behave X. Li
Internet-Draft C. Bao
Obsoletes: 2765 (if approved) CERNET Center/Tsinghua University
Intended status: Standards Track F. Baker
Expires: October 5, 2010 Cisco Systems
April 3, 2010
IP/ICMP Translation Algorithm
draft-ietf-behave-v6v4-xlate-16
Abstract
This document forms a replacement of the Stateless IP/ICMP
Translation Algorithm (SIIT) described in RFC 2765. The algorithm
translates between IPv4 and IPv6 packet headers (including ICMP
headers).
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 5, 2010.
Copyright Notice
Copyright (c) 2010 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
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
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described in the Simplified BSD License.
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
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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. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Translating from IPv4 to IPv6 . . . . . . . . . . . . . . . . 5
3.1. Translating IPv4 Headers into IPv6 Headers . . . . . . . . 7
3.2. Translating ICMPv4 Headers into ICMPv6 Headers . . . . . . 9
3.3. Translating ICMPv4 Error Messages into ICMPv6 . . . . . . 13
3.4. Translator Sending ICMPv4 Error Message . . . . . . . . . 14
3.5. Transport-layer Header Translation . . . . . . . . . . . . 14
3.6. Knowing When to Translate . . . . . . . . . . . . . . . . 14
4. Translating from IPv6 to IPv4 . . . . . . . . . . . . . . . . 14
4.1. Translating IPv6 Headers into IPv4 Headers . . . . . . . . 17
4.2. Translating ICMPv6 Headers into ICMPv4 Headers . . . . . . 19
4.3. Translating ICMPv6 Error Messages into ICMPv4 . . . . . . 22
4.4. Translator Sending ICMPv6 Error Message . . . . . . . . . 23
4.5. Transport-layer Header Translation . . . . . . . . . . . . 23
4.6. Knowing When to Translate . . . . . . . . . . . . . . . . 24
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
6. Security Considerations . . . . . . . . . . . . . . . . . . . 24
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
8. Appendix: Stateless translation workflow example . . . . . . . 25
8.1. H6 establishes communication with H4 . . . . . . . . . . . 26
8.2. H4 establishes communication with H6 . . . . . . . . . . . 27
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
9.1. Normative References . . . . . . . . . . . . . . . . . . . 28
9.2. Informative References . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30
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1. Introduction and Motivation
This document is a product of the 2008-2010 effort to define a
replacement for NAT-PT [RFC2766]. It is directly derivative from
Erik Nordmark's "Stateless IP/ICMP Translation Algorithm (SIIT)"
[RFC2765], which provides stateless translation between IPv4
[RFC0791] and IPv6 [RFC2460], and between ICMPv4 [RFC0792] and ICMPv6
[RFC4443].
Readers of this document are expected to have read and understood the
framework described in [I-D.ietf-behave-v6v4-framework].
Implementations of this IPv4/IPv6 translation specification MUST also
support the address translation algorithms in
[I-D.ietf-behave-address-format]. Implementations MAY also support
stateful translation [I-D.ietf-behave-v6v4-xlate-stateful].
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: IPv6/IPv4
| IPv4 +----+ IPv6 | Translator
| Domain | | Domain |
| | | |
\ | | /
\\ // \\ //
-------- ---------
Figure 1: IPv4-IPv6 Translation Model
The scenarios of the translation model are discussed in
[I-D.ietf-behave-v6v4-framework].
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 fragmentation header.
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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 will not be translated by the IP/ICMP translator.
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
[I-D.ietf-behave-address-format].
Translator SHOULD make sure that the packets belonging to the same
flow leave the translator in the same order in which they arrived.
1.3. Stateless vs. Stateful Mode
An IP/ICMP translator has two possible modes of operation: stateless
and stateful [I-D.ietf-behave-v6v4-framework]. 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 is 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 [I-D.ietf-behave-address-format].
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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 MSS
option [RFC0879]. 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 3 and 4 of this document. The reassembling of
fragmented packets in the stateful translator is discussed in
[I-D.ietf-behave-v6v4-xlate-stateful], since it requires state
maintenance in the translator.
2. 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].
3. 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. Since the ICMPv6
[RFC4443], TCP [RFC0793], UDP [RFC0768] and DCCP [RFC4340] headers
contain checksums that cover IP header information, if the address
mapping algorithm is not checksum-neutral, the checksum MUST be
evaluated before translation and the ICMPv6 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 IPv6 destination address.
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+-------------+ +-------------+
| 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 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 Don't Fragment (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 so that it fits in 1280-byte IPv6 packets, since that is the
minimum IPv6 MTU. Also, when the IPv4 sender does not set the DF bit
the translator MUST always include an IPv6 fragment header to
indicate that the sender allows fragmentation.
