IP/ICMP Translation AlgorithmCERNET Center/Tsinghua UniversityRoom 225, Main Building, Tsinghua UniversityBeijing100084China+86 10-62785983congxiao@cernet.edu.cnCERNET Center/Tsinghua UniversityRoom 225, Main Building, Tsinghua UniversityBeijing100084China+86 10-62785983xing@cernet.edu.cnRedpill LinproVitaminveien 1A0485 OsloNorway+47 959 31 212tore@redpill-linpro.comhttp://www.redpill-linpro.comSI6 NetworksSegurola y Habana 4310, 7mo PisoVilla DevotoCiudad Autonoma de Buenos AiresArgentinafgont@si6networks.comhttps://www.si6networks.comThe IPv6 CompanyMolino de la Navata, 75La Navata - GalapagarMadrid28420Spainjordi.palet@theipv6company.comhttp://www.theipv6company.com/
Ops
v6opsSIITinternetprotocolcontrolmessageIPv4IPv6Stateless IP/ICMP Translation Algorithm
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.
This document obsoletes
.
Readers of this document are expected to have read and understood the
framework described in
.
Implementations of this IPv4/IPv6 translation specification MUST support
one or more address mapping algorithms, which are defined in .
The translation model consists of two or more network domains
connected by one or more IP/ICMP translators (XLATs) as shown
in .
The scenarios of the translation model are discussed in
.
This document specifies the
translation algorithms between IPv4 packets and IPv6 packets.
As with
,
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
.
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 ().
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
cannot be mapped to IPv6 multicast addresses
based on the unicast mapping rule
. An example of experiments of the multicast address mapping can be found in
.
An IP/ICMP translator has two possible modes of operation:
stateless and stateful
.
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)
, 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
,
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 .
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 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 .
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
.
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 and
of this document.
The reassembling of fragmented packets in the stateful translator is discussed in
,
since it requires state maintenance in the translator.
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
.
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.
| Header |
| Header | | (if needed) |
+-------------+ +-------------+
| | | Transport- |
~ Data ~ | Layer |
| | | Header |
+-------------+ +-------------+
| |
~ Data ~
| |
+-------------+
]]>
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 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.
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: 6By default, copied from the IP Type Of
Service (TOS) octet. According to , 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 may be applicable.
0 (all zero bits)Total length value from the IPv4 header,
minus the size of the IPv4 header and IPv4 options, if
present.
For ICMPv4 (1), it is changed to ICMPv6 (58);
otherwise, the protocol field MUST be copied from the IPv4 header.
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.
Mapped to an IPv6 address based on the algorithms presented in
.
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
).
Note when translating ICMPv4 Error Messages into ICMPv6, the
"illegal" source address will be translated for the purpose of trouble shooting.
Mapped to an IPv6 address based on the algorithms presented in
.
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: 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.Fragment Header (44).
For ICMPv4 (1), it is changed to ICMPv6 (58); otherwise, the protocol
field MUST be copied from the IPv4 header.
Fragment Offset copied from the
IPv4 header.More Fragments bit copied from the IPv4
header.The low-order 16 bits copied from
the Identification field in the IPv4 header. The high-order 16
bits set to zero.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: 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.Obsoleted
in ICMPv6. Silently drop.Obsoleted
in ICMPv6. Silently drop.Obsoleted
in ICMPv6. Silently drop.Single-hop
message. Silently drop.Single-hop
message. Silently drop.Silently drop.While the Multicast
Listener Discovery (MLD) messages specified in , , and 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.
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:
Set
the Code to 0 (No route to destination).Translate
to an ICMPv6 Parameter Problem (Type 4, Code 1) and
make the Pointer point to the IPv6 Next Header
field.Set the Code to 4
(Port unreachable).
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
,
then the translator MUST use the
plateau values specified in
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 .
Set the Code
to 0 (No route to destination). Note that this error
is unlikely since source routes are not
translated.Set the Code to 0 (No route to
destination).(Communication with Destination
Host Administratively Prohibited): Set the Code to 1 (Communication with destination
administratively prohibited).
Set the Code to 0 (No route
to
destination).Set the Code to 1 (Communication with destination administratively prohibited). Silently drop. Set the Code to 1 (Communication with destination administratively prohibited). Silently drop.Single-hop message.
Silently drop.
