<- RFC Index (6101..6200)
RFC 6145
Obsoletes RFC 2765
Obsoleted by RFC 7915
Updated by RFC 6791, RFC 7757
Internet Engineering Task Force (IETF) X. Li
Request for Comments: 6145 C. Bao
Obsoletes: 2765 CERNET Center/Tsinghua
Category: Standards Track University
ISSN: 2070-1721 F. Baker
Cisco Systems
April 2011
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 2765.
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.
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/rfc6145.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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.
Li, et al. Standards Track [Page 1]
RFC 6145 IPv4/IPv6 Translation April 2011
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction and Motivation . . . . . . . . . . . . . . . . . 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 2765 . . . . . . . . . . . . . . . . . . . . 5
3. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Translating from IPv4 to IPv6 . . . . . . . . . . . . . . . . 6
4.1. Translating IPv4 Headers into IPv6 Headers . . . . . . . . 7
4.2. Translating ICMPv4 Headers into ICMPv6 Headers . . . . . . 10
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 . . . . . . 20
5.3. Translating ICMPv6 Error Messages into ICMPv4 . . . . . . 22
5.4. Generation of ICMPv6 Error Messages . . . . . . . . . . . 23
5.5. Transport-Layer Header Translation . . . . . . . . . . . . 24
5.6. Knowing When to Translate . . . . . . . . . . . . . . . . 24
6. Special Considerations for ICMPv6 Packet Too Big . . . . . . . 24
7. Security Considerations . . . . . . . . . . . . . . . . . . . 25
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
9.1. Normative References . . . . . . . . . . . . . . . . . . . 26
9.2. Informative References . . . . . . . . . . . . . . . . . . 28
Appendix A. Stateless Translation Workflow Example . . . . . . . 30
A.1. H6 Establishes Communication with H4 . . . . . . . . . . . 30
A.2. H4 Establishes Communication with H6 . . . . . . . . . . . 32
Li, et al. Standards Track [Page 2]
RFC 6145 IPv4/IPv6 Translation April 2011
1. Introduction and Motivation
This document is a product of the 2008-2010 effort to define a
replacement for NAT-PT [RFC2766] (which was changed to Historic
status when [RFC4966] was published in 2007). It is directly derived
from Erik Nordmark's "Stateless IP/ICMP Translation Algorithm (SIIT)"
[RFC2765], which provides stateless translation between IPv4
[RFC791] and IPv6 [RFC2460], and between ICMPv4 [RFC792] and ICMPv6
[RFC4443]. This document obsoletes RFC 2765 [RFC2765]. The changes
from RFC 2765 [RFC2765] are listed in Section 2.
Readers of this document are expected to have read and understood the
framework described in [RFC6144]. Implementations of this IPv4/IPv6
translation specification MUST also support the address translation
algorithms in [RFC6052]. Implementations MAY also support stateful
translation [RFC6146].
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.
--------- ---------
// \\ // \\
/ +----+ \
| |XLAT| | XLAT: IP/ICMP
| IPv4 +----+ IPv6 | Translator
| Domain | | Domain |
| | | |
\ | | /
\\ // \\ //
-------- ---------
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 [RFC2765], 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.
Li, et al. Standards Track [Page 3]
RFC 6145 IPv4/IPv6 Translation April 2011
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 in 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 the IP/ICMP
translator.
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].
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 and hence are forced to have their
communications translated.
In the stateless mode, 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. Note
that IPv4-translatable addresses are a subset of IPv4-converted
addresses. In general, there is no need to concern oneself with
translation tables, as the IPv4 and IPv6 counterparts are
algorithmically related.
In the stateful mode, a specific IPv6 address range will represent
IPv4 systems (IPv4-converted addresses), but the IPv6 systems may use
any IPv6 addresses [RFC4291] except in that range. In this case, a
translation table is required to bind the IPv6 systems' addresses to
the IPv4 addresses maintained in the translator.
The address translation mechanisms for the stateless and the stateful
translations are defined in [RFC6052].
