Rule IPv6 prefix 4. Well-Known Ports (WKPs) authorized (OPTIONAL) B. Domain Path MTU (PMTU) C. Hub-and-spoke topology (Yes or No) D. Tunnel Traffic Class (OPTIONAL) "Rule IPv4 prefix" is used to find, by a longest match, which Mapping rule applies to a 4rd IPv4 address (Section 4.5). A Mapping rule whose Rule IPv4 prefix is longer than /0 is a CE Mapping rule. BR and NAT64+ Mapping rules, which must apply to all off-domain IPv4 addresses, have /0 as their Rule IPv4 prefixes. "EA-bits length" is the number of bits that are common to 4rd IPv4 addresses and 4rd IPv6 addresses derived from them. In a CE Mapping rule, it is also the number of bits that are common to a CE-delegated IPv6 prefix and the 4rd IPv4 prefix derived from it. BR and NAT64+ Mapping rules have EA-bits lengths equal to 32. "Rule IPv6 prefix" is the prefix that is used as a substitute for the Rule IPv4 prefix when a 4rd IPv6 address is derived from a 4rd IPv4 address (Section 4.5). In a BR Mapping rule or a NAT64+ Mapping rule, it MUST be a /80 prefix whose bits 64-79 are the 4rd Tag. "WKPs authorized" may be set for Mapping rules that assign shared IPv4 addresses to CEs. (These rules are those whose length of the Rule IPv4 prefix plus the EA-bits length exceeds 32.) If set, well-known ports may be assigned to some CEs having particular IPv6 prefixes. If not set, fairness is privileged: all IPv6 prefixes concerned with the Mapping rule have port sets having identical values (no port set includes any of the well-known ports).
"Domain PMTU" is the IPv6 Path MTU that the ISP can guarantee for all of its IPv6 paths between CEs and between BRs and CEs. It MUST be at least 1280 octets [RFC2460]. "Hub-and-spoke topology", if set to Yes, requires CEs to tunnel all IPv4 packets via BRs. If set to No, CE-to-CE packets take the same routes as native IPv6 packets between the same CEs (mesh topology). "Tunnel Traffic Class", if provided, is the IPv6 traffic class that BRs and CEs MUST set in Tunnel packets. In this case, evolutions of the IPv6 traffic class that may occur during Domain traversal are not reflected in TOS fields of IPv4 packets at Domain exit (Section 4.7).4.3. Reversible Packet Translations at Domain Entries and Exits
R-3: Domain-entry nodes that receive IPv4 packets with IPv4 options MUST discard these packets and return ICMPv4 error messages to signal IPv4-option incompatibility (Type = 12, Code = 0, Pointer = 20) [RFC792]. This limitation is acceptable because there are a lot of firewalls in the current IPv4 Internet that also filter IPv4 packets with IPv4 options. R-4: Domain-entry nodes that receive IPv4 packets without IPv4 options MUST convert them to Tunnel packets, with or without IPv6 fragment headers, depending on what is needed to ensure IPv4 transparency (Figure 2). Domain-exit nodes MUST convert them back to IPv4 packets. An IPv6 fragmentation header MUST be included at tunnel entry (Figure 2) if and only if one or several of the following conditions hold: * The Tunnel Traffic Class option applies to the Domain. * TTL = 1 OR TTL = 255. * The IPv4 packet is already fragmented, or may be fragmented later on, i.e., if MF = 1 OR offset > 0 OR (total length > 68 AND DF = 0). In order to optimize cases where fragmentation headers are unnecessary, the NAT44 of a CE that has one SHOULD send packets with TTL = 254.
R-5: In Domains whose chosen topology is hub-and-spoke, BRs that
receive 4rd IPv6 packets whose embedded destination IPv4
addresses match a CE Mapping rule MUST do the equivalent of
reversibly translating their headers to IPv4 and then
reversibly translate them back to IPv6 as though packets would
be entering the Domain.
(A) Without IPv6 fragment header
IPv4 packet Tunnel packet
+--------------------+ : : +--------------------+
20| IPv4 Header | : <==> : | IPv6 Header | 40
+--------------------+ : : +--------------------+
| IP Payload | <==> | IP Payload |
| | Layer 4 | |
+--------------------+ unchanged +--------------------+
(B) With IPv6 fragment header
Tunnel packet
: +--------------------+
IPv4 packet : | IPv6 Header | 40
+--------------------+ : : +--------------------+
20| IPv4 Header | : <==> : |IPv6 Fragment Header| 8
+--------------------+ : : +--------------------+
| IP Payload | <==> | IP Payload |
| | Layer 4 | |
+--------------------+ unchanged +--------------------+
Figure 2: Reversible Packet Translation
R-6: Values to be set in IPv6 header fields at Domain entry are
detailed in Table 1 (no fragment header) and Table 2 (with
fragment header). Those to be set in IPv4 header fields at
Domain exit are detailed in Table 3 (no fragment header) and
Table 4 (with fragment header).