The rules in section 3.1 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 3.1 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).
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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.
3.1. 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 MUST 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
is fragmented prior to the translation, the IPv4 packets must 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 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 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 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 IPv4 header, minus the size
of the IPv4 header and IPv4 options, if present.
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Next Header: For ICMPv4 (1) changed to ICMPv6 (58), otherwise
protocol field MUST be copied from IPv4 header.
Hop Limit: The hop limit is derived from the TTL value in the IPv4
header. Since the translator is a router, as part of forwarding
the packet it needs to decrement either the IPv4 TTL (before the
translation) or the IPv6 Hop Limit (after the translation). As
part of decrementing the TTL or Hop Limit the translator (as any
router) MUST check for zero and send the ICMPv4 "TTL Exceeded" or
ICMPv6 "Hop Limit Exceeded" error.
Source Address: The IPv4-converted address derived from the IPv4
source address per [I-D.ietf-behave-address-format] section 2.1.
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 [I-D.ietf-behave-address-format] section 2.1. A
workflow example of stateless translation is shown in the Appendix
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 [I-D.ietf-behave-v6v4-xlate-stateful].
If any IPv4 options are present in the IPv4 packet, the IPv4 options
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:
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IPv6 fields:
Payload Length: Total length value from 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) changed to ICMPv6 (58), otherwise
protocol field MUST be copied from IPv4 header.
Fragment Offset: Fragment Offset copied from the IPv4 header.
M flag: More Fragments bit copied from the IPv4 header.
Identification: The low-order 16 bits copied from the
Identification field in the IPv4 header. The high-order 16
bits set to zero.
3.2. 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 value 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.
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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 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 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 field as
described below, set the Type field 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 field as follows:
Code 0, 1 (Net, host unreachable): Set Code value to 0 (no
route to destination).
Code 2 (Protocol unreachable): Translate to an ICMPv6
Parameter Problem (Type 4, Code value 1) and make the
Pointer point to the IPv6 Next Header field.
Code 3 (Port unreachable): Set Code value to 4 (port
unreachable).
Code 4 (Fragmentation needed and DF set): Translate to an
ICMPv6 Packet Too Big message (Type 2) with Code value
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
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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.) The translator
MUST provide a configuration function to adjust the MTU
from a value smaller than 1280 to 1280, which is required
for the workaround ([RFC2460] related issue) discussed in
Section 4.
Code 5 (Source route failed): Set Code value to 0 (No route
to destination). Note that this error is unlikely since
source routes are not translated.
Code 6, 7, 8: Set Code value to 0 (No route to
destination).
Code 9, 10 (Communication with destination host
administratively prohibited): Set Code value to 1
(Communication with destination administratively
prohibited)
Code 11, 12: Set Code value to 0 (no route to destination).
Code 13 (Communication Administratively Prohibited): Set
Code value to 1 (Communication with destination
administratively prohibited).
Code 14 (Host Precedence Violation): Silently drop.
Code 15 (Precedence cutoff in effect): Set Code value 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 field to 3, and adjust
the ICMP checksum both to take the type change into account
and to include the ICMPv6 pseudo-header. The Code field is
unchanged.
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Parameter Problem (Type 12): Set the Type field 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 field as follows:
Code 0 (Pointer indicates the error): Set the Code value to
0 (Erroneous header field encountered) and update the
pointer as defined in Figure 3 (If the Original IPv4
Pointer Value is not listed or the Translated IPv6
Pointer Value is listed as "n/a", silently drop the
packet).
Code 1 (Missing a required option): Silently drop
Code 2 (Bad length): Set the Code value 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
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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.
3.3. Translating ICMPv4 Error Messages into ICMPv6
There are some differences between the ICMPv4 and the ICMPv6 error
message formats as detailed above. In addition, the ICMP error
messages contain the packet in error, which MUST be translated just
like a normal IP 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.
+-------------+ +-------------+
| 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.
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3.4. Translator Sending 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 value of 3 (Destination
Unreachable) and a Code value of 13 (Communication Administratively
Prohibited), unless otherwise specified in this document or in
[I-D.ietf-behave-v6v4-xlate-stateful]. The translator SHOULD allow
an administrator to configure whether the ICMPv4 error messages are
sent, rate-limited, or not sent.
3.5. Transport-layer Header Translation
If the address translation algorithm is not checksum neutral, the
recalculation and updating of the transport-layer headers which
contain pseudo headers (e.g. of TCP, UDP and DCCP) MUST be performed.