Li, et al. Standards Track [Page 4]
RFC 6145 IPv4/IPv6 Translation April 2011
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 [RFC879]. 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 2765
The changes from RFC 2765 are the following:
1. Redescribing the network model to map to present and projected
usage. The scenarios, applicability, and limitations originally
presented in RFC 2765 [RFC2765] are moved to the framework
document [RFC6144].
2. Moving the address format to the address format document
[RFC6052], to coordinate with other documents on the topic.
3. Describing the header translation for the stateless and stateful
operations. The details of the session database and mapping
table handling of the stateful translation is in the stateful
translation document [RFC6146].
4. Having refined the header translation, fragmentation handling,
ICMP translation and ICMP error translation in the IPv4-to-IPv6
direction, as well as the IPv6-to-IPv4 direction.
5. Adding more discussion on transport-layer header translation.
6. Adding Section 5.1.1 for "IPv6 Fragment Processing".
7. Adding Section 6 for "Special Considerations for ICMPv6 Packet
Too Big".
8. Updating Section 7 for "Security Considerations".
9. Adding Appendix A "Stateless translation workflow example".
Li, et al. Standards Track [Page 5]
RFC 6145 IPv4/IPv6 Translation April 2011
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.
+-------------+ +-------------+
| IPv4 | | IPv6 |
| Header | | Header |
+-------------+ +-------------+
| Transport- | | Fragment |
| Layer | ===> | Header |
| Header | | (if needed) |
+-------------+ +-------------+
| | | Transport- |
~ Data ~ | Layer |
| | | Header |
+-------------+ +-------------+
| |
~ Data ~
| |
+-------------+
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
Li, et al. Standards Track [Page 6]
RFC 6145 IPv4/IPv6 Translation April 2011
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 MAY 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 SHOULD
always include an IPv6 Fragment Header to indicate that the sender
allows fragmentation. The translator MAY provide a configuration
function that allows the translator not to 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. In addition, the rules in Section 4.1 use
the presence of an IPv6 Fragment Header to indicate that the sender
might not be using path MTU discovery (i.e., the packet should not
have the DF flag set should it later be translated back to IPv4).
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 1280 bytes, the packet SHOULD be fragmented so the
resulting IPv6 packet (with Fragment Header added to each fragment)
will be less than or equal to 1280 bytes. For example, if the packet
Li, et al. Standards Track [Page 7]
RFC 6145 IPv4/IPv6 Translation April 2011
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 MAY provide a configuration
function for the network administrator to adjust the threshold of the
minimum IPv6 MTU to a value greater than 1280-byte 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.
If the DF bit is set and the packet is not a fragment (i.e., the More
Fragments (MF) flag is not set and the Fragment Offset is equal to
zero), then the translator SHOULD NOT add a Fragment Header to the
resulting packet. 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.
Li, et al. Standards Track [Page 8]
RFC 6145 IPv4/IPv6 Translation April 2011
Source Address: The IPv4-converted address derived from the IPv4
source address per [RFC6052], Section 2.3.
If the translator gets an illegal source address (e.g., 0.0.0.0,
127.0.0.1, etc.), the translator SHOULD silently drop the packet
(as discussed in Section 5.3.7 of [RFC1812]).
Destination Address: In the stateless mode, which is to say that if
the IPv4 destination address is within a range of configured IPv4
stateless translation prefix, the IPv6 destination address is the
IPv4-translatable address derived from the IPv4 destination
address per [RFC6052], Section 2.3. A workflow example of
stateless translation is shown in Appendix A of this document.
In the stateful mode (which is to say that if the IPv4 destination
address is not within the range of any configured IPv4 stateless
translation prefix), the IPv6 destination address and
corresponding transport-layer destination port are derived from
the Binding Information Bases (BIBs) reflecting current session
state in the translator as described in [RFC6146].
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 DF bit is not set or
the packet is a fragment), 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.
