behave X. Li
Internet-Draft C. Bao
Obsoletes: 2765 (if approved) CERNET Center/Tsinghua University
Intended status: Standards Track F. Baker
Expires: May 24, 2010 Cisco Systems
November 20, 2009
IP/ICMP Translation Algorithm
draft-ietf-behave-v6v4-xlate-04
Abstract
This document specifies an update to the Stateless IP/ICMP
Translation Algorithm (SIIT) described in RFC 2765. The algorithm
translates between IPv4 and IPv6 packet headers (including ICMP
headers).
This specification addresses both a stateless and a stateful mode.
In the stateless mode, translation information is carried in the
address itself, permitting both IPv4->IPv6 and IPv6->IPv4 session
establishment without maintaining state in the IP/ICMP translator.
In the stateful mode, translation state is maintained between IPv4
address/transport port tuples and IPv6 address/transport port tuples,
enabling IPv6 systems to open sessions with IPv4 systems. The choice
of operational mode is made by the operator deploying the network and
is critical to the operation of the applications using it.
Significant issues exist in the stateless and stateful modes that are
not addressed in this document, related to the address assignment and
the maintenance of the translation tables, respectively. This
document confines itself to the actual translation.
Acknowledgement of previous work
This document is a product of the 2008-2009 effort to define a
replacement for NAT-PT. It is an update to and directly derivative
from Erik Nordmark's [RFC2765], which similarly provides both
stateless and stateful translation between IPv4 [RFC0791] and IPv6
[RFC2460], and between ICMPv4 [RFC0792] and ICMPv6 [RFC4443]. The
original document was a product of the NGTRANS working group.
The changes in this document reflect five components:
1. Redescribing the network model to map to present and projected
usage [I-D.ietf-behave-v6v4-framework].
Li, et al. Expires May 24, 2010 [Page 1]
Internet-Draft IPv4/IPv6 Translation November 2009
2. Moving the address format to the address format document
[I-D.ietf-behave-address-format], to coordinate with other drafts
on the topic.
3. Describing both stateful and stateless operation.
4. Some changes in ICMP.
5. Updating references.
Requirements
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 RFC 2119.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on May 24, 2010.
Copyright Notice
Copyright (c) 2009 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
Li, et al. Expires May 24, 2010 [Page 2]
Internet-Draft IPv4/IPv6 Translation November 2009
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 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
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.
Li, et al. Expires May 24, 2010 [Page 3]
Internet-Draft IPv4/IPv6 Translation November 2009
Table of Contents
1. Introduction and Motivation . . . . . . . . . . . . . . . . . 4
1.1. Translation Model . . . . . . . . . . . . . . . . . . . . 4
1.2. Applicability and Limitations . . . . . . . . . . . . . . 5
1.3. Stateless vs. Stateful Mode . . . . . . . . . . . . . . . 6
1.4. Path MTU discovery and fragmentation . . . . . . . . . . . 6
2. Translating from IPv4 to IPv6 . . . . . . . . . . . . . . . . 7
2.1. Translating IPv4 Headers into IPv6 Headers . . . . . . . . 8
2.2. Translating UDP over IPv4 . . . . . . . . . . . . . . . . 10
2.3. Translating ICMPv4 Headers into ICMPv6 Headers . . . . . . 11
2.4. Translating ICMPv4 Error Messages into ICMPv6 . . . . . . 13
2.5. Translator sending ICMP error message . . . . . . . . . . 14
2.6. Transport-layer Header Translation . . . . . . . . . . . . 14
2.7. Knowing when to Translate . . . . . . . . . . . . . . . . 14
3. Translating from IPv6 to IPv4 . . . . . . . . . . . . . . . . 15
3.1. Translating IPv6 Headers into IPv4 Headers . . . . . . . . 16
3.2. Translating ICMPv6 Headers into ICMPv4 Headers . . . . . . 18
3.3. Translating ICMPv6 Error Messages into ICMPv4 . . . . . . 20
3.4. Translator sending ICMPv6 error message . . . . . . . . . 20
3.5. Transport-layer Header Translation . . . . . . . . . . . . 21
3.6. Knowing when to Translate . . . . . . . . . . . . . . . . 21
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
5. Security Considerations . . . . . . . . . . . . . . . . . . . 21
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.1. Normative References . . . . . . . . . . . . . . . . . . . 22
7.2. Informative References . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
Li, et al. Expires May 24, 2010 [Page 4]
Internet-Draft IPv4/IPv6 Translation November 2009
1. Introduction and Motivation
An understanding of the framework presented in
[I-D.ietf-behave-v6v4-framework] is presumed in this document.
