wdiff rfc7915v1.txt rfc7915.txt

Internet Engineering Task Force (IETF) C. Bao
Request for Comments: 7915 X. Li
Obsoletes: 6145 CERNET Center/Tsinghua University
Category: Standards Track F. Baker
ISSN: 2070-1721 Cisco Systems
T. Anderson
Redpill Linpro
F. Gont
Huawei Technologies
June 2016

IP/ICMP Translation Algorithm

Abstract

This document describes the Stateless IP/ICMP Translation Algorithm
(SIIT), which translates between IPv4 and IPv6 packet headers
(including ICMP headers). This document obsoletes RFC 6145.

Status of This Memo

This is an Internet Standards Track document.

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). Further information on
Internet Standards is available in Section 2 of RFC 5741. 7841.

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/rfc7915.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction and Motivation . . . . . . . . . . . . . . . . . 2 3
1.1. IPv4-IPv6 Translation Model . . . . . . . . . . . . . . . 3
1.2. Applicability and Limitations . . . . . . . . . . . . . . 3
1.3. Stateless vs. Stateful Mode . . . . . . . . . . . . . . . 4
1.4. Path MTU Discovery and Fragmentation . . . . . . . . . . 5
2. Changes from RFC 6145 . . . . . . . . . . . . . . . . . . . . 5
3. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Translating from IPv4 to IPv6 . . . . . . . . . . . . . . . . 5 6
4.1. Translating IPv4 Headers into IPv6 Headers . . . . . . . 7
4.2. Translating ICMPv4 Headers into ICMPv6 Headers . . . . . 9
4.3. Translating ICMPv4 Error Messages into ICMPv6 . . . . . . 13
4.4. Generation of ICMPv4 Error Message . . . . . . . . . . . 14
4.5. Transport-Layer Header Translation . . . . . . . . . . . 14
4.6. Knowing When to Translate . . . . . . . . . . . . . . . . 15
5. Translating from IPv6 to IPv4 . . . . . . . . . . . . . . . . 15
5.1. Translating IPv6 Headers into IPv4 Headers . . . . . . . 17
5.1.1. IPv6 Fragment Processing . . . . . . . . . . . . . . 19
5.2. Translating ICMPv6 Headers into ICMPv4 Headers . . . . . 19
5.3. Translating ICMPv6 Error Messages into ICMPv4 . . . . . . 22
5.4. Generation of ICMPv6 Error Messages . . . . . . . . . . . 23
5.5. Transport-Layer Header Translation . . . . . . . . . . . 23
5.6. Knowing When to Translate . . . . . . . . . . . . . . . . 23 24
6. Mapping of IP Addresses . . . . . . . . . . . . . . . . . . . 23 24
7. Special Considerations for ICMPv6 Packet Too Big . . . . . . 24
8. Security Considerations . . . . . . . . . . . . . . . . . . . 24
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 24 25
9.1. Normative References . . . . . . . . . . . . . . . . . . 24 25
9.2. Informative References . . . . . . . . . . . . . . . . . 26 27
Appendix A. Stateless Translation Workflow Example . . . . . . . 28 30
A.1. H6 Establishes Communication with H4 . . . . . . . . . . 29 30
A.2. H4 Establishes Communication with H6 . . . . . . . . . . 30 32
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 31 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32 33

1. Introduction and Motivation

This document obsoletes [RFC6145].

Readers of this document are expected to have read and understood the
framework described in [RFC6144]. Implementations of this IPv4/IPv6
translation specification MUST support one or more address mapping
algorithms, which are defined in Section 6.

1.1. IPv4-IPv6 Translation Model

The translation model consists of two or more network domains
connected by one or more IP/ICMP translators (XLATs) as shown in
Figure 1.

Figure 1: IPv4-IPv6 Translation Model

The scenarios of the translation model are discussed in [RFC6144].

1.2. Applicability and Limitations

This document specifies the translation algorithms between IPv4
packets and IPv6 packets.

As with [RFC6145], the translating function specified in this
document does not translate any IPv4 options, and it does not
translate IPv6 extension headers except the Fragment Header.

The issues and algorithms in the translation of datagrams containing
TCP segments are described in [RFC5382].

Fragmented IPv4 UDP packets that do not contain a UDP checksum (i.e.,
the UDP checksum field is zero) are not of significant use on the
Internet, and in general will not be translated by the IP/ICMP
translator (Section 4.5). However, when the translator is configured
to forward the packet without a UDP checksum, the fragmented IPv4 UDP
packets will be translated.

Fragmented ICMP/ICMPv6 packets will not be translated by IP/ICMP
translators.

The IP/ICMP header translation specified in this document is
consistent with requirements of multicast IP/ICMP headers. However,
IPv4 multicast addresses [RFC5771] cannot be mapped to IPv6 multicast
addresses [RFC3307] based on the unicast mapping rule [RFC6052]. An
example of experiments of the multicast address mapping can be found
in [RFC6219].

1.3. Stateless vs. Stateful Mode

An IP/ICMP translator has two possible modes of operation: stateless
and stateful [RFC6144]. In both cases, we assume that a system (a
node or an application) that has an IPv4 address but not an IPv6
address is communicating with a system that has an IPv6 address but
no IPv4 address, or that the two systems do not have contiguous
routing connectivity, or they might have contiguous routing
connectivity but are interacting via masking addresses (i.e.,
hairpinning) [RFC4787], and hence are forced to have their
communications translated.

In the stateless mode, an IP/ICMP translator will convert IPv4
addresses to IPv6 and vice versa solely based on the configuration of
the stateless IP/ICMP translator and information contained within the
packet being translated. For example, for the default behavior
defined in [RFC6052], a specific IPv6 address range will represent
IPv4 systems (IPv4-converted addresses), and the IPv6 systems have
addresses (IPv4-translatable addresses) that can be algorithmically
mapped to a subset of the service provider's IPv4 addresses. Other
stateless translation algorithms are defined in Section 6. The
stateless translator does not keep any dynamic session or binding
state, thus there is no requirement that the packets in a single
session or flow traverse a single translator.

In the stateful mode, a specific IPv6 address range (consisting of
IPv4-converted IPv6 addresses) will typically represent IPv4 systems.
The IPv6 nodes may use any IPv6 addresses [RFC4291] except in that
range. A stateful IP/ICMP translator continuously maintains a
dynamic translation table containing bindings between the IPv4 and
IPv6 addresses, and likely also the Layer-4 identifiers, that are
used in the translated packets. The exact address translations of
any given packet thus become dependent on how packets belonging to
the same session or flow have been translated. For this reason,
stateful translation generally requires that all packets belonging to
a single flow must traverse the same translator.

In order to be able to successfully translate a packet from IPv4 to
IPv6 or vice versa, the translator must implement an address mapping
algorithm. This document does not specify any such algorithms,
instead these are referenced from Section 6.

