As per Relevance of the word destination, we have this rfc below:
Network Working Group E.
Request for Comments: 2765 Sun
Category: Standards Track February 2000
Stateless IP/ICMP Translation Algorithm (SIIT
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited
Copyright
Copyright (C) The Internet Society (2000). All Rights Reserved
This document specifies a transition mechanism algorithm in
to the mechanisms already specified in [TRANS-MECH]. The
translates between IPv4 and IPv6 packet headers (including
headers) in separate translator "boxes" in the network
requiring any per-connection state in those "boxes". This
algorithm can be used as part of a solution that allows IPv6 hosts
which do not have a permanently assigned IPv4 addresses,
communicate with IPv4-only hosts. The document neither
address assignment nor routing to and from the IPv6 hosts when
communicate with the IPv4-only hosts
This document is a product of the NGTRANS working group. Some
has been extracted from an old Internet Draft titled "IPAE: The
Interoperability and Transition Mechanism" authored by R. Gilligan
E. Nordmark, and B. Hinden. George Tsirtsis provides the figures
Section 1. Keith Moore provided a careful review of the document
Nordmark Standards Track [Page 1]
RFC 2765 SIIT February 2000
Table of
1. Introduction and Motivation.............................. 2
1.1. Applicability and Limitations....................... 5
1.2. Assumptions......................................... 7
1.3. Impact Outside the Network Layer.................... 7
2. Terminology.............................................. 8
2.1. Addresses........................................... 9
2.2. Requirements........................................ 9
3. Translating from IPv4 to IPv6............................ 9
3.1. Translating IPv4 Headers into IPv6 Headers.......... 11
3.2. Translating UDP over IPv4........................... 13
3.3. Translating ICMPv4 Headers into ICMPv6 Headers...... 13
3.4. Translating ICMPv4 Error Messages into ICMPv6....... 16
3.5. Knowing when to Translate........................... 16
4. Translating from IPv6 to IPv4............................ 17
4.1. Translating IPv6 Headers into IPv4 Headers.......... 18
4.2. Translating ICMPv6 Headers into ICMPv4 Headers...... 20
4.3. Translating ICMPv6 Error Messages into ICMPv4....... 22
4.4. Knowing when to Translate........................... 22
5. Implications for IPv6-Only Nodes......................... 22
6. Security Considerations.................................. 23
References................................................... 24
Author's Address............................................. 25
Full Copyright Statement..................................... 26
1. Introduction and
The transition mechanisms specified in [TRANS-MECH] handle the
of dual IPv4/IPv6 hosts interoperating with both dual hosts
IPv4-only hosts, which is needed early in the transition to IPv6.
The dual hosts are assigned both an IPv4 and one or more IPv
addresses. As the number of available globally unique IPv4
becomes smaller and smaller as the Internet grows there will be
desire to take advantage of the large IPv6 address and not
that every new Internet node have a permanently assigned IPv
address
There are several different scenarios where there might be IPv6-
hosts that need to communicate with IPv4-only hosts. These IPv
hosts might be IPv4-capable, i.e. include an IPv4 implementation
not be assigned an IPv4 address, or they might not even include
IPv4 implementation
- A completely new network with new devices that all support IPv6.
In this case it might be beneficial to not have to configure
routers within the new network to route IPv4 since none of
Nordmark Standards Track [Page 2]
RFC 2765 SIIT February 2000
hosts in the new network are configured with IPv4 addresses.
these new IPv6 devices might occasionally need to communicate
some IPv4 nodes out on the Internet
- An existing network where a large number of IPv6 devices
added. The IPv6 devices might have both an IPv4 and an IPv
protocol stack but there is not enough global IPv4 address
to give each one of them a permanent IPv4 address. In this
it is more likely that the routers in the network already
IPv4 and are upgraded to dual routers
However, there are other potential solutions in this area
- If there is no IPv4 routing inside the network i.e., the
that contains the new devices, some possible solutions are
either use the translators specified in this document at
boundary of the cloud, or to use Application Layer Gateways (ALG
on dual nodes at the cloud's boundary. The ALG solution is
flexible in that it is application protocol specific and it
also less robust since an ALG box is likely to be a single
of failure for a connection using that box
- Otherwise, if IPv4 routing is supported inside the cloud and
implementations support both IPv6 and IPv4 it might suffice
have a mechanism for allocating a temporary address IPv4 and
IPv4 end to end when communicating with IPv4-only nodes. However
it would seem that such a solution would require the pool
temporary IPv4 addresses to be partitioned across all the
in the cloud which would either require a larger pool of IPv
addresses or result in cases where communication would fail due
no available IPv4 address for the node's subnet
This document specifies an algorithm that is one of the
needed to make IPv6-only nodes interoperate with IPv4-only nodes
Other components, not specified in this document, are a mechanism
the IPv6-only node to somehow acquire a temporary IPv4 address, and
mechanism for providing routing (perhaps using tunneling) to and
the temporary IPv4 address assigned to the node
The temporary IPv4 address will be used as an IPv4-translated IPv
address and the packets will travel through a stateless IP/
translator that will translate the packet headers between IPv4
IPv6 and translate the addresses in those headers between IPv
addresses on one side and IPv4-translated or IPv4-mapped IPv
addresses on the other side
Nordmark Standards Track [Page 3]
RFC 2765 SIIT February 2000
This specification does not cover how an IPv6 node can acquire
temporary IPv4 address and how such a temporary address be
in the DNS. The DHCP protocol, perhaps with some extensions,
probably be used to acquire temporary addresses with short leases
that is outside the scope of this document. Also, the mechanism
routing this IPv4-translated IPv6 address in the site is
specified in this document
The figures below show how the Stateless IP/ICMP
algorithm (SIIT) can be used initially for small networks (e.g.,
single subnet) and later for a site which has IPv6-only hosts in
dual IPv4/IPv6 network. This use assumes a mechanism for the IPv
nodes to acquire a temporary address from the pool of IPv4 addresses
Note that SIIT is not likely to be useful later during
when most of the Internet is IPv6 and there are only small islands
IPv4 nodes, since such use would either require the IPv6 nodes
acquire temporary IPv4 addresses from a "distant" SIIT box
by a different administration, or require that the IPv6
contain routes for IPv6-mapped addresses. (The latter is known to
a very bad idea due to the size of the IPv4 routing table that
potentially be injected into IPv6 routing in the form of IPv4-
addresses.)