The transition mechanisms specified in [RFC4213] handle the case of
dual IPv4/IPv6 hosts interoperating with both dual IPv4/IPv6 hosts
and IPv4-only hosts, which is needed early in the transition to IPv6.
The dual IPv4/IPv6 hosts are assigned both an IPv4 and one or more
IPv6 addresses. The number of available globally unique IPv4
addresses is becoming smaller and smaller as the Internet grows; we
expect that there will be a desire to take advantage of the large
IPv6 address space and not require that every new Internet node have
a permanently assigned IPv4 address.
SIIT [RFC2765] is designed for the case of small networks (e.g., a
single subnet) and for a site that has IPv6-only hosts in a dual
IPv4/IPv6 network. This use assumes a mechanism for IPv6 nodes to
acquire a temporary address from the pool of IPv4 addresses.
However, SIIT is not useful in the case when the IPv6 nodes need to
acquire temporary IPv4 addresses from a "distant" SIIT box operated
by a different administration, or require that the IPv6 Internet
contain routes for IPv4-mapped addresses (The latter is known to be a
very bad idea due to the size of the IPv4 routing table that would
potentially be injected into IPv6 routing in the form of IPv4-mapped
addresses.)
In addition, due to the IPv4 address depletion problem, it is
desirable that a single IPv4 address needs to be shared via transport
port multiplexing for different IPv6 nodes when they communicate with
other IPv4 hosts.
Furthermore, in SIIT [RFC2765], an IPv6-only node that works through
SIIT translators needs some modifications beyond a normal IPv6-only
node. These modifications are not strictly implied in this document,
since normal IPv6 addresses can be used in the IPv6 end nodes.
A detailed discussion of translation scenarios is presented in
[I-D.ietf-behave-v6v4-framework], while the technical specification
of the translation algorithm itself is covered in this document.
1.1. Translation Model
This document specifies the translation algorithm that is one of the
components described in [I-D.ietf-behave-v6v4-framework] needed to
make IPv6-only nodes interoperate with IPv4-only nodes as shown in
Figure 1.
Li, et al. Expires May 24, 2010 [Page 5]
Internet-Draft IPv4/IPv6 Translation November 2009
-------- --------
// IPv4 \\ // IPv6 \\
/ Domain \ / Domain \
/ +----+ +--+ \
| |XLAT| |S2| | Sn: Servers
| +--+ +----+ +--+ | Hn: Clients
| |S1| +----+ |
| +--+ |DNS | +--+ | XLAT: V4/V6 Translator
\ +--+ +----+ |H2| / DNS: DNS Server
\ |H1| / \ +--+ /
\\ +--+ // \\ //
-------- --------
Figure 1: Translation Model
The translation model consists of two or more network domains
connected by one or more IP/ICMP translators. One of those networks
either routes IPv4 but not IPv6, or contains some hosts that only
implement IPv4 or have IPv4 only applications (even if the host and
the network support IPv6). The other network either routes IPv6 but
not IPv4, or contains some hosts that only implement IPv6 or has IPv6
only applications. Both networks contain clients, servers, and
peers.
1.2. Applicability and Limitations
The use of this translation algorithm assumes that the IPv6 network
is somehow well-connected i.e., when an IPv6 node wants to
communicate with another IPv6 node there is an IPv6 path between
them. Various tunneling schemes exist that can provide such a path,
but those mechanisms and their use is outside the scope of this
document and [RFC2765].
The translation algorithm can be used not only in a subnet, but can
also be used in service provider's backbone network.
The translating function specified in this document does not
translate any IPv4 options and it does not translate IPv6 routing
headers, hop-by-hop extension headers, destination options headers or
source routing headers [RFC2765].
The issues and algorithms in the translation of datagram containing
TCP segments are described in [RFC5382]. The considerations of that
document are applicable in this case as well.