1.4. Path MTU Discovery and Fragmentation

Due to the different sizes of the IPv4 and IPv6 header, which are 20+
octets and 40 octets respectively, handling the maximum packet size
is critical for the operation of the IPv4/IPv6 translator. There are
three mechanisms to handle this issue: path MTU discovery (PMTUD),
fragmentation, and transport-layer negotiation such as the TCP
Maximum Segment Size (MSS) option [RFC0879]. [RFC6691]. Note that the
translator MUST behave as a router, i.e., the translator MUST send a
Packet Too Big error message or fragment the packet when the packet
size exceeds the MTU of the next-hop interface.

Don't Fragment, ICMP Packet Too Big, and packet fragmentation are
discussed in Sections 4 and 5 of this document. The reassembling of
fragmented packets in the stateful translator is discussed in
[RFC6146], since it requires state maintenance in the translator.

2. Changes from RFC 6145

The changes from RFC 6145 are the following:

1. Inserted the notes about IPv6 extension header handling:
[Err3059], [Err3060], [Err3061], and [Err3061]. [Err4090].

2. Deprecated the algorithm that generates the IPv6 atomic
fragments, as a result of the analysis in [ATOMIC] and the
specification in [IPv6].

3. Inserted the notes for stateless source address mapping for
ICMPv6 packets [RFC6791].

4. Supported new address mapping algorithms and moved the discussion
of these algorithms to Section 6.

3. Conventions

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].

4. Translating from IPv4 to IPv6

When an IP/ICMP translator receives an IPv4 datagram addressed to a
destination towards the IPv6 domain, it translates the IPv4 header of
that packet into an IPv6 header. The original IPv4 header on the
packet is removed and replaced by an IPv6 header, and the transport
checksum is updated as needed, if that transport is supported by the
translator. The data portion of the packet is left unchanged. The
IP/ICMP translator then forwards the packet based on the IPv6
destination address.

Figure 2: IPv4-to-IPv6 Translation

Path MTU discovery is mandatory in IPv6, but it is optional in IPv4.
IPv6 routers never fragment a packet -- only the sender can do
fragmentation.

When an IPv4 node performs path MTU discovery (by setting the Don't
Fragment (DF) bit in the header), path MTU discovery can operate end-
to-end, i.e., across the translator. In this case, either IPv4 or
IPv6 routers (including the translator) might send back ICMP Packet
Too Big messages to the sender. When the IPv6 routers send these
ICMPv6 errors, they will pass through a translator that will
translate the ICMPv6 error to a form that the IPv4 sender can
understand. As a result, an IPv6 Fragment Header is only included if
the IPv4 packet is already fragmented.

However, when the IPv4 sender does not set the DF bit, the translator
MUST ensure that the packet does not exceed the path MTU on the IPv6
side. This is done by fragmenting the IPv4 packet (with Fragment
Headers) so that it fits in 1280-byte IPv6 packets, since that is the
minimum IPv6 MTU. The IPv6 Fragment Header has been shown to cause
operational difficulties in practice due to limited firewall
fragmentation support, etc. In an environment where the network
owned/operated by the same entity that owns/operates the translator,
the translator MUST provide a configuration function for the network
administrator to adjust the threshold of the minimum IPv6 MTU to a
value that reflects the real value of the minimum IPv6 MTU in the
network (greater than 1280 bytes). This will help reduce the chance
of including the Fragment Header in the packets.

When the IPv4 sender does not set the DF bit, the translator MUST NOT
include the Fragment Header for the non-fragmented IPv6 packets.

The rules in Section 4.1 ensure that when packets are fragmented,
either by the sender or by IPv4 routers, the low-order 16 bits of the
fragment identification are carried end-to-end, ensuring that packets
are correctly reassembled.

Other than the special rules for handling fragments and path MTU
discovery, the actual translation of the packet header consists of a
simple translation as defined below. Note that ICMPv4 packets
require special handling in order to translate the content of ICMPv4
error messages and also to add the ICMPv6 pseudo-header checksum.

The translator SHOULD make sure that the packets belonging to the
same flow leave the translator in the same order in which they
arrived.

4.1. Translating IPv4 Headers into IPv6 Headers

If the DF flag is not set and the IPv4 packet will result in an IPv6
packet larger than a user-defined length (hereinafter referred to as
"lowest-ipv6-mtu", and which defaults to 1280 bytes), the packet
SHOULD be fragmented so that the resulting IPv6 packet (with Fragment
Header added to each fragment) will be less than or equal to lowest-
ipv6-mtu, For example, if the packet is fragmented prior to the
translation, the IPv4 packets should be fragmented so that their
length, excluding the IPv4 header, is at most 1232 bytes (1280 minus
40 for the IPv6 header and 8 for the Fragment Header). The
translator MUST provide a configuration function for the network
administrator to adjust the threshold of the minimum IPv6 MTU to a
value greater than 1280 bytes if the real value of the minimum IPv6
MTU in the network is known to the administrator. The resulting
fragments are then translated independently using the logic described
below.

If the DF bit is set and the MTU of the next-hop interface is less
than the total length value of the IPv4 packet plus 20, the
translator MUST send an ICMPv4 "Fragmentation Needed" error message
to the IPv4 source address.

The IPv6 header fields are set as follows:

Version: 6

Traffic Class: By default, copied from the IP Type Of Service (TOS)
octet. According to [RFC2474], the semantics of the bits are
identical in IPv4 and IPv6. However, in some IPv4 environments
these fields might be used with the old semantics of "Type Of
Service and Precedence". An implementation of a translator SHOULD
support an administratively configurable option to ignore the IPv4
TOS and always set the IPv6 traffic class (TC) to zero. In
addition, if the translator is at an administrative boundary, the
filtering and update considerations of [RFC2475] may be
applicable.

Flow Label: 0 (all zero bits)

Payload Length: Total length value from the IPv4 header, minus the
size of the IPv4 header and IPv4 options, if present.

Next Header: For ICMPv4 (1), it is changed to ICMPv6 (58);
otherwise, the protocol field MUST be copied from the IPv4 header.

Hop Limit: The hop limit is derived from the TTL value in the IPv4
header. Since the translator is a router, as part of forwarding
the packet it needs to decrement either the IPv4 TTL (before the
translation) or the IPv6 Hop Limit (after the translation). As
part of decrementing the TTL or Hop Limit, the translator (as any
router) MUST check for zero and send the ICMPv4 "TTL Exceeded" or
ICMPv6 "Hop Limit Exceeded" error.

Source Address: Mapped to an IPv6 address based on the algorithms
presented in Section 6.