Silently drop.Obsoleted in ICMPv6.
Silently drop.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.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:
Set the Code to 0 (Erroneous header field encountered) and update the pointer as defined in .
(If the Original IPv4 Pointer Value is not listed or the Translated IPv6 Pointer
Value is listed as "n/a", silently drop the packet.)
Silently drop.
Set the Code to 0 (Erroneous header field encountered) and update the pointer as defined in .
(If the Original IPv4 Pointer Value is not listed or the Translated IPv6 Pointer
Value is listed as "n/a", silently drop the packet.)
Silently drop.
Silently drop.
If the received ICMPv4 packet contains an ICMPv4 Extension
,
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
,
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.
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.
| IPv6 |
| Header | | Header |
+-------------+ +-------------+
| Partial | | Partial |
| Transport- | | Transport- |
| Layer | | Layer |
| Header | | Header |
+-------------+ +-------------+
]]>
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.
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
.
The translator SHOULD allow an administrator to configure whether the ICMPv4 error messages are sent, rate-limited, or not sent.
If the address translation algorithm is not checksum neutral
(see Section 4.1 of
),
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:
Dropping the packet and generating a system management
event that specifies at least the IP addresses and port numbers of the
packet.
Calculating an UDP 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
,
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 .
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.
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 ,
TCP ,
UDP , and
DCCP
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. | Layer |
|(if present) | | Header |
+-------------+ +-------------+
| Transport | | |
| Layer | ~ Data ~
| Header | | |
+-------------+ +-------------+
| |
~ Data ~
| |
+-------------+
]]>
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:
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
.
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 .
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
.
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.
If there is no IPv6 Fragment Header, the IPv4 header fields are set
as follows: 45 (no IPv4 options)By default, copied from
the IPv6 Traffic Class (all 8 bits). According to , 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 may be
applicable.Payload length value from the IPv6 header,
plus the size of the IPv4 header.
Set according to a Fragment Identification generator at the translator.
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.
All zeros.
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.
The IPv6-Frag (44) header is handled as discussed in
.
ICMPv6 (58) is changed to ICMPv4 (1), and the
payload is translated as discussed in .
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.
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
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 ).
Computed once the IPv4 header has
been created.
Mapped to an IPv4 address based on the algorithms presented in
.
If the translator gets an illegal source address (e.g., ::1,
etc.), the translator SHOULD silently drop the packet.
Mapped to an IPv4 address based on the algorithms presented in
.
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.If the IPv6 packet contains a Fragment Header, the header fields are
set as above with the following exceptions:
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.
Copied from the low-order 16
bits in
the Identification field in the Fragment Header.
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.
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.
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.
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:(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.(Type
130, 131, 132): Single-hop message. Silently drop.(Type 133 through 137):
Single-hop message. Silently drop.Silently
drop.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:
Set the Code
to 1 (Host unreachable).(Communication with destination
administratively prohibited): Set the
Code to 10 (Communication with destination host
administratively prohibited).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.Set the Code to 1
(Host unreachable).Set the Code to 3
(Port unreachable).
Silently drop.
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 .
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. 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:
Set to Type 12, Code 0, and update the pointer as
defined in . (If the Original
IPv6 Pointer Value is not listed or the Translated
IPv4 Pointer Value is listed as "n/a", silently drop the packet.)
Translate this to an
ICMPv4 protocol unreachable (Type 3, Code 2).
Silently drop.
Silently drop.
If the received ICMPv6 packet contains an ICMPv6 Extension
,
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
,
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.
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. | IPv4 |
| Header | | Header |
+-------------+ +-------------+
| Partial | | Partial |
| Transport- | | Transport- |
| Layer | | Layer |
| Header | | Header |
+-------------+ +-------------+
]]>
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.
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
.
The translator SHOULD allow an administrator to configure whether the ICMPv6 error messages are sent, rate-limited, or not sent.
If the address translation algorithm is not checksum neutral
(see Section 4.1 of
),
the
recalculation and updating of the transport-layer headers that
contain pseudo-headers need to be performed. Translators MUST do this for TCP, UDP,
and ICMP.
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 .
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.
The translator MUST support the stateless address mapping algorithm defined in , which is the default behavior. A workflow example is shown in
of this document.
Note that
updates
, 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
also complies with the IPv6 address architecture.