When a translator receives an unfragmented UDP IPv4 packet and the
checksum field is zero, the translator SHOULD compute the missing UDP
checksum as part of translating the packet. Also, the translator
SHOULD maintain a counter of how many UDP checksums are generated in
this manner.
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 specifying at least the IP addresses and port numbers in the
packet. When it receives fragments other than the first, it SHOULD
silently drop the packet, since there is no port information to log.
For stateful translator, the handling of fragmented UDP IPv4 packets
with a zero checksum is discussed in
[I-D.ietf-behave-v6v4-xlate-stateful] section 3.1.
3.6. Knowing When to Translate
If the IP/ICMP translator also provides 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.
4. Translating from IPv6 to IPv4
When an IP/ICMP translator receives an IPv6 datagram addressed to a
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destination towards the IPv4 domain, it translates the IPv6 header of
the received IPv6 packet into an IPv4 header. The original IPv6
header on the packet is removed and replaced by an IPv4 header.
Since the ICMPv6 [RFC4443], TCP [RFC0793], UDP [RFC0768] and DCCP
[RFC4340] headers contain checksums that cover the IP header, if the
address mapping algorithm is not checksum-neutral, the checksum MUST
be evaluated before translation and the ICMP and transport-layer
headers MUST be updated. The data portion of the packet is left
unchanged. The IP/ICMP translator then forwards the packet based on
the IPv4 destination address.
+-------------+ +-------------+
| 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 area 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. Thus, unless there were
special measures, it would not be possible to do end-to-end path MTU
discovery when the path includes a translator, since the IPv6 node
might receive ICMPv6 "Packet Too Big" messages originated by an IPv4
router that report an MTU less than 1280. However, [RFC2460] section
5 requires that IPv6 nodes handle such an ICMPv6 "Packet Too Big"
message by reducing the path MTU to 1280 and including an IPv6
fragment header with each packet. In this case, the translator
SHOULD set DF to 0 and take the identification value from the IPv6
fragment header when a fragmentation header with (MF=0; Offset=0) is
present or set DF to 1 otherwise. This allows end-to-end path MTU
discovery across the translator as long as the path MTU is 1280 bytes
or greater. When the path MTU drops below the 1280 limit, the IPv6
sender will originate 1280-byte packets that will be fragmented by
IPv4 routers along the path after being translated to IPv4.
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The drawback with this scheme is that it is not possible to use PMTU
discovery to do optimal UDP fragmentation (as opposed to completely
avoiding fragmentation) at the sender, since the presence of an IPv6
Fragment header is interpreted that it is okay to fragment the packet
on the IPv4 side. Thus if a UDP application wants to send large
packets independent of the PMTU, the sender will only be able to
determine the path MTU on the IPv6 side of the translator. If the
path MTU on the IPv4 side of the translator is smaller, then the IPv6
sender will not receive any ICMPv6 "Too Big" errors and cannot adjust
the size fragments it is sending.
On the other hand, a recent study indicates that only 43.46% of IPv6-
capable web servers include an IPv6 fragmentation header in their
respond packets after they were sent an ICMPv6 "Packet Too Big"
message specifying an MTU<1280 bytes. A workaround to this problem
(ICMPv6 "Packet Too Big" message with MTU<1280) is that (1) in the
IPv4 to IPv6 direction, the translator can adjust MTU in "Packet Too
Big" message from a value smaller than 1280 to 1280; (2) in the IPv6
to IPv4 direction, if there is no fragmentation header in the IPv6
packet, the translator SHOULD set DF to 0 for the packets equal to or
smaller than 1280 bytes and set DF to 1 for packets larger than 1280
bytes. In addition, the translator SHOULD take the identification
value from the IPv6 fragmentation header if present or generate the
identification value otherwise. This avoids the introduction of the
path MTU discovery black hole. Note that translator generating the
IPv4 identification value is tricky in stateless mode. The Internet
Protocol standard [RFC0791] specifies:
"The choice of the Identifier for a datagram is based on the need
to provide a way to uniquely identify the fragments of a
particular datagram. The protocol module assembling fragments
judges fragments to belong to the same datagram if they have the
same source, destination, protocol, and Identifier. Thus, the
sender must choose the Identifier to be unique for this source,
destination pair and protocol for the time the datagram (or any
fragment of it) could be alive in the Internet."
Therefore, the translator may require states per three tuple IPv4
identification field. However, this does not prevent the deployment
of the stateless translator, since as discussed in
[I-D.ietf-behave-v6v4-framework], the stateless translation can be
used in scenarios 1, 2, 5 and 6. All of these scenarios involve "An
IPv6 network" which are managed networks and network firewall, host
firewall or host misbehavior can be controlled. In such a controlled
environment, it can be assured that hosts and firewalls properly
process ICMPv6 messages as described in Section 5 of [RFC2460].