Fragmented IPv4 UDP packets that do not contain a UDP checksum (i.e.
the UDP checksum field is zero) are not of significant use over wide-
areas in the Internet and will not be translated by the IP/ICMP
Li, et al. Expires May 24, 2010 [Page 6]
Internet-Draft IPv4/IPv6 Translation November 2009
translator [Miller][Dongjin].
IPv4 multicast addresses [RFC3171] cannot be mapped to IPv6 multicast
addresses [RFC3307] based on the unicast mapping rule. However, a
special rule for address translation can be created for the multicast
packet translation algorithm; if that is done, the IP/ICMP header
translation aspect of this memo works.
1.3. Stateless vs. Stateful Mode
The IP/ICMP translator has two possible modes of operation: stateless
and stateful. In both cases, we assume that a system 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 that can be algorithmatically mapped to a subset of the
service provider's IPv4 addresses (IPv4-translatable addresses). In
this mode, there is no need to concern oneself with port translation
or 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 [RFC4291] addresses 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].
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: the path MTU discovery,
fragmentation and transport layer negotiation such as the TCP MSS
option. Note that the translator MUST behave as a router, i.e. the
translator MUST send packet to big error message when the packet size
exceeds the MTU of the next hop interface.
"Don't Fragment", ICMP "too big", and packet fragmentation are
discussed in "Translating from IPv4 to IPv6" and "Translating from
IPv6 to IPv4" sections of this document. The reassembling of the
Li, et al. Expires May 24, 2010 [Page 7]
Internet-Draft IPv4/IPv6 Translation November 2009
fragmented packets in the stateful translator is discussed in
[I-D.ietf-behave-v6v4-xlate-stateful], since it requires the state
maintenance in the translator.
2. 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. Since the ICMP [RFC0792][RFC4443],
TCP [RFC0793] and UDP [RFC0768] headers contain checksums that cover
IP header information, if the address mapping algorithm is not
checksum-neutral, 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 IPv6
destination address. The original IPv4 header on the packet is
removed and replaced by an IPv6 header.
+-------------+ +-------------+
| IPv4 | | IPv6 |
| Header | | Header |
+-------------+ +-------------+
| Transport | | Fragment |
| Layer | ===> | Header |
| Header | |(not always) |
+-------------+ +-------------+
| | | Transport |
~ Data ~ | Layer |
| | | Header |
+-------------+ +-------------+
| |
~ Data ~
| |
+-------------+
Figure 2: IPv4-to-IPv6 Translation
One of the differences between IPv4 and IPv6, is that in IPv6 path
MTU discovery is mandatory but it is optional in IPv4. This implies
that IPv6 routers will never fragment a packet - only the sender can
do fragmentation.
When the IPv4 node performs path MTU discovery (by setting the DF bit
in the header) the 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 ICMP errors
they will pass through a translator that will translate the ICMP
Li, et al. Expires May 24, 2010 [Page 8]
Internet-Draft IPv4/IPv6 Translation November 2009
error to a form that the IPv4 sender can understand. In this case,
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
has to 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 packet
size. 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. That is needed should the
packet pass through an IP/ICMP translator.
The above rules 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 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 mapping as defined below. Note that ICMP packets require
special handling in order to translate the content of ICMP error
message and also to add the ICMP pseudo-header checksum.
2.1. 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 IPv4 packet MUST be fragmented
prior to translating it. Since IPv4 packets with DF not set will
always result in a fragment header being added to the packet 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 or equal to the total length value of the IPv4 packet minus 20,
Send ICMP (packet too big) to 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 zero) then the translator
SHOULD NOT add a fragment header to the packet. The IPv6 header
fields are set as follows:
Li, et al. Expires May 24, 2010 [Page 9]
Internet-Draft IPv4/IPv6 Translation November 2009
Version: 6
Traffic Class: By default, copied from IP Type Of Service 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 provide the
ability to ignore the IPv4 "TOS" and always set the IPv6 traffic
class 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.
Next Header: Protocol field copied from IPv4 header
Hop Limit: TTL value copied from 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) needs to check for
zero and send the ICMPv4 "ttl exceeded" or ICMPv6 "hop limit
exceeded" error.