If the translator gets an illegal source address (e.g., 0.0.0.0,
127.0.0.1, etc.), the translator SHOULD silently discard the
packet (as discussed in Section 5.3.7 of [RFC1812]). Note when
translating ICMPv4 Error Messages into ICMPv6, the "illegal"
source address will be translated for the purpose of trouble
shooting.

Destination Address: Mapped to an IPv6 address based on the
algorithms presented in Section 6.

If any IPv4 options are present in the IPv4 packet, they MUST be
ignored and the packet translated normally; there is no attempt to
translate the options. However, if an unexpired source route option
is present, then the packet MUST instead be discarded, and an ICMPv4
"Destination Unreachable, Source Route Failed" (Type 3, Code 5) error
message SHOULD be returned to the sender.

If there is a need to add a Fragment Header (the packet is a fragment
or the DF bit is not set and the packet size is greater than the
minimum IPv6 MTU in the network set by the translator configuration
function), the header fields are set as above with the following
exceptions:

IPv6 fields:

Payload Length: Total length value from the IPv4 header, plus 8
for the Fragment Header, minus the size of the IPv4 header and
IPv4 options, if present.

Next Header: Fragment Header (44).

Fragment Header fields:

Next Header: For ICMPv4 (1), it is changed to ICMPv6 (58);
otherwise, the protocol field MUST be copied from the IPv4
header.

Fragment Offset: Fragment Offset copied from the IPv4 header.

M flag: More Fragments bit copied from the IPv4 header.

Identification: The low-order 16 bits copied from the
Identification field in the IPv4 header. The high-order 16
bits set to zero.

4.2. Translating ICMPv4 Headers into ICMPv6 Headers

All ICMPv4 messages that are to be translated require that the ICMPv6
checksum field be calculated as part of the translation since ICMPv6,
unlike ICMPv4, has a pseudo-header checksum just like UDP and TCP.

In addition, all ICMPv4 packets MUST have the Type translated and,
for ICMPv4 error messages, the included IP header also MUST be
translated.

The actions needed to translate various ICMPv4 messages are as
follows:

ICMPv4 query messages:

Echo and Echo Reply (Type 8 and Type 0): Adjust the Type values
to 128 and 129, respectively, and adjust the ICMP checksum both
to take the type change into account and to include the ICMPv6
pseudo-header.

Information Request/Reply (Type 15 and Type 16): Obsoleted in
ICMPv6. Silently drop.

Timestamp and Timestamp Reply (Type 13 and Type 14): Obsoleted in
ICMPv6. Silently drop.

Address Mask Request/Reply (Type 17 and Type 18): Obsoleted in
ICMPv6. Silently drop.

ICMP Router Advertisement (Type 9): Single-hop message. Silently
drop.

ICMP Router Solicitation (Type 10): Single-hop message. Silently
drop.

Unknown ICMPv4 types: Silently drop.

IGMP messages: While the Multicast Listener Discovery (MLD)
messages specified in [RFC2710], [RFC3590], and [RFC3810] are
the logical IPv6 counterparts for the IPv4 IGMP messages, all
the "normal" IGMP messages are single-hop messages and SHOULD
be silently dropped by the translator. Other IGMP messages
might be used by multicast routing protocols and, since it
would be a configuration error to try to have router
adjacencies across IP/ICMP translators, those packets SHOULD
also be silently dropped.

ICMPv4 error messages:

Destination Unreachable (Type 3): Translate the Code as
described below, set the Type to 1, and adjust the ICMP
checksum both to take the type/code change into account and
to include the ICMPv6 pseudo-header.

Translate the Code as follows:

Code 0, 1 (Net Unreachable, Host Unreachable): Set the Code
to 0 (No route to destination).

Code 2 (Protocol Unreachable): Translate to an ICMPv6
Parameter Problem (Type 4, Code 1) and make the Pointer
point to the IPv6 Next Header field.

Code 3 (Port Unreachable): Set the Code to 4 (Port
unreachable).

Code 4 (Fragmentation Needed and DF was Set): Translate to
an ICMPv6 Packet Too Big message (Type 2) with Code set
to 0. The MTU field MUST be adjusted for the difference
between the IPv4 and IPv6 header sizes, but MUST NOT be
set to a value smaller than the minimum IPv6 MTU (1280
bytes). That is, it should be set to

maximum(1280,
minimum((MTU value in the Packet Too Big Message) + 20,
MTU_of_IPv6_nexthop,
(MTU_of_IPv4_nexthop) + 20)).

Note that if the IPv4 router set the MTU field to zero,
i.e., the router does not implement [RFC1191], then the
translator MUST use the plateau values specified in
[RFC1191] to determine a likely path MTU and include that
path MTU in the ICMPv6 packet. (Use the greatest plateau
value that is less than the returned Total Length field,
but that is larger than or equal to 1280.)

See also the requirements in Section 7.

Code 5 (Source Route Failed): Set the Code to 0 (No route
to destination). Note that this error is unlikely since
source routes are not translated.

Code 6, 7, 8: Set the Code to 0 (No route to destination).

Code 9, 10 (Communication with Destination Host
Administratively Prohibited): Set the Code to 1
(Communication with destination administratively
prohibited).

Code 11, 12: Set the Code to 0 (No route to destination).

Code 13 (Communication Administratively Prohibited): Set
the Code to 1 (Communication with destination
administratively prohibited).

Code 14 (Host Precedence Violation): Silently drop.

Code 15 (Precedence cutoff in effect): Set the Code to 1
(Communication with destination administratively
prohibited).

Other Code values: Silently drop.

Redirect (Type 5): Single-hop message. Silently drop.

Alternative Host Address (Type 6): Silently drop.

Source Quench (Type 4): Obsoleted in ICMPv6. Silently drop.

Time Exceeded (Type 11): Set the Type to 3, and adjust the
ICMP checksum both to take the type change into account and
to include the ICMPv6 pseudo-header. The Code is unchanged.

Parameter Problem (Type 12): Set the Type to 4, and adjust the
ICMP checksum both to take the type/code change into account
and to include the ICMPv6 pseudo-header.

Translate the Code as follows:

Code 0 (Pointer indicates the error): Set the Code to 0
(Erroneous header field encountered) and update the
pointer as defined in Figure 3. (If the Original IPv4
Pointer Value is not listed or the Translated IPv6
Pointer Value is listed as "n/a", silently drop the
packet.)

Code 1 (Missing a required option): Silently drop.

Code 2 (Bad length): Set the Code to 0 (Erroneous header
field encountered) and update the pointer as defined in
Figure 3. (If the Original IPv4 Pointer Value is not
listed or the Translated IPv6 Pointer Value is listed as
"n/a", silently drop the packet.)

Other Code values: Silently drop.

Unknown ICMPv4 types: Silently drop.