The translator does not translate IPv6 routing headers, hop-by-hop
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extension headers, destination options headers, source routing
headers, or any layer 4 protocol (e.g., TCP header, IPsec
authentication header (AH) or encapsulating security payload (ESP)
header). However, the translator needs to traverse the IPv6 'next
header' chain and copy the next header value (which contains the
transport protocol number) in the last known 'next header' to the
protocol field in the IPv4 header. This means that the translator
MUST forward all protocols to avoid black holing. Some protocols are
known to fail when translated (e.g., IPsec AH) and will fail at the
receiver.
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.
4.1. 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 IPv6 header, plus the size
of the IPv4 header.
Identification: If the packet size is equal to or smaller than 1280
bytes and greater than 88 bytes, generate the identification
value. If the packet size is greater than 1280 bytes or smaller
than 88 bytes, set the Identification field to all zeros
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Flags: The More Fragments (MF) flag is set to zero. If the packet
size is equal to or smaller than 1280 bytes and greater than 88
bytes, the Don't Fragments (DF) flag is set to zero. If the
packet size is greater than 1280 bytes or smaller than 88 bytes,
the Don't Fragments (DF) flag is set to one.
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: For ICMPv6 (58) changed to ICMPv4 (1), otherwise skip
extension headers, copy the Next Header field (ESP, transport
protocol or undefined next header value) in the last known next
header.
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 [I-D.ietf-behave-address-format] section
2.1. Note that the original IPv6 source address is an IPv4-
translatable address. A workflow example of stateless translation
is shown in Appendix 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 Type=1, Code=5
(Destination Unreachable, Source address failed ingress/egress
policy).
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 [I-D.ietf-behave-v6v4-xlate-stateful].
In stateless and stateful modes, if the translator gets an illegal
source address (e.g. ::1, etc.), the translator SHOULD silently
drop the packet.
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Destination Address: The IPv4 destination address is derived from
the IPv6 destination address of the datagram being translated per
[I-D.ietf-behave-address-format] section 2.1. Note that the
original IPv6 destination address is an IPv4-converted address.
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:
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 More Fragments (MF) flag is copied from the M flag in the
Fragment header. The Don't Fragments (DF) flag is set to zero
allowing this packet to be fragmented if required by IPv4 routers.
Fragment Offset: Copied from the Fragment Offset field in the
Fragment header.
Protocol: For ICMPv6 (58) changed to ICMPv4 (1), otherwise skip
extension headers, Next Header field 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.
4.2. Translating ICMPv6 Headers into ICMPv4 Headers
All ICMPv6 messages that are to be translated require that the ICMPv4
checksum field be updated as part of the translation since ICMPv6
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(unlike ICMPv4) includes a pseudo-header in the checksum just like
UDP and TCP.
In addition all ICMP packets MUST have the Type value 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 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 field 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 field as follows:
Code 0 (no route to destination): Set Code value to 1 (Host
unreachable).
Code 1 (Communication with destination administratively
prohibited): Set Code value to 10 (Communication with
destination host administratively prohibited).
Code 2 (Beyond scope of source address): Set Code value to 1
(Host unreachable). Note that this error is very unlikely
since an IPv4-translatable source address is typically
considered to have global scope.
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Code 3 (Address unreachable): Set Code value to 1 (Host
unreachable).
Code 4 (Port unreachable): Set Code value to 3 (Port
unreachable).
Other Code values: Silently drop.
Packet Too Big (Type 2): Translate to an ICMPv4 Destination
Unreachable (Type 3) with Code value equal to 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)
Time Exceeded (Type 3): Set the Type value to 11, and adjust the
ICMPv4 checksum both to take the type change into account and
to exclude the ICMPv6 pseudo-header. The Code field is
unchanged.
Parameter Problem (Type 4): Translate the Type and Code field 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 field as follows:
Code 0 (Erroneous header field encountered): Set 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.
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| 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
those containing IPv6 address literals will not have those
addresses translated to IPv4 address literals; this may cause
problems with processing of those ICMP extensions.
4.3. Translating ICMPv6 Error Messages into ICMPv4
There are some differences between the ICMPv4 and the ICMPv6 error
message formats as detailed above. In addition, the ICMP error
messages contain the packet in error, which MUST be translated just
like a normal IP packet. 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.
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+-------------+ +-------------+
| 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 first embedded header and drop the packet if it contains
more. 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.