___________
/ \
[IPv6 Host]---[SIIT]---------< IPv4 network>--[IPv4 Host
| \___________/
(pool of IPv4 addresses
IPv4-translatable -> IPv4->IPv4
IPv4-
Figure 1. Using SIIT for a single IPv6-only subnet
___________ ___________
/ \ / \
[IPv6 Host]--< Dual network>--[SIIT]--< IPv4 network>--[IPv4 Host
\___________/ | \___________/
(pool of IPv4 addresses
IPv4-translatable -> IPv4->IPv4
IPv4-
Figure 2. Using SIIT for an IPv6-only or dual cloud (e.g. a site
which contains some IPv6-only hosts as well as IPv4 hosts
Nordmark Standards Track [Page 4]
RFC 2765 SIIT February 2000
The protocol translators are assumed to fit around some piece
topology that includes some IPv6-only nodes and that may also
IPv4 nodes as well as dual nodes. There has to be a translator
each path used by routing the "translatable" packets in and out
this cloud to ensure that such packets always get translated.
does not require a translator at every physical connection
the cloud and the rest of the Internet since the routing can be
to deliver the packets to the translator
The IPv6-only node communicating with an IPv4 node through
translator will see an IPv4-mapped address for the peer and use
IPv4-translatable address for its local address for
communication. When the IPv6-only node sends packets the IPv4-
address indicates that the translator needs to translate the packets
When the IPv4 node sends packets those will translated to have
IPv4-translatable address as a destination; it is not possible to
an IPv4-mapped or an IPv4-compatible address as a destination
that would either route the packet back to the translator (for
IPv4-mapped address) or make the packet be encapsulated in IPv4 (
the IPv4-compatible address). Thus this specification introduces
new notion of an IPv4-translatable address
1.1. Applicability and
The use of this translation algorithm assumes that the IPv6
is somehow well connected i.e. when an IPv6 node wants to
with another IPv6 node there is an IPv6 path between them.
tunneling schemes exist that can provide such a path, but
mechanisms and their use is outside the scope of this document
The IPv6 protocol [IPv6] has been designed so that the TCP and
pseudo-header checksums are not affected by the
specified in this document, thus the translator does not need
modify normal TCP and UDP headers. The only exceptions
unfragmented IPv4 UDP packets which need to have a UDP
computed since a pseudo-header checksum is required for UDP in IPv6.
Also, ICMPv6 include a pseudo-header checksum but it is not
in ICMPv4 thus the checksum in ICMP messages need to be modified
the translator. In addition, ICMP error messages contain an
header as part of the payload thus the translator need to
those parts of the packets to make the receiver be able to
the included IP header. However, all of the translator's operations
including path MTU discovery, are stateless in the sense that
translator operates independently on each packet and does not
any state from one packet to another. This allows
translator boxes without any coordination and a given TCP
can have the two directions of packets go through
translator boxes
Nordmark Standards Track [Page 5]
RFC 2765 SIIT February 2000
The translating function as specified in this document does
translate any IPv4 options and it does not translate IPv6
headers, hop-by-hop extension headers, or destination
headers. It could be possible to define a translation between
routing in IPv4 and IPv6. However such a translation would not
semantically correct due to the slight differences between the IPv
and IPv6 source routing. Also, the usefulness of source routing
going through a header translator might be limited since all
IPv6-only routers would need to have an IPv4-translated IPv6
since the IPv4-only node will send a source route option
only IPv4 addresses
At first sight it might appear that the IPsec functionality [IPv6-SA
IPv6-ESP, IPv6-AH] can not be carried across the translator
However, since the translator does not modify any headers above
logical IP layer (IP headers, IPv6 fragment headers, and
messages) packets encrypted using ESP in Transport-mode can
carried through the translator. [Note that this assumes that the
management can operate between the IPv6-only node and the IPv4-
node.] The AH computation covers parts of the IPv4 header
such as IP addresses, and the identification field (fields that
either immutable or predictable by the sender) [IPv6-AUTH].