+--------------------------------+--------------------------------+
| Original IPv4 Pointer Value | Translated IPv6 Pointer Value |
+--------------------------------+--------------------------------+
| 0 | Version/IHL | 0 | Version/Traffic Class |
| 1 | Type Of Service | 1 | Traffic Class/Flow Label |
| 2,3 | Total Length | 4 | Payload Length |
| 4,5 | Identification | n/a | |
| 6 | Flags/Fragment Offset | n/a | |
| 7 | Fragment Offset | n/a | |
| 8 | Time to Live | 7 | Hop Limit |
| 9 | Protocol | 6 | Next Header |
|10,11| Header Checksum | n/a | |
|12-15| Source Address | 8 | Source Address |
|16-19| Destination Address | 24 | Destination Address |
+--------------------------------+--------------------------------+

Figure 3: Pointer Value for Translating from IPv4 to IPv6

ICMP Error Payload: If the received ICMPv4 packet contains an
ICMPv4 Extension [RFC4884], the translation of the ICMPv4
packet will cause the ICMPv6 packet to change length. When
this occurs, the ICMPv6 Extension length attribute MUST be
adjusted accordingly (e.g., longer due to the translation
from IPv4 to IPv6). If the ICMPv4 Extension exceeds the
maximum size of an ICMPv6 message on the outgoing interface,
the ICMPv4 extension SHOULD be simply truncated. For
extensions not defined in [RFC4884], the translator passes
the extensions as opaque bit strings, and those containing
IPv4 address literals will not have their included addresses
translated to IPv6 address literals; this may cause problems
with processing of those ICMP extensions.

4.3. Translating ICMPv4 Error Messages into ICMPv6

There are some differences between the ICMPv4 and the ICMPv6 error
message formats as detailed above. The ICMP error messages
containing the packet in error MUST be translated just like a normal
IP packet (except the TTL value of the inner IPv4/IPv6 packet). If
the translation of this "packet in error" changes the length of the
datagram, the Total Length field in the outer IPv6 header MUST be
updated.

Figure 4: IPv4-to-IPv6 ICMP Error Translation

The translation of the inner IP header can be done by invoking the
function that translated the outer IP headers. This process MUST
stop at the first embedded header and drop the packet if it contains
more embedded headers.

4.4. Generation of ICMPv4 Error Message

If the IPv4 packet is discarded, then the translator SHOULD be able
to send back an ICMPv4 error message to the original sender of the
packet, unless the discarded packet is itself an ICMPv4 error
message. The ICMPv4 message, if sent, has a Type of 3 (Destination
Unreachable) and a Code of 13 (Communication Administratively
Prohibited), unless otherwise specified in this document or in
[RFC6146]. The translator SHOULD allow an administrator to configure
whether the ICMPv4 error messages are sent, rate-limited, or not
sent.

4.5. Transport-Layer Header Translation

If the address translation algorithm is not checksum neutral (see
Section 4.1 of [RFC6052]), the recalculation and updating of the
transport-layer headers that contain pseudo-headers need to be
performed. Translators MUST do this for TCP and ICMP packets and for
UDP packets that contain a UDP checksum (i.e., the UDP checksum field
is not zero).

For UDP packets that do not contain a UDP checksum (i.e., the UDP
checksum field is zero), the translator SHOULD provide a
configuration function to allow:

1. Dropping the packet and generating a system management event that
specifies at least the IP addresses and port numbers of the
packet.

2. Calculating an IPv6 checksum and forwarding the packet (which has
performance implications).

A stateless translator cannot compute the UDP checksum of
fragmented packets, so when a stateless translator receives the
first fragment of a fragmented UDP IPv4 packet and the checksum
field is zero, the translator SHOULD drop the packet and generate
a system management event that specifies at least the IP
addresses and port numbers in the packet.

For a stateful translator, the handling of fragmented UDP IPv4
packets with a zero checksum is discussed in [RFC6146],
Section 3.4.

Other transport protocols (e.g., the Datagram Congestion Control
Protocol (DCCP)) are OPTIONAL to support. In order to ease debugging
and troubleshooting, translators MUST forward all transport protocols
as described in the "Next Header" step of Section 4.1.

4.6. Knowing When to Translate

If the IP/ICMP translator also provides a normal forwarding function,
and the destination IPv4 address is reachable by a more specific
route without translation, the translator MUST forward it without
translating it. Otherwise, when an IP/ICMP translator receives an
IPv4 datagram addressed to an IPv4 destination representing a host in
the IPv6 domain, the packet MUST be translated to IPv6.

5. Translating from IPv6 to IPv4

When an IP/ICMP translator receives an IPv6 datagram addressed to a
destination towards the IPv4 domain, it translates the IPv6 header of
the received IPv6 packet into an IPv4 header. The original IPv6
header on the packet is removed and replaced by an IPv4 header.
Since the ICMPv6 [RFC4443], TCP [RFC0793], UDP [RFC0768], and DCCP
[RFC4340] headers contain checksums that cover the IP header, if the
address mapping algorithm is not checksum neutral, the checksum MUST
be evaluated before translation and the ICMP and transport-layer
headers MUST be updated. The data portion of the packet is left
unchanged. The IP/ICMP translator then forwards the packet based on
the IPv4 destination address.

Figure 5: IPv6-to-IPv4 Translation

There are some differences between IPv6 and IPv4 (in the areas of
fragmentation and the minimum link MTU) that affect the translation.
An IPv6 link has to have an MTU of 1280 bytes or greater. The
corresponding limit for IPv4 is 68 bytes. Path MTU discovery across
a translator relies on ICMP Packet Too Big messages being received
and processed by IPv6 hosts.

The difference in the minimum MTUs of IPv4 and IPv6 is accommodated
as follows:

o When translating an ICMPv4 "Fragmentation Needed" packet, the
indicated MTU in the resulting ICMPv6 "Packet Too Big" will never
be set to a value lower than 1280. This ensures that the IPv6
nodes will never have to encounter or handle Path MTU values lower
than the minimum IPv6 link MTU of 1280. See Section 4.2.

o When the resulting IPv4 packet is smaller than or equal to 1260
bytes, the translator MUST send the packet with a cleared Don't
Fragment bit. Otherwise, the packet MUST be sent with the Don't
Fragment bit set. See Section 5.1.

This approach allows Path MTU Discovery to operate end-to-end for
paths whose MTU are not smaller than the minimum IPv6 MTU of 1280
(which corresponds to an MTU of 1260 in the IPv4 domain). On paths
that have IPv4 links with MTU < 1260, the IPv4 router(s) connected to
those links will fragment the packets in accordance with Section 2.3
of [RFC0791].

Other than the special rules for handling fragments and path MTU
discovery, the actual translation of the packet header consists of a
simple translation as defined below. Note that ICMPv6 packets
require special handling in order to translate the contents of ICMPv6
error messages and also to remove the ICMPv6 pseudo-header checksum.