To convey IPv4 header information that has no equivalent in
IPv6, some ad hoc fields are placed in IPv6 flow labels and in
Identification fields of IPv6 fragment headers, as detailed in
Figure 3.
|0 |4 19|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | Addr_Prot_Cksm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IPv6 Flow Label
0 1 2 |8 |16 31|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|.|.|.| 0 | IPv4_TOS | IPv4_ID |
/-+-\-\-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ \ TTL_255 IPv6 Identification Field
IPv4_DF TTL_1 (in fragment header if needed)
Figure 3: 4rd Identification Fields of IPv6 Fragment Headers
+---------------------+----------------------------------------+
| IPv6 Field | Value (fields from IPv4 header) |
+---------------------+----------------------------------------+
| Version | 6 |
| Traffic Class | TOS |
| Addr_Prot_Cksm | Sum of addresses and Protocol (Note 1) |
| Payload length | Total length - 20 |
| Next header | Protocol |
| Hop limit | Time to Live |
| Source address | See Section 4.5 |
| Destination address | See Section 4.5 |
+---------------------+----------------------------------------+
Table 1: IPv4-to-IPv6 Reversible Header Translation
(without Fragment Header)
+-----------------+----------------------------------------------+ | IPv6 Field | Value (fields from IPv4 header) | +-----------------+----------------------------------------------+ | Version | 6 | | Traffic Class | TOS OR Tunnel Traffic Class (Section 4.7) | | Addr_Prot_Cksm | Sum of addresses and Protocol (Note 1) | | Payload length | Total length - 12 | | Next header | 44 (fragment header) | | Hop limit | IF Time to Live = 1 or 255 THEN 254 | | | ELSE Time to Live (Note 2) | | Source address | See Section 4.5 | | Dest. address | See Section 4.5 | | 2nd next header | Protocol | | Fragment offset | IPv4 fragment offset | | M | More Fragments flag (MF) | | IPv4_DF | Don't Fragment flag (DF) | | TTL_1 | IF Time to Live = 1 THEN 1 ELSE 0 (Note 2) | | TTL_255 | IF Time to Live = 255 THEN 1 ELSE 0 (Note 2) | | IPv4_TOS | Type of Service (TOS) | | IPv4_ID | Identification | +-----------------+----------------------------------------------+ Table 2: IPv4-to-IPv6 Reversible Header Translation (with Fragment Header) +-----------------+------------------------------------+ | IPv4 Field | Value (fields from IPv6 header) | +-----------------+------------------------------------+ | Version | 4 | | Header length | 5 | | TOS | Traffic Class | | Total length | Payload length + 20 | | Identification | 0 | | DF | 1 | | MF | 0 | | Fragment offset | 0 | | Time to Live | Hop count | | Protocol | Next header | | Header checksum | Computed as per [RFC791] (Note 3) | | Source address | Bits 80-111 of source address | | Dest. address | Bits 80-111 of destination address | +-----------------+------------------------------------+ Table 3: IPv6-to-IPv4 Reversible Header Translation (without Fragment Header)
+-----------------------+-----------------------------------------+ | IPv4 Field | Value (fields from IPv6 header) | +-----------------------+-----------------------------------------+ | Version | 4 | | Header length | 5 | | TOS | Traffic Class OR IPv4_TOS (Section 4.7) | | Total length | Payload length + 12 | | Identification | IPv4_ID | | DF | IPv4_DF | | MF | M | | Fragment offset | Fragment offset | | Time to Live (Note 2) | IF TTL_255 = 1 THEN 255 | | | ELSEIF TTL_1 = 1 THEN 1 | | | ELSE hop count | | Protocol | 2nd next header | | Header checksum | Computed as per [RFC791] (Note 3) | | Source address | Bits 80-111 of source address | | Destination address | Bits 80-111 of destination address | +-----------------------+-----------------------------------------+ Table 4: IPv6-to-IPv4 Reversible Header Translation (with Fragment Header) NOTE 1: The need to save in the IPv6 header a checksum of both IPv4 addresses and the IPv4 protocol field results from the following facts: (1) header checksums, present in IPv4 but not in IPv6, protect addresses or protocol integrity; (2) in IPv4, ICMP messages and null-checksum UDP datagrams depend on this protection because, unlike other datagrams, they have no other address-and-protocol integrity protection. The sum MUST be performed in ordinary two's complement arithmetic. IP-layer Packet length is another field covered by the IPv4 header checksum. It is not included in the saved checksum because (1) doing so would have conflicted with [RFC6437] (flow labels must be the same in all packets of each flow); (2) ICMPv4 messages have good enough protection with their own checksums; (3) the UDP length field provides to null-checksum UDP datagrams the same level of protection after Domain traversal as without Domain traversal (consistency between IP-layer and UDP-layer lengths can be checked).