the SIIT algorithm is specified so that those IPv4 fields can
predicted by the IPv6 sender it is not possible for the IPv6
to determine the value of the IPv4 Identification field in
sent by the IPv4 node. Thus as the translation algorithm
specified in this document it is not possible to use end-to-end
through the translator
For ESP Tunnel-mode to work through the translator the IPv6
would have to be able to both parse and generate "inner" IPv4
since the inner IP will be encrypted together with the
protocol
Thus in practise, only ESP transport mode is relatively easy to
work through a translator
IPv4 multicast addresses can not be mapped to IPv6
addresses. For instance, ::ffff:224.1.2.3 is an IPv4 mapped IPv
address with a class D address, however it is not an IPv6
address. While the IP/ICMP header translation aspect of this memo
theory works for multicast packets this address mapping
makes it impossible to apply the techniques in this memo
multicast traffic
Nordmark Standards Track [Page 6]
RFC 2765 SIIT February 2000
1.2.
The IPv6 nodes using the translator must have an IPv4-translated IPv
address while it is communicating with IPv4-only nodes
The use of the algorithm assumes that there is an IPv4 address
used to generate IPv4-translated addresses. Routing needs to be
to route any IPv4 packets, whether generated "outside" or "inside
the translator, destined to addresses in this pool towards
translator. This implies that the address pool can not be
to subnets but must be separated from the IPv4 subnets used on
"inside" of the translator
Fragmented IPv4 UDP packets that do not contain a UDP checksum (i.e
the UDP checksum field is zero) are not of significant use
wide-areas in the Internet and will not be translated by
translator. An informal trace [MILLER] in the backbone showed
out of 34,984,468 IP packets there were 769 fragmented UDP
with a zero checksum. However, all of them were due to malicious
broken behavior; a port scan and first fragments of IP packets
are not a multiple of 8 bytes
1.3. Impact Outside the Network
The potential existence of stateless IP/ICMP translators is
taken care of from a protocol perspective in [IPv6]. However,
IPv6 node that wants to be able to use translators needs
additional logic in the network layer
The network layer in an IPv6-only node, when presented by
application with either an IPv4 destination address or an IPv4-
IPv6 destination address, is likely to drop the packet and
some error message to the application. In order to take advantage
translators such a node should instead send an IPv6 packet where
destination address is the IPv4-mapped address and the source
is the node's temporarily assigned IPv4-translated address. If
node does not have a temporarily assigned IPv4-translated address
should acquire one using mechanisms that are not discussed in
document
Note that the above also applies to a dual IPv4/IPv6
node which is not configured with any IPv4 address
There are no extra changes needed to applications to operate
a translator beyond what applications already need to do to
on a dual node. The applications that have been modified to work
a dual node already have the mechanisms to determine whether they
communicating with an IPv4 or an IPv6 peer. Thus if the
Nordmark Standards Track [Page 7]
RFC 2765 SIIT February 2000
need to modify their behavior depending on the type of the peer,
as ftp determining whether to fallback to using the PORT/PASV
when EPRT/EPSV fails (as specified in [FTPEXT]), they already need
do that when running on dual nodes and the presense of
does not add anything. For example, when using the socket
[BSDAPI] the applications know that the peer is IPv6 if they get
AF_INET6 address from the name service and the address is not
IPv4-mapped address (i.e., IN6_IS_ADDR_V4MAPPED returns false).
this is not the case, i.e., the address is AF_INET or an IPv4-
IPv6 address, the peer is IPv4.
One way of viewing the translator, which might help clarify
applications do not need to know that a translator is used, is
look at the information that is passed from the transport layer
the network layer. If the transport passes down an IPv4
(whether or not is in the IPv4-mapped encoding) this means that
some point there will be IPv4 packets generated. In a dual node
generation of the IPv4 packets takes place in the sending node.
an IPv6-only node conceptually the only difference is that the IPv
packet is generated by the translator - all the information that
transport layer passed to the network layer will be conveyed to
translator in some form. That form just "happens" to be in the
of an IPv6 header
2.
This documents uses the terminology defined in [IPv6]
[TRANS-MECH] with these clarifications
IPv4 capable node
A node which has an IPv4 protocol stack
In order for the stack to be usable the node must
assigned one or more IPv4 addresses
IPv4 enabled node
A node which has an IPv4 protocol
and is assigned one or more IPv4 addresses.
IPv4-only and IPv6/IPv4 nodes are IPv4 enabled
IPv6 capable node
A node which has an IPv6 protocol stack
In order for the stack to be usable the node must
assigned one or more IPv6 addresses
IPv6 enabled node
A node which has an IPv6 protocol
and is assigned one or more IPv6 addresses.