Source Address: The IPv6 source address is derived from the IPv4
source address. Note that the IPv6 source address is the IPv4-
converted address.
Destination Address: In stateless mode, which is to say that if the
IPv4 destination address is within the range of the IPv4 stateless
translation prefix, the IPv6 destination address is derived from
the IPv4 destination address. Note that the IPv6 destination
address is the IPv4-translatable address.
In stateful mode, which is to say that if the IPv4 destination
address is not within the range of the IPv4 stateless translation
prefix, the IPv6 address and corresponding transport-layer
destination port are derived from the database reflecting current
session state in the translator. Database maintenance is as
described in [I-D.ietf-behave-v6v4-xlate-stateful].
If the destination address is in the multicast range, the
multicast address mapping method should be applied.
If IPv4 options are present in the IPv4 packet, they are ignored
Li, et al. Expires May 24, 2010 [Page 10]
Internet-Draft IPv4/IPv6 Translation November 2009
i.e., there is no attempt to translate them. 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 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: Protocol field 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.
2.2. Translating UDP over IPv4
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.
When a stateful translator receives fragmented UDP IPv4 packets and
the checksum field is zero, if the translator has enough resource to
reassemble the packets, the stateful translator SHOULD reassemble the
Li, et al. Expires May 24, 2010 [Page 11]
Internet-Draft IPv4/IPv6 Translation November 2009
packets and SHOULD calculate the checksum. Otherwise, the stateful
translator MAY drop the packets.
2.3. Translating ICMPv4 Headers into ICMPv6 Headers
All ICMP messages that are to be translated require that the ICMP
checksum field be updated as part of the translation since ICMPv6,
unlike ICMPv4, has a pseudo-header checksum just like UDP and TCP.
In addition, all ICMP packets need to have the Type value translated
and, for ICMP error messages, the included IP header also needs
translation.
The actions needed to translate various ICMPv4 messages are:
ICMPv4 query messages:
Echo and Echo Reply (Type 8 and Type 0) Adjust the type 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 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
[I-D.venaas-behave-v4v6mc-framework]. 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.
Li, et al. Expires May 24, 2010 [Page 12]
Internet-Draft IPv4/IPv6 Translation November 2009
ICMPv4 error messages:
Destination Unreachable (Type 3) For all codes that are not
explicitly listed below, set the Type to 1.
Translate the code field as follows:
Code 0, 1 (net, host unreachable): Set 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 Code to 4 (port
unreachable).
Code 4 (fragmentation needed and DF set): Translate to an
ICMPv6 Packet Too Big message (Type 2) with code 0. The
MTU field needs to be adjusted for the difference between
the IPv4 and IPv6 header sizes (the original advertised
MTU+20). Note that if the IPv4 router did not set the
MTU field, 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.)
Code 5 (source route failed): Set Code to 0 (no route to
destination). Note that this error is unlikely since
source routes are not translated.
Code 6,7: Set Code to 0 (no route to destination).
Code 8: Set Code to 0 (no route to destination).
Code 9, 10 (communication with destination host
administratively prohibited): Set Code to 1 (communication
with destination administratively prohibited)
Code 11, 12: Set Code to 0 (no route to destination).
Redirect (Type 5) Single hop message. Silently drop.
Li, et al. Expires May 24, 2010 [Page 13]
Internet-Draft IPv4/IPv6 Translation November 2009
Source Quench (Type 4) Obsoleted in ICMPv6. Silently drop.
Time Exceeded (Type 11) Set the Type field to 3. The Code
field is unchanged.
Parameter Problem (Type 12) Set the Type field to 4. The
Pointer needs to be updated to point to the corresponding
field in the translated include IP header.
ICMP Error Payload If the received ICMP packet contains an
ICMP Extension [RFC4884] the translation of the ICMP packet
will cause the ICMP packet to change length. When this
occurs, the ICMP Extension length attribute MUST be adjusted
accordingly (e.g., longer due to the translation from IPv4
to IPv6). If the ICMP Extension exceeds the maximum size of
an ICMPv6 message on the outgoing interface, the ICMP
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 is likely to cause problems with
processing of those ICMP extensions.