NOTE 2: TTL treatment has been chosen to permit adjacency tests between two IPv4 nodes situated at both ends of a 4rd tunnel. TTL values to be preserved for this are TTL = 255 and TTL = 1. For other values, TTL decreases between two IPv4 nodes as though the traversed IPv6 routers were IPv4 routers. The effect of this TTL treatment on IPv4 traceroute is specific: (1) the number of routers of the end-to-end path includes traversed IPv6 routers; (2) IPv6 routers of a Domain are listed after IPv4 routers of Domain entry and exit; (3) the IPv4 address shown for an IPv6 router is the IPv6-only dummy IPv4 address (Section 4.8); (4) the response time indicated for an IPv6 router is that of the next router. NOTE 3: Provided the sum of obtained IPv4 addresses and protocol matches Addr_Prot_Cksm. If not, the packet MUST be silently discarded.
4.4. Address Mapping from CE IPv6 Prefixes to 4rd IPv4 Prefixes
+--------------------------------------+ | CE IPv6 prefix | +--------------------------+-----------+ : Longest match : : : with a Rule IPv6 prefix : : : || : EA-bits : : \/ : length : +--------------------------+ | : | Rule IPv6 prefix |<----'---->: +--------------------------+ : || : : \/ : : +-----------------+-----------+ |Rule IPv4 prefix | EA bits | +-----------------+-----------+ : : +-----------------------------+ | CE 4rd IPv4 prefix | +-----------------------------+ ________/ \_________ : / \ : : ____:________________/ \__ : / : \ : <= 32 : : > 32 : +----------------+ +-----------------+----+ |IPv4 prfx or add| OR | IPv4 address |PSID| +----------------+ +-----------------+----+ : 32 : || : \/ (by default) (If WKPs authorized) : : : : +---+----+---------+ +----+-------------+ Ports in |> 0|PSID|any value| OR |PSID| any value | the CE port set +---+----+---------+ +----+-------------+ : 4 : 12 : : 16 : Figure 4: From CE IPv6 Prefix to 4rd IPv4 Address and Port Set R-7: A CE whose delegated IPv6 prefix matches the Rule IPv6 prefix of one or several Mapping rules MUST select the CE Mapping rule for which the match is the longest. It then derives its 4rd IPv4 prefix as shown in Figure 4: (1) The CE replaces the Rule IPv6 prefix with the Rule IPv4 prefix. The result is the CE 4rd IPv4 prefix. (2) If this CE 4rd IPv4 prefix has less than 32 bits, the CE takes it as its assigned IPv4 prefix. If it has exactly 32 bits, the CE takes it as its IPv4 address. If
it has more than 32 bits, the CE MUST take the first 32 bits as
its shared public IPv4 address and bits beyond the first 32 as
its Port-Set identifier (PSID). Ports of its restricted port
set are by default those that have any non-zero value in their
first 4 bits (the PSID offset), followed by the PSID, and
followed by any values in remaining bits. If the WKP
authorized option applies to the Mapping rule, there is no
4-bit offset before the PSID so that all ports can be assigned.
NOTE: The choice of the default PSID position in port fields
has been guided by the following objectives: (1) for fairness,
avoid having any of the well-known ports 0-1023 in the port set
specified by any PSID value; (2) for compatibility with RTP/
RTCP [RFC4961], include in each port set pairs of consecutive
ports; (3) in order to facilitate operation and training, have
the PSID at a fixed position in port fields; (4) in order to
facilitate documentation in hexadecimal notation, and to
facilitate maintenance, have this position nibble-aligned.
Ports that are excluded from assignment to CEs are 0-4095,
instead of just 0-1023, in a trade-off to favor nibble
alignment of PSIDs and overall simplicity.