IPv6-only and IPv6/IPv4 nodes are IPv6 enabled
Nordmark Standards Track [Page 8]
RFC 2765 SIIT February 2000
2.1.
In addition to the forms of addresses defined in [ADDR-ARCH]
document also introduces the new form of IPv4-translated address
This is needed to avoid using IPv4-compatible addresses outside
intended use of automatic tunneling. Thus the address forms are
IPv4-mapped
An address of the form 0::ffff:a.b.c.d which
to a node that is not IPv6-capable. In addition
its use in the API this protocol uses IPv4-
addresses in IPv6 packets to refer to an IPv4 node
IPv4-compatible
An address of the form 0::0:a.b.c.d which refers
an IPv6/IPv4 node that supports automatic tunneling
Such addresses are not used in this protocol
IPv4-translated
An address of the form 0::ffff:0:a.b.c.d which
to an IPv6-enabled node. Note that the
0::ffff:0:0:0/96 is chosen to checksum to zero
avoid any changes to the transport protocol's
header checksum
2.2.
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in
document, are to be interpreted as described in [KEYWORDS].
3. Translating from IPv4 to IPv
When an IPv4-to-IPv6 translator receives an IPv4 datagram
to a destination that lies outside of the attached IPv4 island,
translates the IPv4 header of that packet into an IPv6 header.
then forwards the packet based on the IPv6 destination address.
original IPv4 header on the packet is removed and replaced by an IPv
header. Except for ICMP packets the transport layer header and
portion of the packet are left unchanged
Nordmark Standards Track [Page 9]
RFC 2765 SIIT February 2000
+-------------+ +-------------+
| IPv4 | | IPv6 |
| Header | | Header |
+-------------+ +-------------+
| Transport | | Fragment |
| Layer | ===> | Header |
| Header | |(not always) |
+-------------+ +-------------+
| | | Transport |
~ Data ~ | Layer |
| | | Header |
+-------------+ +-------------+
| |
~ Data ~
| |
+-------------+
IPv4-to-IPv6
One of the differences between IPv4 and IPv6 is that in IPv6 path
discovery is mandatory but it is optional in IPv4. This implies
IPv6 routers will never fragment a packet - only the sender can
fragmentation
When the IPv4 node performs path MTU discovery (by setting the DF
in the header) the path MTU discovery can operate end-to-end i.e
across the translator. In this case either IPv4 or IPv6
might send back ICMP "packet too big" messages to the sender.
these ICMP errors are sent by the IPv6 routers they will pass
a translator which will translate the ICMP error to a form that
IPv4 sender can understand. In this case an IPv6 fragment header
only included if the IPv4 packet is already fragmented
However, when the IPv4 sender does not perform path MTU discovery
translator has to ensure that the packet does not exceed the path
on the IPv6 side. This is done by fragmenting the IPv4 packet
that it fits in 1280 byte IPv6 packet since IPv6 guarantees that 1280
byte packets never need to be fragmented. Also, when the IPv4
does not perform path MTU discovery the translator MUST
include an IPv6 fragment header to indicate that the sender
fragmentation. That is needed should the packet pass through
IPv6-to-IPv4 translator
The above rules ensure that when packets are fragmented either by
sender or by IPv4 routers that the low-order 16 bits of the
identification is carried end-end to ensure that packets
correctly reassembled. In addition, the rules use the presence of
Nordmark Standards Track [Page 10]
RFC 2765 SIIT February 2000
IPv6 fragment header to indicate that the sender might not be
path MTU discovery i.e. the packet should not have the DF flag
should it later be translated back to IPv4.
Other than the special rules for handling fragments and path
discovery the actual translation of the packet header consists of
simple mapping as defined below. Note that ICMP packets
special handling in order to translate the content of ICMP
message and also to add the ICMP pseudo-header checksum
3.1. Translating IPv4 Headers into IPv6
If the DF flag is not set and the IPv4 packet will result in an IPv
packet larger than 1280 bytes the IPv4 packet MUST be
prior to translating it. Since IPv4 packets with DF not set
always result in a fragment header being added to the packet the IPv
packets must be fragmented so that their length, excluding the IPv
header, is at most 1232 bytes (1280 minus 40 for the IPv6 header
8 for the Fragment header). The resulting fragments are
translated independently using the logic described below
If the DF bit is set and the packet is not a fragment (i.e., the
flag is not set and the Fragment Offset is zero) then there is
need to add a fragment header to the packet. The IPv6 header
are set as follows
Version
6
Traffic Class
By default, copied from IP Type Of Service
Precedence field (all 8 bits are copied).