4.4. Translator Sending ICMPv6 Error Message
If the IPv6 packet is discarded, then the translator SHOULD be able
to 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, has a Type value 1 and Code
value 5 (Source address failed ingress/egress policy). In other
cases, the ICMPv6 message has a Type value of 1 (Destination
Unreachable) and a Code value of 1 (Communication with destination
administratively prohibited), unless otherwise specified in this
document or [I-D.ietf-behave-v6v4-xlate-stateful]. The translator
SHOULD allow an administrator to configure whether the ICMPv6 error
messages are sent, rate-limited, or not sent.
4.5. Transport-layer Header Translation
If the address translation algorithm is not checksum neutral, the
recalculation and updating of the known transport-layer headers which
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contain pseudo headers (e.g. of TCP, UDP and DCCP) MUST be performed.
For ESP or undefined transport protocol, the translator MUST forward
the packets to the destination without touching the transport-layer
header.
4.6. 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.
5. IANA Considerations
This memo adds no new IANA considerations.
Note to RFC Editor: This section will have served its purpose if it
correctly tells IANA that no new assignments or registries are
required, or if those assignments or registries are created during
the RFC publication process. From the author's perspective, it may
therefore be removed upon publication as an RFC at the RFC Editor's
discretion.
6. 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. The [I-D.ietf-behave-address-format] addresses these issues.
As the Authentication Header [RFC4302] is specified to include the
IPv4 Identification field and the translating function is not able to
always preserve the Identification field, it is not possible for an
IPv6 endpoint to verify the AH on received packets that have been
translated from IPv4 packets. Thus AH does not work through a
translator.
Packets with ESP can be translated since ESP does not depend on
header fields prior to the ESP header. Note that ESP transport mode
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is easier to handle than ESP tunnel mode; in order to use ESP tunnel
mode, the IPv6 node MUST be able to generate an inner IPv4 header
when transmitting packets and remove such an IPv4 header when
receiving packets.
7. Acknowledgements
This is under development by a large group of people. Those who have
posted to the list during the discussion include 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, Senthil Sivakumar, Simon
Perreault and Zen Cao.
8. Appendix: 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 [I-D.ietf-behave-address-format] 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.
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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.
8.1. 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:DB8:1C6:3364:200:: is formed from
198.51.100.2 based on the address mapping algorithm
[I-D.ietf-behave-address-format].
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:DB8:1C6:3364:200::.
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 source address in the packet
and 198.51.100.2 as 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:DB8:1C6:3364:200::
(IPv4-converted address) of the received IPv6 packet that is
being translated.
5. The XLAT sends the translated packet out 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:
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* 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 destination address
in the packet and 2001:DB8:1C6:3364:200:: as source address in
the packet. Note that 2001:DB8:1C0:2:21:: and
2001:DB8:1C6:3364:200::
are formed directly from the destination IPv4
address 192.0.2.33 and source IPv4 address 198.51.100.2 of the
received IPv4 packet that is being translated.
8. The translated packet is sent out the IPv6 interface to H6.
The packet exchange between H6 and H4 continues until the session is
finished.
8.2. 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 IPv4-translatable address 2001:DB8:1C0:2:21:: based
on the address mapping algorithm
[I-D.ietf-behave-address-format].
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:DB8:1C6:3364:200:: as source address in
the packet and 2001:DB8:1C0:2:21:: as destination address in
the packet. Note that 2001:DB8:1C6:3364:200:: (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:DB8:1C6:3364:200:: and source address 2001:DB8:1C0:2:21::.
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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 destination address in the
packet and 192.0.2.33 as 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:DB8:1C6:3364:200:: and source
IPv6 address 2001:DB8:1C0:2:21:: of the received IPv6 packet
that is being translated.
8. The translated packet is sent out the IPv4 interface to H4.
The packet exchange between H4 and H6 continues until the session
finished.
9. References
9.1. Normative References
[I-D.ietf-behave-address-format]
Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators",
draft-ietf-behave-address-format-06 (work in progress),
March 2010.
[I-D.ietf-behave-v6v4-xlate-stateful]
Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers",
draft-ietf-behave-v6v4-xlate-stateful-11 (work in
progress), March 2010.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
Li, et al. Expires October 5, 2010 [Page 28]
Internet-Draft IPv4/IPv6 Translation April 2010
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995.
[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.
[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.
9.2. Informative References
[I-D.ietf-behave-v6v4-framework]
Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
IPv4/IPv6 Translation",
draft-ietf-behave-v6v4-framework-08 (work in progress),
March 2010.
[RFC0879] 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.
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[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.
[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.
[RFC5737] Arkko, J., Cotton, M., and L. Vegoda, "IPv4 Address Blocks
Reserved for Documentation", RFC 5737, January 2010.
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
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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
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