R-8: A CE whose delegated IPv6 prefix has its longest match with the
Rule IPv6 prefix of the BR Mapping rule MUST take as its IPv4
address the 32 bits that, in the delegated IPv6 prefix, follow
this Rule IPv6 prefix. If this is the case while the hub-and-
spoke option applies to the Domain, or if the Rule IPv6 prefix
is not a /80, there is a configuration error in the Domain. An
implementation-dependent administrative action MAY be taken.
A CE whose delegated IPv6 prefix does not match the Rule IPv6
prefix of either any CE Mapping rule or the BR Mapping rule,
and is in a Domain that has a NAT64+ Mapping rule, MUST be
noted as having the unspecified IPv4 address.
4.5. Address Mapping from 4rd IPv4 Addresses to 4rd IPv6 Addresses
: 32 : : 16 : \ +----------------------------+ +---------------+ | | IPv4 address | |Port_or_ICMP_ID| | Shared-address +----------------------------+ +---+------+----+ | case : Longest match : : 4 : PSID : | (PSID length : with a Rule IPv4 prefix : :length: | of the rule > 0) : || : : : | with WKPs : \/ : : : | not authorized +----------------+-----------+ +------+ | (PSID offset = 4) |Rule IPv4 prefix|IPv4 suffix| | PSID | | +----------------+-----------+ +------+ | : || \_______ \____ | | | : \/ \ \| / | +--------------------------+--------+-----+ / | Rule IPv6 prefix | EA bits | +--------------------------+--------------+ : : +-----------------------------------------+ | IPv6 prefix | +-----------------------------------------+ :\_______________________________ / \ : ___________________\______/ \_______________ : / \ \ : / (CE Mapping rule) \ (BR Mapping rule) \ : <= 64 : : 112 : +----------+---+---+------+---+ +--------------+---+------+---+ |CE v6 prfx| 0 |tag|v4 add|CNP| |BR IPv6 prefix|tag|v4 add|CNP| +----------+-|-+---+------+---+ +--------------+---+------+---+ : <= 64 : | :16 : 32 :16 : : 64 :16 : 32 :16 : | Padding to /64 Figure 5: From 4rd IPv4 Address to 4rd IPv6 Address
R-9: BRs, and CEs that are assigned public IPv4 addresses, shared or
not, MUST derive 4rd IPv6 addresses from 4rd IPv4 addresses via
the steps below or their functional equivalent (Figure 5
details the shared public IPv4 address case):
NOTE: The rules for forming 4rd-specific Interface Identifiers
(IIDs) are to obey [RFC7136]:
"Specifications of other forms of 64-bit IIDs MUST specify how
all 64 bits are set."
and
"the whole IID value MUST be viewed as an opaque bit string by
third parties, except possibly in the local context."
(1) If hub-and-spoke topology does not apply to the Domain, or
if it applies but the IPv6 address to be derived is a
source address from a CE or a destination address from a
BR, find the CE Mapping rule whose Rule IPv4 prefix has
the longest match with the IPv4 address.
If no Mapping rule is thus obtained, take the BR Mapping
rule.
If the obtained Mapping rule assigns IPv4 prefixes to CEs,
i.e., if the length of the Rule IPv4 prefix plus EA-bits
length is 32 - k, with k >= 0, delete the last k bits of
the IPv4 address.
Otherwise, if the length of the Rule IPv4 prefix plus the
EA-bits length is 32 + k, with k > 0, take k as the PSID
length and append to the IPv4 address the PSID copied from
bits p to p+3 of the Port_or_ICMP_ID field where (1) p,
the PSID offset, is 4 by default and 0 if the WKPs
authorized option applies to the rule; (2) the
Port_or_ICMP_ID field is in bits of the IP payload that
depend on whether the address is source or destination, on
whether the packet is ICMP or not, and, if it is ICMP,
whether it is an error message or an Echo message. This
field is:
a. If the packet Protocol is not ICMP, the port field
associated with the address (bits 0-15 for a source
address and bits 16-31 for a destination address).
b. If the packet is an ICMPv4 Echo or Echo reply message,
the ICMPv4 Identification field (bits 32-47).
c. If the packet is an ICMPv4 error message, the port
field associated with the address in the returned
packet header (bits 240-255 for a source address and
bits 224-239 for a destination address).
NOTE 1: Using Identification fields of ICMP messages as
port fields permits the exchange of Echo requests and Echo
replies between shared-address CEs and IPv4 hosts having
exclusive IPv4 addresses. Echo exchanges between two
shared-address CEs remain impossible, but this is a
limitation inherent in address sharing (one reason among
many to use IPv6).