to [DIFFSERV] the semantics of the bits are
in IPv4 and IPv6. However, in some IPv4
these fields might be used with the old semantics
"Type Of Service and Precedence". An
of a translator SHOULD provide the ability to
the IPv4 "TOS" and always set the IPv6 traffic
to zero
Flow Label
0 (all zero bits
Payload Length
Total length value from IPv4 header, minus the
of the IPv4 header and IPv4 options, if present
Nordmark Standards Track [Page 11]
RFC 2765 SIIT February 2000
Next Header
Protocol field copied from IPv4
Hop Limit
TTL value copied from IPv4 header. Since
translator is a router, as part of forwarding
packet it needs to decrement either the IPv4
(before the translation) or the IPv6 Hop Limit (
the translation). As part of decrementing the TTL
Hop Limit the translator (as any router) needs
check for zero and send the ICMPv4 or ICMPv6 "
exceeded" error
Source Address
The low-order 32 bits is the IPv4 source address
The high-order 96 bits is the IPv4-mapped
(::ffff:0:0/96)
Destination Address
The low-order 32 bits is the IPv4
address. The high-order 96 bits is the IPv4-
translated prefix (0::ffff:0:0:0/96)
If IPv4 options are present in the IPv4 packet, they are
i.e., there is no attempt to translate them. However, if
unexpired source route option is present then the packet MUST
be discarded, and an ICMPv4 "destination unreachable/source
failed" (Type 3/Code 5) error message SHOULD be returned to
sender
If there is need to add a fragment header (the DF bit is not set
the packet is a fragment) the header fields are set as above with
following exceptions
IPv6 fields
Payload Length
Total length value from IPv4 header, plus 8 for
fragment header, minus the size of the IPv4
and IPv4 options, if present
Next Header
Fragment Header (44).
Fragment header fields
Next Header
Protocol field copied from IPv4 header
Nordmark Standards Track [Page 12]
RFC 2765 SIIT February 2000
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
field in the IPv4 header. The high-order 16 bits
to zero
3.2. Translating UDP over IPv
If a UDP packet has a zero UDP checksum then a valid checksum must
calculated in order to translate the packet. A stateless
can not do this for fragmented packets but [MILLER] indicates
fragmented UDP packets with a zero checksum appear to only be
for malicious purposes. Thus this is not believed to be a
limitation
When a translator receives the first fragment of a fragmented
IPv4 packet and the checksum field is zero the translator SHOULD
the packet and generate a system management event specifying at
the IP addresses and port numbers in the packet. When it
fragments other than the first it SHOULD silently drop the packet
since there is no port information to log
When a translator receives an unfragmented UDP IPv4 packet and
checksum field is zero the translator MUST compute the missing
checksum as part of translating the packet. Also, the
SHOULD maintain a counter of how many UDP checksums are generated
this manner
3.3. Translating ICMPv4 Headers into ICMPv6
All ICMP messages that are to be translated require that the
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
and for ICMP error messages the included IP header also
translation
Nordmark Standards Track [Page 13]
RFC 2765 SIIT February 2000
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
ICMP checksum both to take the type change into account
to include the ICMPv6 pseudo-header
Information Request/Reply (Type 15 and Type 16)
Obsoleted in ICMPv4. 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
Silently drop
IGMP messages
While the MLD messages [MLD] are the logical IPv
counterparts for the IPv4 IGMP messages all the "normal"
messages are single-hop messages and should be
dropped by the translator. Other IGMP messages might be
by multicast routing protocols and, since it would be
configuration error to try to have router adjacencies
IPv4/IPv6 translators those packets should also be
dropped
ICMPv4 error messages
Destination Unreachable (Type 3)
For all that are not explicitly listed below set the Type
1.
Translate the code field as follows
Code 0, 1 (net, host unreachable):
Set Code to 0 (no route to destination).
Nordmark Standards Track [Page 14]
RFC 2765 SIIT February 2000
Code 2 (protocol unreachable):
Translate to an ICMPv6 Parameter Problem (Type 4,
Code 1) and make the Pointer point to the IPv6
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 (
2) with code 0. The MTU field needs to be
for the difference between the IPv4 and IPv6
sizes. Note that if the IPv4 router did not
the MTU field i.e. the router does not
[PMTUv4], then the translator must use the
values specified in [PMTUv4] to determine a
path MTU and include that path MTU in the ICMPv
packet. (Use the greatest plateau value that
less than the returned Total Length field.)
Code 5 (source route failed):
Set Code to 0 (no route to destination). Note
this error is unlikely since source routes are
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
administratively prohibited):
Set Code to 1 (communication with
administratively prohibited
Code 11, 12:
Set Code to 0 (no route to destination).