The translator SHOULD make sure that the packets belonging to the
same flow leave the translator in the same order in which they
arrived.

5.1. Translating IPv6 Headers into IPv4 Headers

If there is no IPv6 Fragment Header, the IPv4 header fields are set
as follows:

Version: 4

Internet Header Length: 5 (no IPv4 options)

Type of Service (TOS) Octet: By default, copied from the IPv6
Traffic Class (all 8 bits). According to [RFC2474], the semantics
of the bits are identical in IPv4 and IPv6. However, in some IPv4
environments, these bits might be used with the old semantics of
"Type Of Service and Precedence". An implementation of a
translator SHOULD provide the ability to ignore the IPv6 traffic
class and always set the IPv4 TOS Octet to a specified value. In
addition, if the translator is at an administrative boundary, the
filtering and update considerations of [RFC2475] may be
applicable.

Total Length: Payload length value from the IPv6 header, plus the
size of the IPv4 header.

Identification: Set according to a Fragment Identification generator
at the translator.

Flags: The More Fragments flag is set to zero. The Don't Fragment
(DF) flag is set as follows: If the size of the translated IPv4
packet is less than or equal to 1260 bytes, it is set to zero;
otherwise, it is set to one.

Fragment Offset: All zeros.

Time to Live: Time to Live is derived from the Hop Limit value in
the IPv6 header. Since the translator is a router, as part of
forwarding the packet it needs to decrement either the IPv6 Hop
Limit (before the translation) or the IPv4 TTL (after the
translation). As part of decrementing the TTL or Hop Limit, the
translator (as any router) MUST check for zero and send the ICMPv4
"TTL Exceeded" or ICMPv6 "Hop Limit Exceeded" error.

Protocol: The IPv6-Frag (44) header is handled as discussed in
Section 5.1.1. ICMPv6 (58) is changed to ICMPv4 (1), and the
payload is translated as discussed in Section 5.2. The IPv6
headers HOPOPT (0), IPv6-Route (43), and IPv6-Opts (60) are
skipped over during processing as they have no meaning in IPv4.
For the first 'next header' that does not match one of the cases
above, its Next Header value (which contains the transport
protocol number) is copied to the protocol field in the IPv4
header. This means that all transport protocols are translated.

Note: Some translated protocols will fail at the receiver for
various reasons: some are known to fail when translated (e.g.,
IPsec Authentication Header (51)), and others will fail
checksum validation if the address translation is not checksum
neutral [RFC6052] and the translator does not update the
transport protocol's checksum (because the translator doesn't
support recalculating the checksum for that transport protocol;
see Section 5.5).

Header Checksum: Computed once the IPv4 header has been created.

Source Address: Mapped to an IPv4 address based on the algorithms
presented in Section 6.

If the translator gets an illegal source address (e.g., ::1,
etc.), the translator SHOULD silently drop the packet.

Destination Address: Mapped to an IPv4 address based on the
algorithms presented in Section 6.

If any of an IPv6 Hop-by-Hop Options header, Destination Options
header, or Routing header with the Segments Left field equal to zero
are present in the IPv6 packet, those IPv6 extension headers MUST be
ignored (i.e., there is no attempt to translate the extension
headers) and the packet translated normally. However, the Total
Length field and the Protocol field are adjusted to "skip" these
extension headers.

If a Routing header with a non-zero Segments Left field is present,
then the packet MUST NOT be translated, and an ICMPv6 "parameter
problem/erroneous header field encountered" (Type 4, Code 0) error
message, with the Pointer field indicating the first byte of the
Segments Left field, SHOULD be returned to the sender.

5.1.1. IPv6 Fragment Processing

If the IPv6 packet contains a Fragment Header, the header fields are
set as above with the following exceptions:

Total Length: If the Next Header field of the Fragment Header is an
extension header (except ESP, but including the Authentication
Header (AH)), then the packet SHOULD be dropped and logged. For
other cases, the Total Length MUST be set to Payload Length value
from IPv6 header, minus the length of the extension headers up to
the Fragmentation Header, minus 8 for the Fragment Header, plus
the size of the IPv4 header.

Identification: Copied from the low-order 16 bits in the
Identification field in the Fragment Header.

Flags: The IPv4 More Fragments (MF) flag is copied from the M flag
in the IPv6 Fragment Header. The IPv4 Don't Fragment (DF) flag is
cleared (set to zero), allowing this packet to be further
fragmented by IPv4 routers.

Fragment Offset: If the Next Header field of the Fragment Header is
not an extension header (except ESP), then Fragment Offset MUST be
copied from the Fragment Offset field of the IPv6 Fragment Header.
If the Next Header field of the Fragment Header is an extension
header (except ESP), then the packet SHOULD be dropped and logged.

Protocol: For ICMPv6 (58), it is changed to ICMPv4 (1); otherwise,
extension headers are skipped, and the Next Header field is copied
from the last IPv6 header.

If an IPv6 packet that is smaller than or equal to 1280 bytes results
(after translation) in an IPv4 packet that is larger than the MTU of
the next-hop interface, then the translator MUST perform IPv4
fragmentation on that packet such that it can be transferred over the
constricting link.

5.2. Translating ICMPv6 Headers into ICMPv4 Headers

If a non-checksum-neutral translation address is being used, ICMPv6
messages MUST have their ICMPv4 checksum field be updated as part of
the translation since ICMPv6 (unlike ICMPv4) includes a pseudo-header
in the checksum just like UDP and TCP.

In addition, all ICMP packets MUST have the Type translated and, for
ICMP error messages, the included IP header MUST also be translated.

The actions needed to translate various ICMPv6 messages are:

ICMPv6 informational messages:

Echo Request and Echo Reply (Type 128 and 129): Adjust the Type
values to 8 and 0, respectively, and adjust the ICMP checksum
both to take the type change into account and to exclude the
ICMPv6 pseudo-header.

MLD Multicast Listener Query/Report/Done (Type 130, 131, 132):
Single-hop message. Silently drop.

Neighbor Discover messages (Type 133 through 137): Single-hop
message. Silently drop.

Unknown informational messages: Silently drop.

ICMPv6 error messages:

Destination Unreachable (Type 1) Set the Type to 3, and adjust
the ICMP checksum both to take the type/code change into
account and to exclude the ICMPv6 pseudo-header.

Translate the Code as follows:

Code 0 (No route to destination): Set the Code to 1 (Host
unreachable).

Code 1 (Communication with destination administratively
prohibited): Set the Code to 10 (Communication with
destination host administratively prohibited).

Code 2 (Beyond scope of source address): Set the Code to 1
(Host unreachable). Note that this error is very unlikely
since an IPv4-translatable source address is typically
considered to have global scope.

Code 3 (Address unreachable): Set the Code to 1 (Host
unreachable).

Code 4 (Port unreachable): Set the Code to 3 (Port
unreachable).