Li, et al. Standards Track [Page 9]
RFC 6145 IPv4/IPv6 Translation April 2011
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 [RFC2710] [RFC3590] [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
Li, et al. Standards Track [Page 10]
RFC 6145 IPv4/IPv6 Translation April 2011
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, i.e.,
minimum(advertised MTU+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.)
See also the requirements in Section 6.
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).
Li, et al. Standards Track [Page 11]
RFC 6145 IPv4/IPv6 Translation April 2011
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.
Li, et al. Standards Track [Page 12]
RFC 6145 IPv4/IPv6 Translation April 2011
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 those 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. If the translation of this "packet in error" changes the
Li, et al. Standards Track [Page 13]
RFC 6145 IPv4/IPv6 Translation April 2011
length of the datagram, the Total Length field in the outer IPv6
header MUST be updated.
+-------------+ +-------------+
| IPv4 | | IPv6 |
| Header | | Header |
+-------------+ +-------------+
| ICMPv4 | | ICMPv6 |
| Header | | Header |
+-------------+ +-------------+
| IPv4 | ===> | IPv6 |
| Header | | Header |
+-------------+ +-------------+
| Partial | | Partial |
| Transport- | | Transport- |
| Layer | | Layer |
| Header | | Header |
+-------------+ +-------------+
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 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).
Li, et al. Standards Track [Page 14]
RFC 6145 IPv4/IPv6 Translation April 2011
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
3.1.
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 "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 [RFC793], UDP [RFC768], 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
Li, et al. Standards Track [Page 15]
RFC 6145 IPv4/IPv6 Translation April 2011
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.
+-------------+ +-------------+
| IPv6 | | IPv4 |
| Header | | Header |
+-------------+ +-------------+
| Fragment | | Transport |
| Header | ===> | Layer |
|(if present) | | Header |
+-------------+ +-------------+
| Transport | | |
| Layer | ~ Data ~
| Header | | |
+-------------+ +-------------+
| |
~ Data ~
| |
+-------------+
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, including an ICMP Packet Too Big that
indicates the MTU is less than the IPv6 minimum MTU. This
requirement is described in Section 5 of [RFC2460] (for IPv6's
1280-octet minimum MTU) and Section 5 of [RFC1883] (for IPv6's
previous 576-octet minimum MTU).
In an environment where an ICMPv4 Packet Too Big message is
translated to an ICMPv6 Packet Too Big message, and the ICMPv6 Packet
Too Big message is successfully delivered to and correctly processed
by the IPv6 hosts (e.g., a network owned/operated by the same entity
that owns/operates the translator), the translator can rely on IPv6
hosts sending subsequent packets to the same IPv6 destination with
IPv6 Fragment Headers. In such an environment, when the translator
receives an IPv6 packet with a Fragment Header, the translator SHOULD
generate the IPv4 packet with a cleared Don't Fragment bit, and with
its identification value from the IPv6 Fragment Header, for all of
the IPv6 fragments (MF=0 or MF=1).
Li, et al. Standards Track [Page 16]
RFC 6145 IPv4/IPv6 Translation April 2011
In an environment where an ICMPv4 Packet Too Big message is filtered
(by a network firewall or by the host itself) or not correctly
processed by the IPv6 hosts, the IPv6 host will never generate an
IPv6 packet with the IPv6 Fragment Header. In such an environment,
the translator SHOULD set the IPv4 Don't Fragment bit. While setting
the Don't Fragment bit may create PMTUD black holes [RFC2923] if
there are IPv4 links smaller than 1260 octets, this is considered
safer than causing IPv4 reassembly errors [RFC4963].
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: All zero. In order to avoid black holes caused by
ICMPv4 filtering or non-[RFC2460]-compatible IPv6 hosts (a
workaround is discussed in Section 6), the translator MAY provide
a function to generate the identification value if the packet size
is greater than 88 bytes and less than or equal to 1280 bytes.
Li, et al. Standards Track [Page 17]
RFC 6145 IPv4/IPv6 Translation April 2011
The translator SHOULD provide a method for operators to enable or
disable this function.