Redirect (Type 5)
Single hop message. Silently drop
Source Quench (Type 4)
Obsoleted in ICMPv6. Silently drop
Time Exceeded (Type 11)
Set the Type field to 3. The Code field is unchanged
Nordmark Standards Track [Page 15]
RFC 2765 SIIT February 2000
Parameter Problem (Type 12)
Set the Type field to 4. The Pointer needs to be updated
point to the corresponding field in the translated
IP header
3.4. Translating ICMPv4 Error Messages into ICMPv
There are some differences between the IPv4 and the IPv6 ICMP
message formats as detailed above. In addition, the ICMP
messages contain the IP header for the packet in error which needs
be translated just like a normal IP header. The translation of
"packet in error" is likely to change the length of the datagram
the Payload Length field in the outer IPv6 header might need to
updated
+-------------+ +-------------+
| IPv4 | | IPv6 |
| Header | | Header |
+-------------+ +-------------+
| ICMPv4 | | ICMPv6 |
| Header | | Header |
+-------------+ +-------------+
| IPv4 | ===> | IPv6 |
| Header | | Header |
+-------------+ +-------------+
| Partial | | Partial |
| Transport | | Transport |
| Layer | | Layer |
| Header | | Header |
+-------------+ +-------------+
IPv4-to-IPv6 ICMP Error
The translation of the inner IP header can be done by
invoking the function that translated the outer IP headers
3.5. Knowing when to
The translator is assumed to know the pool(s) of IPv4 address
are used to represent the internal IPv6-only nodes. Thus if the IPv
destination field contains an address that falls in these
sets of prefixes the packet needs to be translated to IPv6.
Nordmark Standards Track [Page 16]
RFC 2765 SIIT February 2000
4. Translating from IPv6 to IPv
When an IPv6-to-IPv4 translator receives an IPv6 datagram
to an IPv4-mapped IPv6 address, it translates the IPv6 header of
packet into an IPv4 header. It then forwards the packet based on
IPv4 destination address. The original IPv6 header on the packet
removed and replaced by an IPv4 header. Except for ICMP packets
transport layer header and data portion of the packet are
unchanged
+-------------+ +-------------+
| IPv6 | | IPv4 |
| Header | | Header |
+-------------+ +-------------+
| Fragment | | Transport |
| Header | ===> | Layer |
|(if present) | | Header |
+-------------+ +-------------+
| Transport | | |
| Layer | ~ Data ~
| Header | | |
+-------------+ +-------------+
| |
~ Data ~
| |
+-------------+
IPv6-to-IPv4
There are some differences between IPv6 and IPv4 in the area
fragmentation and the minimum link MTU that effect the translation
An IPv6 link has to have an MTU of 1280 bytes or greater.
corresponding limit for IPv4 is 68 bytes. Thus, unless there
special measures, it would not be possible to do end-to-end path
discovery when the path includes an IPv6-to-IPv4 translator since
IPv6 node might receive ICMP "packet too big" messages originated
an IPv4 router that report an MTU less than 1280. However, [IPv6]
requires that IPv6 nodes handle such an ICMP "packet too big"
by reducing the path MTU to 1280 and including an IPv6
header with each packet. This allows end-to-end path MTU
across the translator as long as the path MTU is 1280 bytes
greater. When the path MTU drops below the 1280 limit the IPv
sender will originate 1280 byte packets that will be fragmented
IPv4 routers along the path after being translated to IPv4.
The only drawback with this scheme is that it is not possible to
PMTU to do optimal UDP fragmentation (as opposed to
avoiding fragmentation) at sender since the presence of an IPv
Nordmark Standards Track [Page 17]
RFC 2765 SIIT February 2000
Fragment header is interpreted that is it OK to fragment the
on the IPv4 side. Thus if a UDP application wants to send
packets independent of the PMTU, the sender will only be able
determine the path MTU on the IPv6 side of the translator. If
path MTU on the IPv4 side of the translator is smaller then the IPv
sender will not receive any ICMP "too big" errors and can not
the size fragments it is sending
Other than the special rules for handling fragments and path
discovery the actual translation of the packet header consists of
simple mapping as defined below. Note that ICMP packets
special handling in order to translate the content of ICMP
message and also to add the ICMP pseudo-header checksum
4.1. Translating IPv6 Headers into IPv4
If there is no IPv6 Fragment header the IPv4 header fields are set
follows
Version
4
Internet Header Length
5 (no IPv4 options
Type of Service and Precedence
By default, copied from the IPv6 Traffic Class (all 8
bits). According to [DIFFSERV] the semantics of
bits are identical in IPv4 and IPv6. However,
some IPv4 environments these bits might be used
the old semantics of "Type Of Service
Precedence". An implementation of a
SHOULD provide the ability to ignore the IPv6
class and always set the IPv4 "TOS" to zero
Total Length
Payload length value from IPv6 header, plus the
of the IPv4 header
Identification
All zero
Flags
The More Fragments flag is set to zero. The Don'
Fragments flag is set to one
Fragment Offset
All zero
Nordmark Standards Track [Page 18]
RFC 2765 SIIT February 2000
Time to Live
Hop Limit value copied from IPv6 header. Since
translator is a router, as part of forwarding
packet it needs to decrement either the IPv6
Limit (before the translation) or the IPv4 TTL (
the translation). As part of decrementing the TTL
Hop Limit the translator (as any router) needs
check for zero and send the ICMPv4 or ICMPv6 "
exceeded" error
Protocol
Next Header field copied from IPv6 header
Header Checksum
Computed once the IPv4 header has been created
Source Address
If the IPv6 source address is an IPv4-
address then the low-order 32 bits of the IPv6
address is copied to the IPv4 source address
Otherwise, the source address is set to 0.0.0.0.
