One document matched: draft-ietf-behave-v6v4-xlate-00.xml
<?xml version="1.0" encoding="US-ASCII"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
<?xml-stylesheet type='text/xsl' href='rfc2629.xslt' ?>
<!-- Some of the more generally applicable PIs that most I-Ds might want to use -->
<!-- Try to enforce the ID-nits conventions and DTD validity -->
<?rfc strict='yes' ?>
<!-- Items used when reviewing the document -->
<?rfc comments='no' ?>
<!-- Controls display of <cref> elements -->
<?rfc inline='no' ?>
<!-- When no, put comments at end in comments section,
otherwise, put inline -->
<?rfc editing='no' ?>
<!-- When yes, insert editing marks: editing marks consist of a
string such as <29> printed in the blank line at the
beginning of each paragraph of text. -->
<!-- Create Table of Contents (ToC) and set some options for it.
Note the ToC may be omitted for very short documents,but idnits insists on a ToC
if the document has more than 15 pages. -->
<?rfc toc='yes'?>
<?rfc tocompact='yes'?>
<!-- If 'yes' eliminates blank lines before main section entries. -->
<?rfc tocdepth='3'?>
<!-- Sets the number of levels of sections/subsections... in ToC -->
<!-- Choose the options for the references.
Some like symbolic tags in the references (and citations) and others prefer
numbers. The RFC Editor always uses symbolic tags.
The tags used are the anchor attributes of the references. -->
<?rfc symrefs='yes'?>
<?rfc sortrefs='yes' ?>
<!-- If 'yes', causes the references to be sorted in order of tags.
This doesn't have any effect unless symrefs is "yes" also. -->
<!-- These two save paper: Just setting compact to 'yes' makes savings by not starting each
main section on a new page but does not omit the blank lines between list items.
If subcompact is also "yes" the blank lines between list items are also omitted. -->
<?rfc compact='yes' ?>
<?rfc subcompact='no' ?>
<!-- end of list of popular I-D processing instructions -->
<!-- end of list of processing instructions -->
<rfc obsoletes="2765" category="std" docName="draft-ietf-behave-v6v4-xlate-00"
ipr="pre5378Trust200902">
<front>
<title abbrev="IPv4/IPv6 Translation">IP/ICMP Translation
Algorithm</title>
<author fullname="Xing Li" initials="X." role="editor" surname="Li">
<organization>CERNET Center/Tsinghua University</organization>
<address>
<postal>
<street>Room 225, Main Building, Tsinghua University</street>
<city>Beijing</city>
<code>100084</code>
<region></region>
<country>China</country>
</postal>
<phone>+86 62785983</phone>
<email>xing@cernet.edu.cn</email>
</address>
</author>
<author fullname="Congxiao Bao" initials="C." role="editor" surname="Bao">
<organization>CERNET Center/Tsinghua University</organization>
<address>
<postal>
<street>Room 225, Main Building, Tsinghua University</street>
<city>Beijing</city>
<code>100084</code>
<region></region>
<country>China</country>
</postal>
<phone>+86 62785983</phone>
<email>congxiao@cernet.edu.cn</email>
</address>
</author>
<author fullname="Fred Baker" initials="F.J." role="editor"
surname="Baker">
<organization>Cisco Systems</organization>
<address>
<postal>
<street></street>
<city>Santa Barbara</city>
<code>93117</code>
<region>California</region>
<country>USA</country>
</postal>
<phone>+1-408-526-4257</phone>
<email>fred@cisco.com</email>
</address>
</author>
<date year="2009" />
<area>Transport</area>
<workgroup>behave</workgroup>
<abstract>
<t>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).</t>
<t>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 with neither state nor configuration 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.</t>
<t>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.</t>
</abstract>
<note title="Acknowledgement of previous work">
<t>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 <xref target="RFC2765"></xref>, which similarly provides
both stateless and stateful translation between IPv4 <xref
target="RFC0791"></xref> and IPv6 <xref target="RFC2460"></xref>, and
between ICMPv4 <xref target="RFC0792"></xref> and ICMPv6 <xref
target="RFC4443"></xref>. The original document was a product of the
NGTRANS working group.</t>
<t>The changes in this document reflect five components:<list
style="numbers">
<t>Redescribing the network model to map to present and projected
usage.</t>
<t>Moving the address format to the framework document, to
coordinate with other drafts on the topic.</t>
<t>Description of both stateful and stateless operation.</t>
<t>Some changes in ICMP.</t>
<t>Updating references.</t>
</list></t>
</note>
<note title="Requirements">
<t>
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.