Other Code values: Silently drop.

Packet Too Big (Type 2): Translate to an ICMPv4 Destination
Unreachable (Type 3) with Code 4, and adjust the ICMPv4
checksum both to take the type change into account and to
exclude the ICMPv6 pseudo-header. The MTU field MUST be
adjusted for the difference between the IPv4 and IPv6 header
sizes, taking into account whether or not the packet in error
includes a Fragment Header, i.e., minimum((MTU value in the
Packet Too Big Message)-20, MTU_of_IPv4_nexthop,
(MTU_of_IPv6_nexthop)-20).

See also the requirements in Section 7.

Time Exceeded (Type 3): Set the Type to 11, and adjust the ICMPv4
checksum both to take the type change into account and to
exclude the ICMPv6 pseudo-header. The Code is unchanged.

Parameter Problem (Type 4): Translate the Type and Code as
follows, and adjust the ICMPv4 checksum both to take the type/
code change into account and to exclude the ICMPv6 pseudo-
header.

Translate the Code as follows:

Code 0 (Erroneous header field encountered): Set to Type 12,
Code 0, and update the pointer as defined in Figure 6. (If
the Original IPv6 Pointer Value is not listed or the
Translated IPv4 Pointer Value is listed as "n/a", silently
drop the packet.)

Code 1 (Unrecognized Next Header type encountered): Translate
this to an ICMPv4 protocol unreachable (Type 3, Code 2).

Code 2 (Unrecognized IPv6 option encountered): Silently drop.

Unknown error messages: Silently drop.

+--------------------------------+--------------------------------+
| Original IPv6 Pointer Value | Translated IPv4 Pointer Value |
+--------------------------------+--------------------------------+
| 0 | Version/Traffic Class | 0 | Version/IHL, Type Of Ser |
| 1 | Traffic Class/Flow Label | 1 | Type Of Service |
| 2,3 | Flow Label | n/a | |
| 4,5 | Payload Length | 2 | Total Length |
| 6 | Next Header | 9 | Protocol |
| 7 | Hop Limit | 8 | Time to Live |
| 8-23| Source Address | 12 | Source Address |
|24-39| Destination Address | 16 | Destination Address |
+--------------------------------+--------------------------------+

Figure 6: Pointer Value for Translating from IPv6 to IPv4

ICMP Error Payload: If the received ICMPv6 packet contains an
ICMPv6 Extension [RFC4884], the translation of the ICMPv6
packet will cause the ICMPv4 packet to change length. When
this occurs, the ICMPv6 Extension length attribute MUST be
adjusted accordingly (e.g., shorter due to the translation from
IPv6 to IPv4). For extensions not defined in [RFC4884], the
translator passes the extensions as opaque bit strings and any
IPv6 address literals contained therein will not be translated
to IPv4 address literals; this may cause problems with
processing of those ICMP extensions.

5.3. Translating ICMPv6 Error Messages into ICMPv4

There are some differences between the ICMPv4 and the ICMPv6 error
message formats as detailed above. The ICMP error messages
containing the packet in error MUST be translated just like a normal
IP packet (except that the TTL/Hop Limit value of the inner IPv4/IPv6
packet are not decremented). The translation of this "packet in
error" is likely to change the length of the datagram; thus, the
Total Length field in the outer IPv4 header MUST be updated.

Figure 7: IPv6-to-IPv4 ICMP Error Translation

5.4. Generation of ICMPv6 Error Messages

If the IPv6 packet is discarded, then the translator SHOULD send back
an ICMPv6 error message to the original sender of the packet, unless
the discarded packet is itself an ICMPv6 message.

The ICMPv6 message MUST have Type 1 (Destination Unreachable) and
Code 1 (Communication with destination administratively prohibited),
unless otherwise specified in this document or [RFC6146]. The
translator SHOULD allow an administrator to configure whether the
ICMPv6 error messages are sent, rate-limited, or not sent.

5.5. Transport-Layer Header Translation

Other transport protocols (e.g., DCCP) are OPTIONAL to support. In
order to ease debugging and troubleshooting, translators MUST forward
all transport protocols as described in the "Protocol" step of
Section 5.1.

5.6. Knowing When to Translate

If the IP/ICMP translator also provides a normal forwarding function,
and the destination address is reachable by a more specific route
without translation, the router MUST forward it without translating
it. When an IP/ICMP translator receives an IPv6 datagram addressed
to an IPv6 address representing a host in the IPv4 domain, the IPv6
packet MUST be translated to IPv4.

6. Mapping of IP Addresses

The translator MUST support the stateless address mapping algorithm
defined in [RFC6052], which is the default behavior. A workflow
example is shown in Appendix A of this document. Note that [RFC7136]
updates [RFC4291], which allows the use of unicast addresses without
u-bit, as long as they're not derived from an IEEE MAC-layer address.
Therefore, the address mapping algorithm defined in [RFC6219] also
complies with the IPv6 address architecture.

The stateless translator SHOULD support the explicit address mapping
algorithm defined in [RFC7757].

The stateless translator SHOULD support [RFC6791] for handling ICMP/
ICMPv6 packets.

Implementations may support both stateless and stateful translation
modes (e.g., Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers (NAT64) [RFC6146]).

Implementations may support stateless NAT64 function, e.g., MAP-T
Customer Edge (CE) or MAP-T Border Relay (BR) [RFC7599].

7. Special Considerations for ICMPv6 Packet Too Big

A number of studies [ATOMIC] indicate that it not unusual for
networks to drop ICMPv6 Packet Too Big error messages. Such packet
drops will result in PMTUD black holes [RFC2923], which can only be
overcome with Packetization Layer Path MTU Discovery (PLPMTUD)
[RFC4821].

8. Security Considerations

The use of stateless IP/ICMP translators does not introduce any new
security issues beyond the security issues that are already present
in the IPv4 and IPv6 protocols and in the routing protocols that are
used to make the packets reach the translator.

There are potential issues that might arise by deriving an IPv4
address from an IPv6 address -- particularly addresses like broadcast
or loopback addresses and the non-IPv4-translatable IPv6 addresses,
etc. [RFC6052] addresses these issues.

As with network address translation of IPv4 to IPv4, the

The IPsec Authentication Header [RFC4302] cannot be used across an IPv6-to-IPv4
translator. for NAT44 or
NAT64.

As with the network address translation of IPv4 to IPv4, packets with
tunnel mode Encapsulating Security Payload (ESP) can be translated
since tunnel mode ESP does not depend on header fields prior to the
ESP header. Similarly, transport mode ESP will fail with IPv6-to-
IPv4 translation unless checksum-neutral addresses are used. In both
cases, the IPsec ESP endpoints will normally detect the presence of
the translator and encapsulate ESP in UDP packets [RFC3948].