The stateless translator SHOULD support the explicit address mapping algorithm defined in .
The stateless translator SHOULD support
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)).
Implementations may support stateless NAT64 function, e.g., MAP-T Customer
Edge (CE) or MAP-T Border Relay (BR) .
A number of studies
indicate that it not unusual for networks to drop ICMPv6 Packet Too Big error
messages. Such packet drops will result in PMTUD black holes ,
which can only be overcome with Packetization Layer Path MTU Discovery (PLPMTUD)
.
Note to RFC Editor: Please remove this section before publication, as it is only intended for the IESG evaluation.This section summarized the known status of existing and interoperable implementations of the protocol subject of this document, as well as closely related protocols. This is following () and intended to assist the relevant WGs, IESG and IETF as a whole, in the evaluation of the document for the document progress through the standardization process.The description of the implementations does not imply any IETF endorsement and is solely based on public available information, which has not been formally confirmed by specific interoperatbility testing for this document publication; however, it is known to be confirmed by existing commercial working deployments worldwide and without knows interoperability issues.IP/ICMP Translation Algorithm () was originally published in April 2011 as (). Then it was republished in June 2016 as the actual ().() needs to be implemented when implementing other related protocols (just to name a few of the most relevant ones) such as:Stateful NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers ().DNS64: DNS Extensions for Network Address Translation from IPv6 Clients to IPv4 Servers ().Mapping of Address and Port using Translation (MAP-T) ().SIIT-DC: Stateless IP/ICMP Translation for IPv6 Data Center Environments ().Stateless IP/ICMP Translation for IPv6 Internet Data Center Environments (SIIT-DC): Dual Translation Mode ().464XLAT: Combination of Stateful and Stateless Translation ().Follows a list of known implementations by different products/vendors, known to be mature and in production products/networks/services worlwide:6Wind. Implemented in multiple products as part of NAT64 support. https://www.6wind.com/6wind-cg-nat-vrouter-with-nat64/.A10. Implemented in multiple products as part of the NAT64 support. https://www.a10networks.com/products/thunder-cgn/.AlliedTelesis. Implemented in multiple products as part of the NAT64 support.https://www.alliedtelesis.com/sites/default/files/documents/configuration-guides/transitioning_ipv4_to_ipv6_feature_overview_guide.pdf.Amazon. Virtual Private Cloud. https://docs.aws.amazon.com/vpc/latest/userguide/nat-gateway-nat64-dns64.html.Android (Google). Implemented since earlier CLAT implementation in 2012. https://android.googlesource.com/platform/external/android-clat. Implemented also in Google Cloud. https://cloud.google.com/vpc/docs/ipv6-to-ipv4-overview.Apple. Implemented since 2016. https://developer.apple.com/library/archive/documentation/NetworkingInternetWeb/Conceptual/NetworkingOverview/UnderstandingandPreparingfortheIPv6Transition/UnderstandingandPreparingfortheIPv6Transition.html.Arista. Implemented in multiple products as part of the NAT64 implementation. https://www.arista.com/en/support/toi/eos-4-24-0f/14495-map-t-border-relay.Bpfnat. Implemented as part of the CLAT support. https://github.com/apalrd/bpfnat.Broadcom. Implemented in VMWare. https://techdocs.broadcom.com/us/en/vmware-cis/nsx/nsxt-dc/3-1/administration-guide/network-address-translation/configure-an-nsx-nat64.html.Cisco. Implemented in multiple series of products since 2010. https://www.cisco.com/c/en/us/td/docs/routers/ios/config/17-x/ip-addressing/b-ip-addressing/m_iadnat-stateless-nat64.html.CLATD. Implemented in Linux, as part of the CLAT implementation. https://github.com/toreanderson/clatd.Ecdysis. Implemented as part of the NAT64 support. http://ecdysis.viagenie.ca/.F5. Implemented in multiple products as part of the NAT64 support. https://techdocs.