</t>
</note>
</front>
<middle>
<section anchor="section1" title="Introduction and Motivation">
<t>An understanding of the framework presented in <xref
target="I-D.ietf-behave-v6v4-framework"></xref> is presumed in this
document. With that remark...</t>
<t>The transition mechanisms specified in <xref target="RFC4213"></xref>
handle the case of dual IPv4/IPv6 hosts interoperating with both dual
hosts and IPv4-only hosts, which is needed early in the transition to
IPv6. The dual hosts are assigned both an IPv4 and one or more IPv6
addresses. The number of available globally unique IPv4 addresses are
becoming smaller and smaller as the Internet grows; we expect that there
will be a desire to take advantage of the large IPv6 address and not
require that every new Internet node have a permanently assigned IPv4
address.</t>
<t>The SIIT <xref target="RFC2765"></xref> is designed for the case for
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 the
IPv6 nodes to acquire a temporary address from the pool of IPv4
addresses. However, SIIT is not to be useful in the case when the IPv6
nodes to acquire temporary IPv4 addresses from a "distant" SIIT box
operated by a different administration, or require that the IPv6 routing
contain routes for IPv6-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.)</t>
<t>In addition, due to the IPv4 address deletion problem, it is
desirable that a single IPv4 address needs to be shared via transport
port multiplexing technique for different IPv6 nodes when they
communicate with other IPv4 hosts.</t>
<t>Furthermore, in the SIIT <xref target="RFC2765"></xref>
implementation, 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 the
normal IPv6 addresses can be used in the IPv6 end nodes.</t>
<t>The detailed discussion of the transition scenarios is presented in
<xref target="I-D.ietf-behave-v6v4-framework"></xref>, the technical
specifications of the translation algorithm itself is illustrated in
this document.</t>
<section anchor="section1.0" title="Translation Model">
<t>This document specifies the translation algorithm that is one of
the components described in <xref
target="I-D.ietf-behave-v6v4-framework"></xref> needed to make
IPv6-only nodes interoperate with IPv4-only nodes as shown in Figure
1.</t>
<figure anchor="cloud1" title="Translation Model">
<artwork align="center"><![CDATA[
-------- --------
// IPv4 \\ // IPv6 \\
/ Domain \ / Domain \
/ +----+ +--+ \
| |XLAT| |S2| | Sn: Servers
| +--+ +----+ +--+ | Hn: Clients
| |S1| +----+ |
| +--+ |DNS | +--+ | XLAT: V4/V6 Translator
\ +--+ +----+ |H2| / DNS: DNS Server
\ |H1| / \ +--+ /
\\ +--+ // \\ //
-------- --------
]]></artwork>
</figure>
<t>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. The other network either routes IPv6 but not IPv4, or
contains some hosts that only implement IPv6. Both networks contain
clients, servers, and peers.</t>
</section>
<section anchor="section1.12" title="Applicability and Limitations">
<t>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 <xref
target="RFC2765"></xref>.</t>
<t>The translation algorithm can be used no only in a subnet or small
networks, but can also be used in the autonomous system scope.</t>
<t>The translating function as 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 <xref target="RFC2765"></xref>.</t>
<t>The issues and algorithms in the translation of datagram containing
TCP segments are described in <xref target="RFC5382"></xref>. The
considerations of that document are applicable in this case as
well.</t>
<t>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
translator <xref target="Miller"></xref>.</t>
<t>The considerations of The IPSec <xref target="RFC4301"></xref>
<xref target="RFC4302"></xref> <xref target="RFC4303"></xref>
functionality discussed in <xref target="RFC2765"></xref> are
applicable in this case as well.</t>
<t>IPv4 multicast addresses <xref target="RFC3171"></xref> cannot be
mapped to IPv6 multicast addresses <xref target="RFC3307"></xref>
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.</t>
</section>
<section anchor="prefix-stateless" title="Stateless vs. Stateful Mode">
<t>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 in either domain
and hence are forced to have their communications translated.</t>
<t>In the stateless mode, one system has an IPv4 address and one has
an address of the form specified in <xref
target="I-D.xli-behave-v4v6-prefix"></xref>, which is explicitly
mapped to an IPv4 address. 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.</t>
<t>In the stateful mode, the same address type will represent the
system with the IPv4 address, but the IPv6 system may use any <xref
target="RFC4291"></xref> address except one in that range. In this