NOTE 2: When the PSID is taken in the port fields of the
IPv4 payload, implementation is kept independent from any
particular Layer 4 protocol having such port fields by not
checking that the protocol is indeed one that has such
port fields. A packet may consequently go, in the case of
a source mistake, from a BR to a shared-address CE with a
protocol that is not supported by this CE. In this case,
the CE NAT44 returns an ICMPv4 "protocol unreachable"
error message. The IPv4 source is thus appropriately
informed of its mistake.
(2) In the result, replace the Rule IPv4 prefix with the Rule
IPv6 prefix.
(3) If the result is shorter than a /64, append to the result
a null padding up to 64 bits, followed by the 4rd Tag
(0x0300), and followed by the IPv4 address.
NOTE: The 4rd Tag is a 4rd-specific mark. Its function is
to ensure that 4rd IPv6 addresses are recognizable by CEs
without any interference with the choice of subnet
prefixes in CE sites. (These choices may have been done
before 4rd is enabled.)
For this, the 4rd Tag has its "u" and "g" bits [RFC4291]
both set to 1, so that they maximally differ from these
existing IPv6 address schemas. So far, u = g = 1 has not
been used in any IPv6 addressing architecture.
With the 4rd Tag, IPv6 packets can be routed to the 4rd
function within a CE node based on a /80 prefix that no
native IPv6 address can contain.
(4) Add to the result a Checksum Neutrality Preserver (CNP).
Its value, in one's complement arithmetic, is the opposite
of the sum of 16-bit fields of the IPv6 address other than
the IPv4 address and the CNP themselves (i.e., five
consecutive fields in address bits 0-79).
NOTE: The CNP guarantees that Tunnel packets are valid
IPv6 packets for all Layer 4 protocols that use the same
checksum algorithm as TCP. This guarantee does not depend
on where the checksum fields of these protocols are placed
in IP payloads. (Today, such protocols are UDP [RFC768],
TCP [RFC793], UDP-Lite [RFC3828], and the Datagram
Congestion Control Protocol (DCCP) [RFC5595]. Should new
ones be specified, BRs will support them without needing
an update.)
R-10: A 4rd-capable CE SHOULD, and a 4rd-enabled CE MUST, always
prohibit all addresses that use its advertised prefix and have
an IID starting with 0x0300 (4rd Tag), by using Duplicate
Address Detection [RFC4862].
R-11: A CE that is assigned the unspecified IPv4 address (see
Section 4.4) MUST use, for packets tunneled between itself and
the Domain NAT64+, addresses as detailed in Figure 6: part (a)
for its IPv6 source, and part (b) as IPv6 destinations that
depend on IPv4 destinations. A NAT64+, being NAT64 conforming
[RFC6146], MUST accept IPv6 packets whose destination conforms
to Figure 6(b) (4rd Tag instead of "u" and 0x00 octets). In
its Binding Information Base, it MUST remember whether a
mapping was created with a "u" or 4rd-tag destination. In the
IPv4-to-IPv6 direction, it MUST use 4rd tunneling, with source
address conforming to Figure 6(b), when using a mapping that
was created with a 4rd-tag destination.
+---------------------+---------+-------+-------------+------+
(a) | CE IPv6 prefix | 0 |4rd Tag| 0 | CNP |
+---------------------+---------+-------+-------------+------+
: <= 64 : >= 0 : 16 : 32 : 16 :
4rd IPv6 address of a CE having no public IPv4 address
<----------- Rule IPv6 prefix --------->:
+-------------------------------+-------+-------------+------+
(b) | NAT64+ IPv6 prefix |4rd Tag|IPv4 address | CNP |
+-------------------------------+-------+-------------+------+
: 64 : 16 : 32 : 16 :
4rd IPv6 address of a host reachable via a NAT64+
Figure 6: Rules for CE and NAT64+
R-12: For anti-spoofing protection, CEs and BRs MUST check that the
IPv6 source address of each received Tunnel packet is that
which, according to R-9, is derived from the source 4rd IPv4
address. For this, the IPv4 address used to obtain the source
4rd IPv4 address is that embedded in the IPv6 source address
(in its bits 80-111). (This verification is needed because
IPv6 ingress filtering [RFC3704] applies only to IPv6 prefixes,
without any guarantee that Tunnel packets are built as
specified in R-9.)
R-13: For additional protection against packet corruption at a link
layer that might be undetected at this layer during Domain
traversal, CEs and BRs SHOULD verify that source and
destination IPv6 addresses have not been modified. This can be
done by checking that they remain checksum neutral (see the
Note above regarding the CNP).
(page 23 continued on part 2)