Flags: The More Fragments flag is set to zero. The Don't Fragment
(DF) flag is set to one. In order to avoid black holes caused by
ICMPv4 filtering or non-[RFC2460]-compatible IPv6 hosts (a
workaround is discussed in Section 6), the translator MAY provide
a function as follows. If the packet size is greater than 88
bytes and less than or equal to 1280 bytes, it sets the DF flag to
zero; otherwise, it sets the DF flag to one. The translator
SHOULD provide a method for operators to enable or disable this
function.
Fragment Offset: All zeros.
Time to Live: Time to Live is derived from Hop Limit value in 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: In the stateless mode (which is to say that if the
IPv6 source address is within the range of a configured IPv6
translation prefix), the IPv4 source address is derived from the
IPv6 source address per [RFC6052], Section 2.3. Note that the
Li, et al. Standards Track [Page 18]
RFC 6145 IPv4/IPv6 Translation April 2011
original IPv6 source address is an IPv4-translatable address. A
workflow example of stateless translation is shown in Appendix A
of this document. If the translator only supports stateless mode
and if the IPv6 source address is not within the range of
configured IPv6 prefix(es), the translator SHOULD drop the packet
and respond with an ICMPv6 "Destination Unreachable, Source
address failed ingress/egress policy" (Type 1, Code 5).
In the stateful mode, which is to say that if the IPv6 source
address is not within the range of any configured IPv6 stateless
translation prefix, the IPv4 source address and transport-layer
source port corresponding to the IPv4-related IPv6 source address
and source port are derived from the Binding Information Bases
(BIBs) as described in [RFC6146].
In stateless and stateful modes, if the translator gets an illegal
source address (e.g., ::1, etc.), the translator SHOULD silently
drop the packet.
Destination Address: The IPv4 destination address is derived from
the IPv6 destination address of the datagram being translated per
[RFC6052], Section 2.3. Note that the original IPv6 destination
address is an IPv4-converted address.
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: Payload length value from IPv6 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: Copied from the Fragment Offset field of the IPv6
Fragment Header.
Li, et al. Standards Track [Page 19]
RFC 6145 IPv4/IPv6 Translation April 2011
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 a translated packet with DF set to 1 will be larger than the MTU
of the next-hop interface, then the translator MUST drop the packet
and send the ICMPv6 Packet Too Big (Type 2, Code 0) error message to
the IPv6 host with an adjusted MTU in the ICMPv6 message.
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 also MUST be translated.
Note that the IPv6 addresses in the IPv6 header may not be IPv4-
translatable addresses and there will be no corresponding IPv4
addresses representing this IPv6 address. In this case, the
translator can do stateful translation. A mechanism by which the
translator can instead do stateless translation of this address is
left for future work.
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.
Li, et al. Standards Track [Page 20]
RFC 6145 IPv4/IPv6 Translation April 2011
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(advertised MTU-20,
MTU_of_IPv4_nexthop, (MTU_of_IPv6_nexthop)-20).
See also the requirements in Section 6.
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.
Li, et al. Standards Track [Page 21]
RFC 6145 IPv4/IPv6 Translation April 2011
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. The translation of this "packet in error" is likely to
Li, et al. Standards Track [Page 22]
RFC 6145 IPv4/IPv6 Translation April 2011
change the length of the datagram; thus, the Total Length field in
the outer IPv4 header MUST be updated.
+-------------+ +-------------+
| IPv6 | | IPv4 |
| Header | | Header |
+-------------+ +-------------+
| ICMPv6 | | ICMPv4 |
| Header | | Header |
+-------------+ +-------------+
| IPv6 | ===> | IPv4 |
| Header | | Header |
+-------------+ +-------------+
| Partial | | Partial |
| Transport- | | Transport- |
| Layer | | Layer |
| Header | | Header |
+-------------+ +-------------+
Figure 7: IPv6-to-IPv4 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. Note that the IPv6 addresses in the IPv6
header may not be IPv4-translatable addresses, and there will be no
corresponding IPv4 addresses. In this case, the translator can do
stateful translation. A mechanism by which the translator can
instead do stateless translation is left for future work.