case, a translation table is required.</t>
</section>
<section anchor="address"
title="IPv4-embedded IPv6 addresses and IPv4-related IPv6 addresses">
<t>In SIIT <xref target="RFC2765"></xref> an IPv6 node should send an
IPv6 packet where the destination address is the IPv4-mapped address
and the source address is the node's temporarily assigned
IPv4-translated address. If the node does not have a temporarily
assigned IPv4-translated address it should acquire one. Different from
the SIIT model, as described in <xref
target="I-D.xli-behave-v4v6-prefix"></xref> the new forms of the
IPv6 addresses are introduced.</t>
<t>IPv4-embedded IPv6 addresses are the IPv6 addresses which have
unique relationship to specific IPv4 addresses. This relationship is
self-described by embedding IPv4 address in the IPv6 address. The
IPv4-embedded IPv6 addresses are used for both the stateless and the
stateful modes.</t>
<t>IPv4-related IPv6 addresses are the IPv6 addresses which have
unique relationship to specific IPv4 addresses. This relationship is
maintained as session-initiated dynamic state (mapping between IPv4
address/transport port and IPv6 address/transport port) in the IP/ICMP
translator. IPv4-related IPv6 addresses are used for the stateful mode
only.</t>
</section>
</section>
<section anchor="section3" title="Translating from IPv4 to IPv6">
<t>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 <xref
target="RFC0792"></xref><xref target="RFC4443"></xref>, TCP <xref
target="RFC0793"></xref> and UDP <xref target="RFC0768"></xref> headers
contain checksums that include IP header information, the ICMP and
transport-layer headers MUST be updated. This is different from <xref
target="RFC2765"></xref>, since <xref target="RFC2765"></xref> uses
special prefix (0::ffff:0:a:b:c:d) to avoid the recalculation of the
transport-layer header checksum. 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.</t>
<figure anchor="v4v6xlat" title="IPv4-to-IPv6 Translation">
<artwork align="center"><![CDATA[
+-------------+ +-------------+
| IPv4 | | IPv6 |
| Header | | Header |
+-------------+ +-------------+
| Transport | | Fragment |
| Layer | ===> | Header |
| Header | |(not always) |
+-------------+ +-------------+
| | | Transport |
~ Data ~ | Layer |
| | | Header |
+-------------+ +-------------+
| |
~ Data ~
| |
+-------------+
]]></artwork>
</figure>
<t>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.</t>
<t>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 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 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.</t>
<t>However, when the IPv4 sender does not perform path MTU discovery 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 accomplishes this, since that is the
minimum IPv6 packet size. Also, when the IPv4 sender does not perform
path MTU discovery 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.</t>
<t>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 is carried end-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.</t>
<t>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.</t>
<section anchor="section3.1"
title="Translating IPv4 Headers into IPv6 Headers">
<t>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.</t>
<t>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 there is no need
to add a fragment header to the packet. The IPv6 header fields are set
as follows: <list style="hanging">
<t hangText="Version:">6</t>
<t hangText="Traffic Class:">By default, copied from IP Type Of
Service octet. According to <xref target="RFC2474"></xref> 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 <xref target="RFC2475"></xref> may be
applicable.</t>
<t hangText="Flow Label:">0 (all zero bits)</t>
<t hangText="Payload Length:">Total length value from IPv4 header,
minus the size of the IPv4 header and IPv4 options, if
present.</t>
<t hangText="Next Header:">Protocol field copied from IPv4
header</t>
<t hangText="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 or ICMPv6 "ttl
exceeded" error.</t>
<t hangText="Source Address:">The source address is derived from
the IPv4 source address to form an IPv4-embedded IPv6 address.</t>
<t hangText="Destination Address:">In stateless mode, which is to
say that if the IPv4 destination address is within the range of
the stateless translation prefix, the destination address is
derived from the IPv4 destination address. <vspace
blankLines="1" /> In stateful mode, which is to say that if the
IPv4 destination address is not within the range of the stateless
translation prefix, the IPv4-related IPv6 address and
corresponding transport layer destination port are derived from
the database reflecting current session state in the
translator.