2.4. Translating ICMPv4 Error Messages into ICMPv6
There are some differences between the IPv4 and the IPv6 ICMP error
message formats as detailed above. In addition, the ICMP error
messages contain the IP header for the packet in error, which needs
to be translated just like a normal IP header. The translation of
this "packet in error" is likely to change the length of the
datagram. Thus the Payload Length field in the outer IPv6 header
might need to be updated.
Li, et al. Expires May 24, 2010 [Page 14]
Internet-Draft IPv4/IPv6 Translation November 2009
+-------------+ +-------------+
| IPv4 | | IPv6 |
| Header | | Header |
+-------------+ +-------------+
| ICMPv4 | | ICMPv6 |
| Header | | Header |
+-------------+ +-------------+
| IPv4 | ===> | IPv6 |
| Header | | Header |
+-------------+ +-------------+
| Partial | | Partial |
| Transport | | Transport |
| Layer | | Layer |
| Header | | Header |
+-------------+ +-------------+
Figure 3: IPv4-to-IPv6 ICMP Error Translation
The translation of the inner IP header can be done by recursively
invoking the function that translated the outer IP headers.
2.5. Translator sending ICMP error message
If the packet is discarded, then the translator SHOULD be able to
send back an ICMP message to the original sender of the packet,
unless the discarded packet is itself an ICMP message. The ICMP
message, if sent, has a type of 3 (Destination Unreachable) and a
code of 13 (Communication Administratively Prohibited). The
translator device MUST allow to configure whether the ICMP error
messages are sent, rate-limited or not sent.
2.6. Transport-layer Header Translation
If the address translation algorithm is not checksum neutral, the
recalculation and updating of the transport-layer headers MUST be
performed. UDP/IPv4 datagrams with a checksum of zero MAY be dropped
and MAY have their checksum calculated for injection into the IPv6
domain. This choice SHOULD be under configuration control.
2.7. Knowing when to Translate
If the IP/ICMP translator is implemented in a router providing both
translation and normal forwarding, and the address is reachable by a
more specific route without translation, the router MUST forward it
without translating it. Otherwise, when an IP/ICMP translator
receives an IPv4 datagram addressed to a destination towards the IPv6
domain, the packet will be translated to IPv6.
Li, et al. Expires May 24, 2010 [Page 15]
Internet-Draft IPv4/IPv6 Translation November 2009
3. 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
that packet into an IPv4 header. Since the ICMP [RFC0792][RFC4443],
TCP [RFC0793] and UDP [RFC0768] headers contain checksums that cover
the IP header, if the address mapping algorithm is not checksum-
neutral, 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. The
original IPv6 header on the packet is removed and replaced by an IPv4
header.
+-------------+ +-------------+
| IPv6 | | IPv4 |
| Header | | Header |
+-------------+ +-------------+
| Fragment | | Transport |
| Header | ===> | Layer |
|(if present) | | Header |
+-------------+ +-------------+
| Transport | | |
| Layer | ~ Data ~
| Header | | |
+-------------+ +-------------+
| |
~ Data ~
| |
+-------------+
Figure 4: 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 ICMP "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 ICMP "packet too big"
message by reducing the path MTU to 1280 and including an IPv6
fragment header with each packet. 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.
Li, et al. Expires May 24, 2010 [Page 16]
Internet-Draft IPv4/IPv6 Translation November 2009
The only drawback with this scheme is that it is not possible to use
PMTU 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 ICMP "too big" errors and cannot adjust
the size fragments it is sending.
Other than the special rules for handling fragments and path MTU
discovery the actual translation of the packet header consists of a
simple mapping as defined below. Note that ICMP packets require
special handling in order to translate the contents of ICMP error
message and also to add the ICMP pseudo-header checksum.
3.1. 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: All zero.
Flags: The More Fragments flag is set to zero. The Don't Fragments
flag is set to one.
Li, et al. Expires May 24, 2010 [Page 17]
Internet-Draft IPv4/IPv6 Translation November 2009
Fragment Offset: All zero.
Time to Live: Hop Limit value copied from 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) needs to check for
zero and send the ICMPv4 "ttl exceeded" or ICMPv6 "hop limit
exceeded" error.