use of 0.0.0.0 is to avoid completely dropping e.g
ICMPv6 error messages sent by IPv6-only routers
makes e.g. traceroute present something for
IPv6-only hops
Destination Address
IPv6 packets that are translated have an IPv4-
destination address. Thus the low-order 32 bits
the IPv6 destination address is copied to the IPv
destination address
If any of an IPv6 hop-by-hop options header, destination
header, or routing header with the Segments Left field equal to
are present in the IPv6 packet, they are ignored i.e., there is
attempt to translate them. However, the Total Length field and
Protocol field would have to be adjusted to "skip" these
headers
If a routing header with a non-zero Segments Left field is
then the packet MUST NOT be translated, and an ICMPv6 "
problem/ erroneous header field encountered" (Type 4/Code 0)
message, with the Pointer field indicating the first byte of
Segments Left field, SHOULD be returned to the sender
Nordmark Standards Track [Page 19]
RFC 2765 SIIT February 2000
If the IPv6 packet contains a Fragment header the header fields
set as above with the following exceptions
Total Length
Payload length value from IPv6 header, minus 8
the Fragment header, plus the size of the IPv
header
Identification
Copied from the low-order 16-bits in
Identification field in the Fragment header
Flags
The More Fragments flag is copied from the M flag
the Fragment header. The Don't Fragments flag is
to zero allowing this packet to be fragmented by IPv
routers
Fragment Offset
Copied from the Fragment Offset field in the
Header
Protocol
Next Header value copied from Fragment header
4.2. Translating ICMPv6 Headers into ICMPv4
All ICMP messages that are to be translated require that the
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
and for ICMP error messages the included IP header also
translation
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 0 and 8, respectively, and adjust the
checksum both to take the type change into account and
exclude the ICMPv6 pseudo-header
MLD Multicast Listener Query/Report/Done (Type 130, 131, 132)
Single hop message. Silently drop
Nordmark Standards Track [Page 20]
RFC 2765 SIIT February 2000
Neighbor Discover messages (Type 133 through 137)
Single hop message. Silently drop
Unknown informational
Silently drop
ICMPv6 error messages
Destination Unreachable (Type 1)
Set the Type field to 3. Translate the code field
follows
Code 0 (no route to destination):
Set Code to 1 (host unreachable).
Code 1 (communication with destination
prohibited):
Set Code to 10 (communication with destination
administratively prohibited).
Code 2 (beyond scope of source address):
Set Code to 1 (host unreachable). Note that
error is very unlikely since the IPv4-
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
the IPv4 and IPv6 header sizes taking into account whether
not the packet in error includes a Fragment header
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
unreachable (Type 3, Code 2). Otherwise set the Type to 12
and the Code to zero. The Pointer needs to be updated
point to the corresponding field in the translated include
header
Unknown error
Silently drop
Nordmark Standards Track [Page 21]
RFC 2765 SIIT February 2000
4.3. Translating ICMPv6 Error Messages into ICMPv
There are some differences between the IPv4 and the IPv6 ICMP
message formats as detailed above. In addition, the ICMP
messages contain the IP header for the packet in error which needs
be translated just like a normal IP header. The translation of
"packet in error" is likely to change the length of the datagram
the Total Length field in the outer IPv4 header might need to
updated
+-------------+ +-------------+
| IPv6 | | IPv4 |
| Header | | Header |
+-------------+ +-------------+
| ICMPv6 | | ICMPv4 |
| Header | | Header |
+-------------+ +-------------+
| IPv6 | ===> | IPv4 |
| Header | | Header |
+-------------+ +-------------+
| Partial | | Partial |
| Transport | | Transport |
| Layer | | Layer |
| Header | | Header |
+-------------+ +-------------+
IPv6-to-IPv4 ICMP Error
The translation of the inner IP header can be done by
invoking the function that translated the outer IP headers
4.4. Knowing when to
When the translator receives an IPv6 packet with an IPv4-
destination address the packet will be translated to IPv4.