Database maintanence is as descrbed in <xref target="I-D.ietf-behave-v6v4-xlate-stateful"/>.</t>
</list></t>
<t>If IPv4 options are present in the IPv4 packet, they are ignored
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.</t>
<t>If there is 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: <list style="hanging">
<t hangText="IPv6 fields:"><list style="hanging">
<t hangText="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.</t>
<t hangText="Next Header:">Fragment Header (44).</t>
</list></t>
<t hangText="Fragment header fields:"><list style="hanging">
<t hangText="Next Header:">Protocol field copied from IPv4
header.</t>
<t hangText="Fragment Offset:">Fragment Offset copied from the
IPv4 header.</t>
<t hangText="M flag">More Fragments bit copied from the IPv4
header.</t>
<t hangText="Identification">The low-order 16 bits copied from
the Identification field in the IPv4 header. The high-order 16
bits set to zero.</t>
</list></t>
</list></t>
</section>
<section anchor="section3.2" title="Translating UDP over IPv4">
<t>If a UDP packet has a zero UDP checksum then a valid checksum must
be calculated in order to translate the packet. A stateless translator
cannot do this for fragmented packets but <xref
target="Miller"></xref> indicates that fragmented UDP packets with a
zero checksum appear to only be used for malicious purposes. Thus this
is not believed to be a noticeable limitation.</t>
<t>When a 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.</t>
<t>When a translator receives an unfragmented UDP IPv4 packet and the
checksum field is zero the translator MUST 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.</t>
</section>
<section anchor="section3.3"
title="Translating ICMPv4 Headers into ICMPv6 Headers">
<t>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.</t>
<t>In addition all ICMP packets need to have the Type value translated
and for ICMP error messages the included IP header also needs
translation.</t>
<t>The actions needed to translate various ICMPv4 messages are: <list
style="hanging">
<t hangText="ICMPv4 query messages:"><list style="hanging">
<t hangText="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.</t>
<t
hangText="Information Request/Reply (Type 15 and Type 16)">Obsoleted
in ICMPv4 Silently drop.</t>
<t
hangText="Timestamp and Timestamp Reply (Type 13 and Type 14)">Obsoleted
in ICMPv6 Silently drop.</t>
<t
hangText="Address Mask Request/Reply (Type 17 and Type 18)">Obsoleted
in ICMPv6 Silently drop.</t>
<t hangText="ICMP Router Advertisement (Type 9)">Single hop
message. Silently drop.</t>
<t hangText="ICMP Router Solicitation (Type 10)">Single hop
message. Silently drop.</t>
<t hangText="Unknown ICMPv4 types">Silently drop.</t>
<t hangText="IGMP messages:">While the MLD messages <xref
target="RFC2710"></xref><xref target="RFC3590"></xref><xref
target="RFC3810"></xref> 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.</t>
<t hangText=" ICMPv4 error messages:"><list style="hanging">
<t hangText="Destination Unreachable (Type 3)">For all
that are not explicitly listed below set the Type to 1.
<vspace blankLines="1" /> Translate the code field as
follows: <list style="hanging">
<t hangText="Code 0, 1 (net, host unreachable):">Set
Code to 0 (no route to destination).</t>
<t hangText="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.</t>
<t hangText="Code 3 (port unreachable):">Set Code to 4
(port unreachable).</t>
<t
hangText="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.