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.
If the ICMPv6 error message is being sent because the IPv6 source
address is not an IPv4-translatable address and the translator is
stateless, the ICMPv6 message (if sent) MUST have Type 1 and Code 5
(Source address failed ingress/egress policy). In other cases, 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.
Li, et al. Standards Track [Page 23]
RFC 6145 IPv4/IPv6 Translation April 2011
5.5. 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, 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
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. Special Considerations for ICMPv6 Packet Too Big
Two recent studies analyzed the behavior of IPv6-capable web servers
on the Internet and found that approximately 95% responded as
expected to an IPv6 Packet Too Big that indicated MTU = 1280, but
only 43% responded as expected to an IPv6 Packet Too Big that
indicated an MTU < 1280. It is believed that firewalls violating
Section 4.3.1 of [RFC4890] are at fault. Both failures (the 5% wrong
response when MTU = 1280 and the 57% wrong response when MTU < 1280)
will cause PMTUD black holes [RFC2923]. Unfortunately, the
translator cannot improve the failure rate of the first case (MTU =
1280), but the translator can improve the failure rate of the second
case (MTU < 1280). There are two approaches to resolving the problem
with sending ICMPv6 messages indicating an MTU < 1280. It SHOULD be
possible to configure a translator for either of the two approaches.
The first approach is to constrain the deployment of the IPv6/IPv4
translator by observing that four of the scenarios intended for
stateless IPv6/IPv4 translators do not have IPv6 hosts on the
Internet (Scenarios 1, 2, 5, and 6 described in [RFC6144], which
refer to "An IPv6 network"). In these scenarios, IPv6 hosts, IPv6-
host-based firewalls, and IPv6 network firewalls can be administered
in compliance with Section 4.3.1 of [RFC4890] and therefore avoid the
problem witnessed with IPv6 hosts on the Internet.
Li, et al. Standards Track [Page 24]
RFC 6145 IPv4/IPv6 Translation April 2011
The second approach is necessary if the translator has IPv6 hosts,
IPv6-host-based firewalls, or IPv6 network firewalls that do not (or
cannot) comply with Section 5 of [RFC2460] -- such as IPv6 hosts on
the Internet. This approach requires the translator to do the
following:
1. In the IPv4-to-IPv6 direction: if the MTU value of ICMPv4 Packet
Too Big (PTB) messages is less than 1280, change it to 1280.
This is intended to cause the IPv6 host and IPv6 firewall to
process the ICMP PTB message and generate subsequent packets to
this destination with an IPv6 Fragment Header.
Note: Based on recent studies, this is effective for 95% of IPv6
hosts on the Internet.
2. In the IPv6-to-IPv4 direction:
A. If there is a Fragment Header in the IPv6 packet, the last 16
bits of its value MUST be used for the IPv4 identification
value.
B. If there is no Fragment Header in the IPv6 packet:
a. If the packet is less than or equal to 1280 bytes:
- The translator SHOULD set DF to 0 and generate an IPv4
identification value.
- To avoid the problems described in [RFC4963], it is
RECOMMENDED that the translator maintain 3-tuple state
for generating the IPv4 identification value.
b. If the packet is greater than 1280 bytes, the translator
SHOULD set the IPv4 DF bit to 1.
7. 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.
Li, et al. Standards Track [Page 25]
RFC 6145 IPv4/IPv6 Translation April 2011
As with network address translation of IPv4 to IPv4, the IPsec
Authentication Header [RFC4302] cannot be used across an IPv6-to-IPv4
translator.
As with 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].