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/cgn-implementations-11-6-0/2.html.Fortinet. Implemented in multiple products as part of the NAT64 support. https://docs.fortinet.com/document/fortigate/7.4.6/fortinet-carrier-grade-nat-field-reference-architecture-guide/891965/nat64.Huawei. Implemented in multiple series of products. https://support.huawei.com/enterprise/en/doc/EDOC1100278545/fe351de4/nat64-configuration.Jool. Implmemented since 2014. https://nicmx.github.io/Jool/en/index.html.Juniper. Implemented in multiple series of products as part of the NAT64 support. https://www.juniper.net/documentation/us/en/software/nce/nce-nat64-ipv6-ipv4-depletion/topics/concept/ipv6-nat64-ipv4-depletion-overview.html.Microsoft. Implemented for the CLAT support in 2016. https://techcommunity.microsoft.com/blog/networkingblog/core-network-stack-features-in-the-creators-update-for-windows-10/339676.Nokia. Implemented in multiple products as part of the NAT64 support. https://documentation.nokia.com/html/0_add-h-f/93-0262-HTML/7750_SR_OS_MSISA_Guide/Application-Assurance-NAT.pdf.OpenWRT. Implemented as part of the support of CLAT. https://github.com/openwrt.Palo Alto. Implemented in muliple products as part of the NAT64 support. https://docs.paloaltonetworks.com/ngfw/networking/nat64.Sophos. Implemented in multiple products as part of the NAT64 support. https://news.sophos.com/en-us/2025/04/08/sophos-firewall-v21-5-early-access-is-now-available/.Tayga. Implemented as part of the NAT64 support. https://github.com/openthread/tayga.VPP. Implemented as part of the NAT64 support. https://docs.fd.io/vpp/17.07/nat64_doc.html.ZTE. Implemented in multiple products as part of the NAT64 support. https://www.zte.com.cn/global/product_index/ip_network_en/68e_e/zxr10-6800e/zxr10-6800e.html.Note that even an effort has been done to compile an extensive list (including a relevant URL), there may be many more implementations not publicly known, so this list doesn't pretent to be exclusive, just an indication of a sufficient number of implementations, as required for the evaluation of the current implementation status.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.
addresses these issues.
The IPsec Authentication Header cannot be
used 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
.
A stateless translation workflow example is depicted in the following figure. The
documentation address blocks 2001:db8::/32
,
192.0.2.0/24, and 198.51.100.0/24
are used in this example.
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
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.
The steps by which H6 establishes communication with H4 are:
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
.
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::.
The packet is routed to the IPv6 interface of the XLAT
(since IPv6 routing is configured that way).
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.
The XLAT sends the translated packet out of its IPv4 interface,
and the packet arrives at H4.
H4 node responds by sending a packet with destination
address 192.0.2.33 and source address 198.51.100.2.
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.
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.
The steps by which H4 establishes communication with H6 are:
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 .
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.
The packet is routed to the IPv4 interface of the XLAT
(since IPv4 routing is configured that way).
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.
The XLAT sends the translated packet out its IPv6 interface,
and the packet arrives at H6.
H6 node responds by sending a packet with destination
address 2001:db8:1c6:3364:2:: and source
address 2001:db8:1c0:2:21::.
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.
The translated packet is sent out the IPv4 interface to H4.
The packet exchange between H4 and H6 continues until the session is finished.
This version basically resolved 1 errata (6955).Updated references.Original authors moved to a Contributors section, to avoid issues in auth-48.Added implementation status section.CERNET Center/Tsinghua UniversityRoom 225, Main Building, Tsinghua UniversityBeijing100084China+86 10-62785983congxiao@cernet.edu.cnCERNET Center/Tsinghua UniversityRoom 225, Main Building, Tsinghua UniversityBeijing100084China+86 10-62785983xing@cernet.edu.cnCisco SystemsSanta Barbara93117CaliforniaUnited States+1-408-526-4257fred@cisco.comRedpill LinproVitaminveien 1A0485 OsloNorway+47 959 31 212tore@redpill-linpro.comhttp://www.redpill-linpro.comHuawei
TechnologiesEvaristo Carriego 26441706HaedoProvincia de Buenos AiresArgentina+54 11 4650 8472fgont@si6networks.comhttp://www.si6networks.comDan Gilboa Waizman reported an errata being resolved by this version.Gandhar Gokhale, Wesley Eddy, and Fernando Gont submitted and
handled the errata reports on . Fernando Gont, Will
(Shucheng) Liu, and Tore Anderson provided the security analysis
and the suggestions for updates concerning atomic fragments. In
addition, Tore Anderson and Alberto Leiva provided the proposal of
the Explicit Address Mapping (EAM) algorithm.