Note that if the IPv4 router did not set the MTU field
i.e. the router does not implement <xref
target="RFC1191"></xref>, then the translator must use
the plateau values specified in <xref
target="RFC1191"></xref> 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.)</t>
<t hangText="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.</t>
<t hangText="Code 6,7:">Set Code to 0 (no route to
destination).</t>
<t hangText="Code 8:">Set Code to 0 (no route to
destination).</t>
<t
hangText="Code 9, 10 (communication with destination host administratively prohibited):">Set
Code to 1 (communication with destination
administratively prohibited)</t>
<t hangText="Code 11, 12:">Set Code to 0 (no route to
destination).</t>
</list></t>
<t hangText="Redirect (Type 5)">Single hop message.
Silently drop.</t>
<t hangText="Source Quench (Type 4)">Obsoleted in ICMPv6
Silently drop.</t>
<t hangText="Time Exceeded (Type 11)">Set the Type field
to 3. The Code field is unchanged.</t>
<t hangText="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.</t>
<t hangText="ICMP Error Payload">The <xref
target="RFC4884"></xref> length field should be updated to
reflect the changed length of the datagram. At the time of
this writing, the authors are not aware of any standard
ICMP extension objects containing realm specific
information.</t>
</list></t>
</list></t>
</list></t>
</section>
<section anchor="section3.4"
title="Translating ICMPv4 Error Messages into ICMPv6">
<t>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.</t>
<figure anchor="v4v6icmp" title="IPv4-to-IPv6 ICMP Error Translation">
<artwork align="center"><![CDATA[
+-------------+ +-------------+
| IPv4 | | IPv6 |
| Header | | Header |
+-------------+ +-------------+
| ICMPv4 | | ICMPv6 |
| Header | | Header |
+-------------+ +-------------+
| IPv4 | ===> | IPv6 |
| Header | | Header |
+-------------+ +-------------+
| Partial | | Partial |
| Transport | | Transport |
| Layer | | Layer |
| Header | | Header |
+-------------+ +-------------+
]]></artwork>
</figure>
<t>The translation of the inner IP header can be done by recursively
invoking the function that translated the outer IP headers.</t>
</section>
<section anchor="transport" title="Transport-layer Header Translation">
<t>For the IPv6 addresses described in <xref
target="I-D.xli-behave-v4v6-prefix"></xref>, 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.</t>
</section>
<section anchor="section3.5" title="Knowing when to Translate">
<t>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 should 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.</t>
</section>
</section>
<section anchor="section4" title="Translating from IPv6 to IPv4">
<t>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 <xref
target="RFC0792"></xref><xref target="RFC4443"></xref>, TCP <xref
target="RFC0793"></xref> and UDP <xref target="RFC0768"></xref> headers
consist of check sums, which include the IP header, the recalculation
and updating of the ICMP header and the transport-layer headers MUST be
performed. This is different from <xref target="RFC2765"></xref>, since
<xref target="RFC2765"></xref> uses special prefix (0::ffff:0:a:b:c:d)
to avoid the recalculation of the transport-layer header checksum. 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.</t>
<figure anchor="v6v4xlat" title="IPv6-to-IPv4 Translation">
<artwork align="center"><![CDATA[
+-------------+ +-------------+
| IPv6 | | IPv4 |
| Header | | Header |
+-------------+ +-------------+
| Fragment | | Transport |
| Header | ===> | Layer |
|(if present) | | Header |
+-------------+ +-------------+
| Transport | | |
| Layer | ~ Data ~
| Header | | |
+-------------+ +-------------+
| |
~ Data ~
| |
+-------------+
]]></artwork>
</figure>
<t>There are some differences between IPv6 and IPv4 in the area of
fragmentation and the minimum link MTU that effect 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 an 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, <xref target="RFC2460"></xref> 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.</t>
<t>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 sender since the presence of an IPv6 Fragment header
is interpreted that is it OK 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.</t>
<t>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.</t>
<section anchor="section4.1"
title="Translating IPv6 Headers into IPv4 Headers">
<t>If there is no IPv6 Fragment header the IPv4 header fields are set
as follows: <list style="hanging">
<t hangText="Version:">4</t>
<t hangText="Internet Header Length:">5 (no IPv4 options)</t>
<t hangText="Type of Service (TOS) Octet:">By default, copied from
the IPv6 Traffic Class (all 8 bits). According to <xref
target="RFC2474"></xref> 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 <xref target="RFC2475"></xref> may be
applicable.</t>
<t hangText="Total Length:">Payload length value from IPv6 header,
plus the size of the IPv4 header.</t>
<t hangText="Identification:">All zero.</t>
<t hangText="Flags:">The More Fragments flag is set to zero. The
Don't Fragments flag is set to one.</t>
<t hangText="Fragment Offset:">All zero.</t>
<t hangText="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 or ICMPv6 "ttl
exceeded" error.</t>
<t hangText="Protocol:">Next Header field copied from IPv6
header.</t>
<t hangText="Header Checksum:">Computed once the IPv4 header has
been created.</t>
<t hangText="Source Address:">In stateless mode, which is to say
that if the IPv6 source address is within the range of the
stateless translation prefix, the source address is derived from
the IPv4-embedded IPv6 address. <vspace blankLines="1" /> In
stateful mode, which is to say that if the IPv6 source address is
not within the range of the 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 maintanence is as descrbed in <xref target="I-D.ietf-behave-v6v4-xlate-stateful"/>.