Protocol: Next Header field copied from IPv6 header.
Header Checksum: Computed once the IPv4 header has been created.
Source Address: In stateless mode, which is to say that if the IPv6
source address is within the range of the IPv6 stateless
translation prefix, the IPv4 source address is derived from the
IPv6 address. Note that the original IPv6 source address is the
IPv4-translatable address.
In stateful mode, which is to say that if the IPv6 source address
is not within the range of the 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 database reflecting current session
state in the translator. Database maintenance is described in
[I-D.ietf-behave-v6v4-xlate-stateful].
Destination Address: The IPv4 destination address is derived from
the IPv6 destination address of the datagram being translated.
Note that the original IPv6 destination address is the IPv4-
converted address.
If the destination address is in the multicast range, the
multicast address mapping method should be applied.
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, they are ignored i.e., there is no
attempt to translate them. However, the Total Length field and the
Protocol field is 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.
Li, et al. Expires May 24, 2010 [Page 18]
Internet-Draft IPv4/IPv6 Translation November 2009
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 flag is copied from the M flag in the
Fragment header. The Don't Fragments flag is set to zero allowing
this packet to be fragmented by IPv4 routers.
Fragment Offset: Copied from the Fragment Offset field in the
Fragment header.
Protocol: Next Header value copied from Fragment header.
3.2. Translating ICMPv6 Headers into ICMPv4 Headers
All ICMP messages that are to be translated require that the ICMP
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 need to have the Type value translated
and, for ICMP error messages, the included IP header also needs
translation. Note that the IPv6 addresses in the IPv6 header may not
be the IPv4-translatable addresses and there will be no corresponding
IPv4 addresses. In this case, a special block of the IPv4 address
can be used to indicate this phenomenon.
The actions needed to translate various ICMPv6 messages are:
ICMPv6 informational messages:
Echo Request and Echo Reply (Type 128 and 129) Adjust the type 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.
Li, et al. Expires May 24, 2010 [Page 19]
Internet-Draft IPv4/IPv6 Translation November 2009
Unknown informational messages Silently drop.
ICMPv6 error messages:
Destination Unreachable (Type 1) Set the Type field to 3.
Translate the code field as follows:
Code 0 (no route to destination): Set Code to 1 (host
unreachable).
Code 1 (communication with destination administratively
prohibited): Set Code to 10 (communication with destination
host administratively prohibited).
Code 2 (beyond scope of source address): Set Code to 1 (host
unreachable). Note that this error is very unlikely since
the IPv4-translatable source address is considered to have
global scope.
Code 3 (address unreachable): Set Code to 1 (host
unreachable).
Code 4 (port unreachable): Set Code to 3 (port unreachable).
Packet Too Big (Type 2) Translate to an ICMPv4 Destination
Unreachable with code 4. The MTU field needs to 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 (the original advertised MTU-20).
Time Exceeded (Type 3) Set the Type to 11. The Code field is
unchanged.
Parameter Problem (Type 4) If the Code is 1, translate this to an
ICMPv4 protocol unreachable (Type 3, Code 2). Otherwise set
the Type to 12 and the Code to zero. The Pointer needs to be
updated to point to the corresponding field in the translated
inner IP header.
Unknown error messages Silently drop.
ICMP Error Payload If the received ICMPv6 packet contains an ICMP
Extension [RFC4884], the translation of the ICMPv6 packet will
cause the ICMPv6 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
Li, et al. Expires May 24, 2010 [Page 20]
Internet-Draft IPv4/IPv6 Translation November 2009
containing IPv6 address literals will not have those addresses
translated to IPv4 address literals; this is likely to cause
problems with processing of those ICMP extensions.
3.3. Translating ICMPv6 Error Messages into ICMPv4
There are some differences between the IPv4 and the IPv6 ICMP error
message formats as detailed above. In addition, the ICMP error
messages contain the IP header for the packet in error, which needs
to be translated just like a normal IP header. 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 might need to be
updated.