5. Implications for IPv6-Only
An IPv6-only node which works through SIIT translators need
modifications beyond a normal IPv6-only node
As specified in Section 1.3 the application protocols need to
operation on a dual stack node. In addition the protocol stack
to be able to
Nordmark Standards Track [Page 22]
RFC 2765 SIIT February 2000
o Determine when an IPv4-translatable address needs to be
and the allocation needs to be refreshed/renewed. This
presumably be done without involving the applications by e.g
handling this under the socket API. For instance, when
connect or sendto socket calls are invoked they could check if
destination is an IPv4-mapped address and in that
allocate/refresh the IPv4-translatable address
o Ensure, as part of the source address selection mechanism,
when the destination address is an IPv4-mapped address the
address MUST be an IPv4-translatable address. And an IPv4-
translatable address MUST NOT be used with other forms of IPv
destination addresses
o Should the peer have AAAA/A6 address records the application (
resolver) SHOULD never fall back to looking for A address
even if communication fails using the available AAAA/A6 records
The reason for this restriction is to prevent traffic between
IPv6 nodes (which AAAA/A6 records in the DNS) from
going through SIIT translators twice; from IPv6 to IPv4 and
IPv6 again. It is considered preferable to instead signal
failure to communicate to the application
6. Security
The use of stateless IP/ICMP translators does not introduce any
security issues beyond the security issues that are already
in the IPv4 and IPv6 protocols and in the routing protocols which
used to make the packets reach the translator
As the Authentication Header [IPv6-AUTH] is specified to include
IPv4 Identification field and the translating function not being
to always preserve the Identification field, it is not possible
an IPv6 endpoint to compute AH on received packets that have
translated from IPv4 packets. Thus AH does not work through
translator
Packets with ESP can be translated since ESP does not depend
header fields prior to the ESP header. Note that ESP transport
is easier to handle than ESP tunnel mode; in order to use ESP
mode the IPv6 node needs to be able to generate an inner IPv4
when transmitting packets and remove such an IPv4 header
receiving packets
Nordmark Standards Track [Page 23]
RFC 2765 SIIT February 2000
[KEYWORDS] Bradner, S., "Key words for use in RFCs to
Requirement Levels", BCP 14, RFC 2119, March 1997.
[IPv6] Deering, S. and R. Hinden, Editors, "Internet Protocol
Version 6 (IPv6) Specification", RFC 2460,
1998.
[IPv4] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[ADDR-ARCH] Deering, S. and R. Hinden, Editors, "IP Version 6
Addressing Architecture", RFC 2373, July 1998.
[TRANS-MECH] Gilligan, R. and E. Nordmark, "Transition Mechanisms
IPv6 Hosts and Routers", RFC 1933, April 1996.
[DISCOVERY] Narten, T., Nordmark, E. and W. Simpson, "
Discovery for IP Version 6 (IPv6)", RFC 2461,
1998.
[IPv6-SA] Atkinson, R., "Security Architecture for the
Protocol", RFC 2401, November 1998.
[IPv6-AUTH] Atkinson, R., "IP Authentication Header", RFC 2402,
November 1998.
[IPv6-ESP] Atkinson, R., "IP Encapsulating Security Payload (ESP)",
RFC 2406, November 1998.
[ICMPv4] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[ICMPv6] Conta, A. and S. Deering, "Internet Control
Protocol (ICMPv6) for the Internet Protocol Version 6
(IPv6)", RFC 2463, December 1998.
[IGMP] Deering, S., "Host extensions for IP multicasting",
5, RFC 1112, August 1989.
[PMTUv4] Mogul, J. and S. Deering, "Path MTU Discovery",
1191, November 1990.
[PMTUv6] McCann, J., Deering, S. and J. Mogul, "Path
Discovery for IP version 6", RFC 1981, August 1996.
Nordmark Standards Track [Page 24]
RFC 2765 SIIT February 2000
[DIFFSERV] Nichols, K., Blake, S., Baker, F. and D. Black
"Definition of the Differentiated Services Field (
Field) in the IPv4 and IPv6 Headers", RFC 2474,
1998.
[MLD] Deering, S., Fenner, W. and B. Haberman, "
Listener Discovery (MLD) for IPv6", RFC 2710,
1999.
[FTPEXT] Allman, M., Ostermann, S. and C. Metz, "FTP
for IPv6 and NATs.", RFC 2428, September 1998.
[MILLER] G. Miller, Email to the ngtrans mailing list on 26
1999.
[BSDAPI] Gilligan, R., Thomson, S., Bound, J. and W. Stevens
"Basic Socket Interface Extensions for IPv6", RFC 2553,
March 1999.
Author's
Erik
Sun Microsystems, Inc
901 San Antonio
Palo Alto, CA 94303
Phone: +1 650 786 5166
Fax: +1 650 786 5896
EMail: nordmark@sun.
Nordmark Standards Track [Page 25]
RFC 2765 SIIT February 2000
Full Copyright
Copyright (C) The Internet Society (2000). All Rights Reserved
This document and translations of it may be copied and furnished
others, and derivative works that comment on or otherwise explain
or assist in its implementation may be prepared, copied,
and distributed, in whole or in part, without restriction of
kind, provided that the above copyright notice and this paragraph
included on all such copies and derivative works. However,
document itself may not be modified in any way, such as by
the copyright notice or references to the Internet Society or
Internet organizations, except as needed for the purpose
developing Internet standards in which case the procedures
copyrights defined in the Internet Standards process must
followed, or as required to translate it into languages other
English
The limited permissions granted above are perpetual and will not
revoked by the Internet Society or its successors or assigns
This document and the information contained herein is provided on
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE
Funding for the RFC Editor function is currently provided by
Internet Society
Nordmark Standards Track [Page 26]
if you see any problems within the linking, don't worry be happy,
this is version 0.1 of the Relevance System and you gotta expect some crappy subroutines sometimes,
just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.
RFC documents can be found at I.E.T.F.
Relevance System Copyright © 2002 Spectrum WorldResearch
other technical nosh by ServerMasters Corporation
collaboration of BobX