</t>
<t hangText="Destination Address:">
The IPv4 destination address is extracted from the
IPv4-mapped destination address of the datagram being translated.</t>
</list></t>
<t>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 would have to be adjusted to "skip" these extension
headers.</t>
<t>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.</t>
<t>If the IPv6 packet contains a Fragment header the header fields are
set as above with the following exceptions: <list style="hanging">
<t hangText="Total Length:">Payload length value from IPv6 header,
minus 8 for the Fragment header, plus the size of the IPv4
header.</t>
<t hangText="Identification:">Copied from the low-order 16-bits in
the Identification field in the Fragment header.</t>
<t hangText="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.</t>
<t hangText="Fragment Offset:">Copied from the Fragment Offset
field in the Fragment Header.</t>
<t hangText="Protocol:">Next Header value copied from Fragment
header.</t>
</list></t>
</section>
<section anchor="section4.2"
title="Translating ICMPv6 Headers into ICMPv4 Headers">
<t>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.</t>
<t>In addition all ICMP packets need to have the Type value translated
and for ICMP error messages the included IP header also needs
translation.</t>
<t>The actions needed to translate various ICMPv6 messages are: <list
style="hanging">
<t hangText="ICMPv6 informational messages:"><list style="hanging">
<t
hangText="Echo Request and Echo Reply (Type 128 and 129)">Adjust
the type to 0 and 8, respectively, and adjust the ICMP
checksum both to take the type change into account and to
exclude the ICMPv6 pseudo-header.</t>
<t
hangText="MLD Multicast Listener Query/Report/Done (Type 130, 131, 132)">Single
hop message. Silently drop.</t>
<t
hangText="Neighbor Discover messages (Type 133 through 137)">Single
hop message. Silently drop.</t>
<t hangText="Unknown informational messages">Silently
drop.</t>
</list></t>
<t hangText="ICMPv6 error messages:"><list style="hanging">
<t hangText="Destination Unreachable (Type 1)">Set the Type
field to 3. Translate the code field as follows: <list
style="hanging">
<t hangText="Code 0 (no route to destination):">Set Code
to 1 (host unreachable).</t>
<t
hangText="Code 1 (communication with destination administratively prohibited):">Set
Code to 10 (communication with destination host
administratively prohibited).</t>
<t hangText="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.</t>
<t hangText="Code 3 (address unreachable):">Set Code to 1
(host unreachable).</t>
<t hangText="Code 4 (port unreachable):">Set Code to 3
(port unreachable).</t>
</list></t>
<t hangText="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.</t>
<t hangText="Time Exceeded (Type 3)">Set the Type to 11. The
Code field is unchanged.</t>
<t hangText="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 include IP header.</t>
<t hangText="Unknown error messages">Silently drop.</t>
<t hangText="ICMP Error Payload">The <xref
target="RFC4884"></xref> length field should be updated to
reflect the changed length of the datagram. At the time of
this writing, the authors are not aware of any standard ICMP
extension objects containing realm specific information.</t>
</list></t>
</list></t>
</section>
<section anchor="section4.3"
title="Translating ICMPv6 Error Messages into ICMPv4">
<t>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.</t>
<figure anchor="v6v4icmp" title="IPv6-to-IPv4 ICMP Error Translation">
<artwork align="center"><![CDATA[
+-------------+ +-------------+
| IPv6 | | IPv4 |
| Header | | Header |