8. Acknowledgements
This is under development by a large group of people. Those who have
posted to the list during the discussion include Alexey Melnikov,
Andrew Sullivan, Andrew Yourtchenko, Brian Carpenter, Dan Wing, Dave
Thaler, David Harrington, Ed Jankiewicz, Hiroshi Miyata, Iljitsch van
Beijnum, Jari Arkko, Jerry Huang, John Schnizlein, Jouni Korhonen,
Kentaro Ebisawa, Kevin Yin, Magnus Westerlund, Marcelo Bagnulo Braun,
Margaret Wasserman, Masahito Endo, Phil Roberts, Philip Matthews,
Reinaldo Penno, Remi Denis-Courmont, Remi Despres, Sean Turner,
Senthil Sivakumar, Simon Perreault, Stewart Bryant, Tim Polk, Tero
Kivinen, and Zen Cao.
9. References
9.1. Normative References
[RFC768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[RFC793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995.
[RFC1883] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 1883, December 1995.
Li, et al. Standards Track [Page 26]
RFC 6145 IPv4/IPv6 Translation April 2011
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm
(SIIT)", RFC 2765, February 2000.
[RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets",
RFC 3948, January 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC4884] Bonica, R., Gan, D., Tappan, D., and C. Pignataro,
"Extended ICMP to Support Multi-Part Messages", RFC 4884,
April 2007.
[RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, October 2008.
[RFC5771] Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
IPv4 Multicast Address Assignments", BCP 51, RFC 5771,
March 2010.
[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
October 2010.
[RFC6146] Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, April 2011.
Li, et al. Standards Track [Page 27]
RFC 6145 IPv4/IPv6 Translation April 2011
9.2. Informative References
[RFC879] Postel, J., "TCP maximum segment size and related topics",
RFC 879, November 1983.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[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,
December 1998.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[RFC2710] Deering, S., Fenner, W., and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710,
October 1999.
[RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address
Translation - Protocol Translation (NAT-PT)", RFC 2766,
February 2000.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, September 2000.
[RFC3307] Haberman, B., "Allocation Guidelines for IPv6 Multicast
Addresses", RFC 3307, August 2002.
[RFC3590] Haberman, B., "Source Address Selection for the Multicast
Listener Discovery (MLD) Protocol", RFC 3590,
September 2003.
[RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
[RFC3849] Huston, G., Lord, A., and P. Smith, "IPv6 Address Prefix
Reserved for Documentation", RFC 3849, July 2004.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering
ICMPv6 Messages in Firewalls", RFC 4890, May 2007.
Li, et al. Standards Track [Page 28]
RFC 6145 IPv4/IPv6 Translation April 2011
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963, July 2007.
[RFC4966] Aoun, C. and E. Davies, "Reasons to Move the Network
Address Translator - Protocol Translator (NAT-PT) to
Historic Status", RFC 4966, July 2007.
[RFC5737] Arkko, J., Cotton, M., and L. Vegoda, "IPv4 Address Blocks
Reserved for Documentation", RFC 5737, January 2010.
[RFC6144] Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
IPv4/IPv6 Translation", RFC 6144, April 2011.
Li, et al. Standards Track [Page 29]
RFC 6145 IPv4/IPv6 Translation April 2011
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
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::.
Li, et al. Standards Track [Page 30]
RFC 6145 IPv4/IPv6 Translation April 2011
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.
Li, et al. Standards Track [Page 31]
RFC 6145 IPv4/IPv6 Translation April 2011
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.
Li, et al. Standards Track [Page 32]
RFC 6145 IPv4/IPv6 Translation April 2011
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.
Authors' Addresses
Xing Li
CERNET Center/Tsinghua University
Room 225, Main Building, Tsinghua University
Beijing, 100084
China
Phone: +86 10-62785983
EMail: xing@cernet.edu.cn
Congxiao Bao
CERNET Center/Tsinghua University
Room 225, Main Building, Tsinghua University
Beijing, 100084
China
Phone: +86 10-62785983
EMail: congxiao@cernet.edu.cn
Fred Baker
Cisco Systems
Santa Barbara, California 93117
USA
Phone: +1-408-526-4257
EMail: fred@cisco.com
Li, et al. Standards Track [Page 33]