+-------------+ +-------------+
| IPv6 | | IPv4 |
| Header | | Header |
+-------------+ +-------------+
| ICMPv6 | | ICMPv4 |
| Header | | Header |
+-------------+ +-------------+
| IPv6 | ===> | IPv4 |
| Header | | Header |
+-------------+ +-------------+
| Partial | | Partial |
| Transport | | Transport |
| Layer | | Layer |
| Header | | Header |
+-------------+ +-------------+
Figure 5: IPv6-to-IPv4 ICMP Error Translation
The translation of the inner IP header can be done by recursively
invoking the function that translated the outer IP headers. Note
that the IPv6 addresses in the IPv6 header may not be the IPv4-
translatable addresses and there will be no corresponding IPv4
addresses. In this case, a special block of the IPv4 address can be
used to indicate this phenomenon.
3.4. Translator sending ICMPv6 error message
If the packet is discarded, then the translator SHOULD be able to
send back an ICMPv6 message to the original sender of the packet,
unless the discarded packet is itself an ICMPv6 message. The ICMPv6
message, if sent, has a type of 1 (Destination Unreachable) and a
code of 1 (Communication with destination administratively
prohibited). The translator device MUST allow configuring whether
the ICMPv6 error messages are sent, rate-limited or not sent.
Li, et al. Expires May 24, 2010 [Page 21]
Internet-Draft IPv4/IPv6 Translation November 2009
3.5. Transport-layer Header Translation
If the address translation algorithm is not checksum neutral, the
recalculation and updating of the transport-layer headers MUST be
performed.
3.6. Knowing when to Translate
If the IP/ICMP translator is implemented in a router providing both
translation and normal forwarding, and the 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 a destination towards the IPv4 domain, the
packet will be translated to IPv4.
4. 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.
5. 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.
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
is easier to handle than ESP tunnel mode; in order to use ESP tunnel
mode, the IPv6 node needs to be able to generate an inner IPv4 header
when transmitting packets and remove such an IPv4 header when
receiving packets.
Li, et al. Expires May 24, 2010 [Page 22]
Internet-Draft IPv4/IPv6 Translation November 2009
6. 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, Congxiao Bao, Dan Wing, Dave
Thaler, Ed Jankiewicz, Fred Baker, Hiroshi Miyata, Iljitsch van
Beijnum, Jari Arkko, Jerry Huang, John Schnizlein, Kevin Yin, Magnus
Westerlund, Marcelo Bagnulo Braun, Margaret Wasserman, Masahito Endo,
Phil Roberts, Philip Matthews, Remi Denis-Courmont, Remi Despres,
Simon Perreault and Xing Li.
7. References
7.1. Normative References
[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.
[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.
[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.
Li, et al. Expires May 24, 2010 [Page 23]
Internet-Draft IPv4/IPv6 Translation November 2009
[RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, October 2008.
7.2. Informative References
[Dongjin] Lee, D., "Email to the behave mailing list", Sept 2009.
[I-D.ietf-behave-address-format]
Huitema, C., Bao, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators",
draft-ietf-behave-address-format-01 (work in progress),
October 2009.
[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-03 (work in progress),
October 2009.
[I-D.ietf-behave-v6v4-xlate-stateful]
Bagnulo, M., Matthews, P., and I. Beijnum, "NAT64: Network
Address and Protocol Translation from IPv6 Clients to IPv4
Servers", draft-ietf-behave-v6v4-xlate-stateful-02 (work
in progress), October 2009.
[I-D.venaas-behave-v4v6mc-framework]
Venaas, S., Li, X., and C. Bao, "Framework for IPv4/IPv6
Multicast Translation",
draft-venaas-behave-v4v6mc-framework-01 (work in
progress), October 2009.
[Miller] Miller, G., "Email to the ngtrans mailing list",
March 1999.
[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
Li, et al. Expires May 24, 2010 [Page 24]
Internet-Draft IPv4/IPv6 Translation November 2009
Listener Discovery (MLD) for IPv6", RFC 2710,
October 1999.
[RFC3171] Albanna, Z., Almeroth, K., Meyer, D., and M. Schipper,
"IANA Guidelines for IPv4 Multicast Address Assignments",
BCP 51, RFC 3171, August 2001.
[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.
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, October 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
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
Li, et al. Expires May 24, 2010 [Page 25]
Internet-Draft IPv4/IPv6 Translation November 2009
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. Expires May 24, 2010 [Page 26]