+-------------+ +-------------+
| ICMPv6 | | ICMPv4 |
| Header | | Header |
+-------------+ +-------------+
| IPv6 | ===> | IPv4 |
| Header | | Header |
+-------------+ +-------------+
| Partial | | Partial |
| Transport | | Transport |
| Layer | | Layer |
| Header | | Header |
+-------------+ +-------------+
]]></artwork>
</figure>
<t>The translation of the inner IP header can be done by recursively
invoking the function that translated the outer IP headers.</t>
</section>
<section anchor="transport2" title="Transport-layer Header Translation">
<t>Stateless and stateful translation using the IPv6 addresses
described in <xref target="I-D.xli-behave-v4v6-prefix"></xref>
requires the recalculation and updating of the transport-layer
checksums.</t>
</section>
<section anchor="section4.4" title="Knowing when to Translate">
<t>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 should 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.</t>
</section>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>This memo adds no new IANA considerations.</t>
<t>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.</t>
</section>
<section anchor="Security" title="Security Considerations">
<t>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.</t>
<t>As the Authentication Header <xref target="RFC4302"></xref> is
specified to include the IPv4 Identification field and the translating
function not being able to always preserve the Identification field, it
is not possible for an IPv6 endpoint to compute AH on received packets
that have been translated from IPv4 packets. Thus AH does not work
through a translator.</t>
<t>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.</t>
</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>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, Ed Jankiewicz, Fred Baker,
Hiroshi Miyata, Iljitsch van Beijnum, John Schnizlein, Kevin Yin, Magnus
Westerlund, Marcelo Bagnulo Braun, Margaret Wasserman, Masahito Endo,
Phil Roberts, Philip Matthews, Remi Denis-Courmont, Remi Despres, and
Xing Li.</t>
</section>
</middle>
<back>
<!-- references split to informative and normative -->
<references title="Normative References">
<?rfc include="reference.RFC.0768" ?>
<?rfc include="reference.RFC.0791" ?>
<?rfc include="reference.RFC.0792" ?>
<?rfc include="reference.RFC.0793" ?>
<?rfc include="reference.RFC.2119" ?>
<?rfc include="reference.RFC.2460" ?>
<?rfc include="reference.RFC.2765" ?>
<?rfc include="reference.RFC.4291" ?>
<?rfc include="reference.RFC.4443" ?>
<?rfc include="reference.RFC.5382" ?>
<?rfc include="reference.RFC.4884" ?>
<?rfc include="reference.I-D.xli-behave-v4v6-prefix" ?>
</references>
<references title="Informative References">
<?rfc include="reference.I-D.ietf-behave-v6v4-xlate-stateful" ?>
<?rfc include="reference.I-D.ietf-behave-v6v4-framework" ?>
<?rfc include="reference.RFC.1191" ?>
<?rfc include="reference.RFC.2474" ?>
<?rfc include="reference.RFC.2475" ?>
<?rfc include="reference.RFC.2710" ?>
<?rfc include="reference.RFC.3171" ?>
<?rfc include="reference.RFC.3307" ?>
<?rfc include="reference.RFC.3590" ?>
<?rfc include="reference.RFC.3810" ?>
<?rfc include="reference.RFC.4213" ?>
<?rfc include="reference.RFC.4301" ?>
<?rfc include="reference.RFC.4302" ?>
<?rfc include="reference.RFC.4303" ?>
<reference anchor="Miller">
<front>
<title>Email to the ngtrans mailing list</title>
<author fullname="Miller" initials="G" surname="Miller">
<organization></organization>
</author>
<date day="26" month="March" year="1999" />
</front>
</reference>
</references>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-24 02:49:06 |