9. References

9.1. Normative References

[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<http://www.rfc-editor.org/info/rfc768>.

[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<http://www.rfc-editor.org/info/rfc791>.

[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>.

[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
RFC 1812, DOI 10.17487/RFC1812, June 1995,
<http://www.rfc-editor.org/info/rfc1812>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets",
RFC 3948, DOI 10.17487/RFC3948, January 2005,
<http://www.rfc-editor.org/info/rfc3948>.

[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <http://www.rfc-editor.org/info/rfc4291>.

[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
DOI 10.17487/RFC4340, March 2006,
<http://www.rfc-editor.org/info/rfc4340>.

[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", RFC 4443,
DOI 10.17487/RFC4443, March 2006,
<http://www.rfc-editor.org/info/rfc4443>.

[RFC4884] Bonica, R., Gan, D., Tappan, D., and C. Pignataro,
"Extended ICMP to Support Multi-Part Messages", RFC 4884,
DOI 10.17487/RFC4884, April 2007,
<http://www.rfc-editor.org/info/rfc4884>.

[RFC5382] Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, DOI 10.17487/RFC5382, October 2008,
<http://www.rfc-editor.org/info/rfc5382>.

[RFC5771] Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
IPv4 Multicast Address Assignments", BCP 51, RFC 5771,
DOI 10.17487/RFC5771, March 2010,
<http://www.rfc-editor.org/info/rfc5771>.

[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
DOI 10.17487/RFC6052, October 2010,
<http://www.rfc-editor.org/info/rfc6052>.

[RFC6145] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
Algorithm", RFC 6145, DOI 10.17487/RFC6145, April 2011,
<http://www.rfc-editor.org/info/rfc6145>.

[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
April 2011, <http://www.rfc-editor.org/info/rfc6146>.

[RFC6791] Li, X., Bao, C., Wing, D., Vaithianathan, R., and G.
Huston, "Stateless Source Address Mapping for ICMPv6
Packets", RFC 6791, DOI 10.17487/RFC6791, November 2012,
<http://www.rfc-editor.org/info/rfc6791>.

[RFC7757] Anderson, T. and A. Leiva Popper, "Explicit Address
Mappings for Stateless IP/ICMP Translation", RFC 7757,
DOI 10.17487/RFC7757, February 2016,
<http://www.rfc-editor.org/info/rfc7757>.

9.2. Informative References

[ATOMIC] Gont, F., LIU, S., and T. Anderson, "Generation of IPv6
Atomic Fragments Considered Harmful", Work in Progress,
draft-ietf-6man-deprecate-atomfrag-generation-06, April
2016.

[Err3059] RFC Errata, Erratum ID 3059, RFC 6145.

[Err3060] RFC Errata, Erratum ID 3060, RFC 6145.

[Err3061] RFC Errata, Erratum ID 3061, RFC 6145.

[Err4090] RFC Errata, Erratum ID 4090, RFC 6145.

[IPv6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", Work in Progress, draft-ietf-6man-
rfc2460bis-04, March 2016.

[RFC0879] Postel, J., "The TCP Maximum Segment Size and Related
Topics", RFC 879, DOI 10.17487/RFC0879, November 1983,
<http://www.rfc-editor.org/info/rfc879>.

[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<http://www.rfc-editor.org/info/rfc1191>.

[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<http://www.rfc-editor.org/info/rfc2474>.

[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<http://www.rfc-editor.org/info/rfc2475>.

[RFC2710] Deering, S., Fenner, W., and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710,
DOI 10.17487/RFC2710, October 1999,
<http://www.rfc-editor.org/info/rfc2710>.

[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, DOI 10.17487/RFC2923, September 2000,
<http://www.rfc-editor.org/info/rfc2923>.

[RFC3307] Haberman, B., "Allocation Guidelines for IPv6 Multicast
Addresses", RFC 3307, DOI 10.17487/RFC3307, August 2002,
<http://www.rfc-editor.org/info/rfc3307>.

[RFC3590] Haberman, B., "Source Address Selection for the Multicast
Listener Discovery (MLD) Protocol", RFC 3590,
DOI 10.17487/RFC3590, September 2003,
<http://www.rfc-editor.org/info/rfc3590>.

[RFC3810] Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
DOI 10.17487/RFC3810, June 2004,
<http://www.rfc-editor.org/info/rfc3810>.

[RFC3849] Huston, G., Lord, A., and P. Smith, "IPv6 Address Prefix
Reserved for Documentation", RFC 3849,
DOI 10.17487/RFC3849, July 2004,
<http://www.rfc-editor.org/info/rfc3849>.

[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
DOI 10.17487/RFC4302, December 2005,
<http://www.rfc-editor.org/info/rfc4302>.

[RFC4787] Audet, F., Ed. and C. Jennings, "Network Address
Translation (NAT) Behavioral Requirements for Unicast
UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
2007, <http://www.rfc-editor.org/info/rfc4787>.

[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<http://www.rfc-editor.org/info/rfc4821>.

[RFC5737] Arkko, J., Cotton, M., and L. Vegoda, "IPv4 Address Blocks
Reserved for Documentation", RFC 5737,
DOI 10.17487/RFC5737, January 2010,
<http://www.rfc-editor.org/info/rfc5737>.

[RFC6144] Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
IPv4/IPv6 Translation", RFC 6144, DOI 10.17487/RFC6144,
April 2011, <http://www.rfc-editor.org/info/rfc6144>.

[RFC6219] Li, X., Bao, C., Chen, M., Zhang, H., and J. Wu, "The
China Education and Research Network (CERNET) IVI
Translation Design and Deployment for the IPv4/IPv6
Coexistence and Transition", RFC 6219,
DOI 10.17487/RFC6219, May 2011,
<http://www.rfc-editor.org/info/rfc6219>.

[RFC6691] Borman, D., "TCP Options and Maximum Segment Size (MSS)",
RFC 6691, DOI 10.17487/RFC6691, July 2012,
<http://www.rfc-editor.org/info/rfc6691>.

[RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6
Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
February 2014, <http://www.rfc-editor.org/info/rfc7136>.

[RFC7599] Li, X., Bao, C., Dec, W., Ed., Troan, O., Matsushima, S.,
and T. Murakami, "Mapping of Address and Port using
Translation (MAP-T)", RFC 7599, DOI 10.17487/RFC7599, July
2015, <http://www.rfc-editor.org/info/rfc7599>.

Appendix A. Stateless Translation Workflow Example

A stateless translation workflow example is depicted in the following
figure. The documentation address blocks 2001:db8::/32 [RFC3849],
192.0.2.0/24, and 198.51.100.0/24 [RFC5737] are used in this example.

+--------------+ +--------------+
| IPv4 network | | IPv6 network |
| | +-------+ | |
| +----+ |-----| XLAT |---- | +----+ |
| | H4 |-----| +-------+ |--| H6 | |
| +----+ | | +----+ |
+--------------+ +--------------+

Figure 8: Stateless Translation Workflow

A translator (XLAT) connects the IPv6 network to the IPv4 network.
This XLAT uses the Network-Specific Prefix (NSP) 2001:db8:100::/40
defined in [RFC6052] to represent IPv4 addresses in the IPv6 address
space (IPv4-converted addresses) and to represent IPv6 addresses
(IPv4-translatable addresses) in the IPv4 address space. In this
example, 192.0.2.0/24 is the IPv4 block of the corresponding
IPv4-translatable addresses.

Based on the address mapping rule, the IPv6 node H6 has an
IPv4-translatable IPv6 address 2001:db8:1c0:2:21:: (address mapping
from 192.0.2.33). The IPv4 node H4 has IPv4 address 198.51.100.2.

The IPv6 routing is configured in such a way that the IPv6 packets
addressed to a destination address in 2001:db8:100::/40 are routed to
the IPv6 interface of the XLAT.

The IPv4 routing is configured in such a way that the IPv4 packets
addressed to a destination address in 192.0.2.0/24 are routed to the
IPv4 interface of the XLAT.

A.1. H6 Establishes Communication with H4

The steps by which H6 establishes communication with H4 are:

1. H6 performs the destination address mapping, so the
IPv4-converted address 2001:db8:1c6:3364:2:: is formed from
198.51.100.2 based on the address mapping algorithm [RFC6052].

2. H6 sends a packet to H4. The packet is sent from a source
address 2001:db8:1c0:2:21:: to a destination address
2001:db8:1c6:3364:2::.

3. The packet is routed to the IPv6 interface of the XLAT (since
IPv6 routing is configured that way).

4. The XLAT receives the packet and performs the following actions:

* The XLAT translates the IPv6 header into an IPv4 header using
the IP/ICMP Translation Algorithm defined in this document.

* The XLAT includes 192.0.2.33 as the source address in the
packet and 198.51.100.2 as the destination address in the
packet. Note that 192.0.2.33 and 198.51.100.2 are extracted
directly from the source IPv6 address 2001:db8:1c0:2:21::
(IPv4-translatable address) and destination IPv6 address
2001:db8:1c6:3364:2:: (IPv4-converted address) of the received
IPv6 packet that is being translated.

5. The XLAT sends the translated packet out of its IPv4 interface,
and the packet arrives at H4.

6. H4 node responds by sending a packet with destination address
192.0.2.33 and source address 198.51.100.2.

7. The packet is routed to the IPv4 interface of the XLAT (since
IPv4 routing is configured that way). The XLAT performs the
following operations:

* The XLAT translates the IPv4 header into an IPv6 header using
the IP/ICMP Translation Algorithm defined in this document.

* The XLAT includes 2001:db8:1c0:2:21:: as the destination
address in the packet and 2001:db8:1c6:3364:2:: as the source
address in the packet. Note that 2001:db8:1c0:2:21:: and
2001:db8:1c6:3364:2:: are formed directly from the destination
IPv4 address 192.0.2.33 and the source IPv4 address
198.51.100.2 of the received IPv4 packet that is being
translated.

8. The translated packet is sent out of the IPv6 interface to H6.

The packet exchange between H6 and H4 continues until the session is
finished.

A.2. H4 Establishes Communication with H6

The steps by which H4 establishes communication with H6 are:

1. H4 performs the destination address mapping, so 192.0.2.33 is
formed from the IPv4-translatable address 2001:db8:1c0:2:21::
based on the address mapping algorithm [RFC6052].

2. H4 sends a packet to H6. The packet is sent from a source
address 198.51.100.2 to a destination address 192.0.2.33.

3. The packet is routed to the IPv4 interface of the XLAT (since
IPv4 routing is configured that way).

4. The XLAT receives the packet and performs the following actions:

* The XLAT translates the IPv4 header into an IPv6 header using
the IP/ICMP Translation Algorithm defined in this document.

* The XLAT includes 2001:db8:1c6:3364:2:: as the source address
in the packet and 2001:db8:1c0:2:21:: as the destination
address in the packet. Note that 2001:db8:1c6:3364:2::
(IPv4-converted address) and 2001:db8:1c0:2:21::
(IPv4-translatable address) are obtained directly from the
source IPv4 address 198.51.100.2 and destination IPv4 address
192.0.2.33 of the received IPv4 packet that is being
translated.

5. The XLAT sends the translated packet out its IPv6 interface, and
the packet arrives at H6.

6. H6 node responds by sending a packet with destination address
2001:db8:1c6:3364:2:: and source address 2001:db8:1c0:2:21::.

7. The packet is routed to the IPv6 interface of the XLAT (since
IPv6 routing is configured that way). The XLAT performs the
following operations:

* The XLAT translates the IPv6 header into an IPv4 header using
the IP/ICMP Translation Algorithm defined in this document.

* The XLAT includes 198.51.100.2 as the destination address in
the packet and 192.0.2.33 as the source address in the packet.
Note that 198.51.100.2 and 192.0.2.33 are formed directly from
the destination IPv6 address 2001:db8:1c6:3364:2:: and source
IPv6 address 2001:db8:1c0:2:21:: of the received IPv6 packet
that is being translated.

8. The translated packet is sent out the IPv4 interface to H4.

The packet exchange between H4 and H6 continues until the session is
finished.

Acknowledgements

The authors would like to thank

Gandhar Gokhale, Wesley Eddy, and Fernando Gont for submitted and handled
the errata report reports on [RFC6145]. Thanks to Fernando Gont, Will (Shucheng) Liu,
and Tore Anderson for provided the security analysis and the suggestions of the
for updates concerning atomic fragments. Thanks to In addition, Tore Anderson
and Alberto Leiva for provided the proposal of the Explicit Address
Mapping (EAM) algorithm.

Authors' Addresses

Congxiao Bao
CERNET Center/Tsinghua University
Room 225, Main Building, Tsinghua University
Beijing 100084
China

Phone: +86 10-62785983
Email: congxiao@cernet.edu.cn

Xing Li
CERNET Center/Tsinghua University
Room 225, Main Building, Tsinghua University
Beijing 100084
China

Phone: +86 10-62785983
Email: xing@cernet.edu.cn

Fred Baker
Cisco Systems
Santa Barbara, California 93117
United States

Phone: +1-408-526-4257
Email: fred@cisco.com
Tore Anderson
Redpill Linpro
Vitaminveien 1A
0485 Oslo
Norway

Phone: +47 959 31 212
Email: tore@redpill-linpro.com
URI: http://www.redpill-linpro.com

Fernando Gont
Huawei Technologies
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina

Phone: +54 11 4650 8472
Email: fgont@si6networks.com
URI: http://www.si6networks.com