Internet DRAFT - draft-ietf-ngtrans-header-trans
draft-ietf-ngtrans-header-trans
HTTP/1.1 200 OK
Date: Tue, 09 Apr 2002 05:53:04 GMT
Server: Apache/1.3.20 (Unix)
Last-Modified: Fri, 14 Aug 1998 13:05:00 GMT
ETag: "304a57-c480-35d435fc"
Accept-Ranges: bytes
Content-Length: 50304
Connection: close
Content-Type: text/plain
INTERNET-DRAFT Erik Nordmark, Sun Microsystems
August 7, 1998
Stateless IP/ICMP Translator (SIIT)
<draft-ietf-ngtrans-header-trans-02.txt>
Status of this Memo
This document is an Internet-Draft. 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.''
To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
Directories on ftp.ietf.org (US East Coast), nic.nordu.net
(Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
Rim).
Distribution of this memo is unlimited.
This Internet Draft expires February 7, 1999.
Abstract
This document specifies a transition mechanism in addition to those
already specified in RFC 1933. The new mechanism can be used as part
of a solution that allows IPv6 hosts that do not have a permanently
assigned IPv4 address to communication with IPv4-only hosts.
Acknowledgements
Some text has been pulled from an old Internet Draft titled "IPAE:
The SIPP Interoperability and Transition Mechanism" authored by R.
Gilligan, E. Nordmark, and B. Hinden. George Tsirtsis provides the
figures for Section 1.
draft-ietf-ngtrans-header-trans-02.txt [Page 1]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
Contents
Status of this Memo.......................................... 1
1. INTRODUCTION AND MOTIVATION.............................. 3
1.1. Applicability and Limitations....................... 5
1.2. Impact Outside the Network Layer.................... 6
2. TERMINOLOGY.............................................. 7
2.1. Addresses........................................... 8
2.2. Requirements........................................ 8
3. OVERVIEW................................................. 8
3.1. Assumptions......................................... 9
4. TRANSLATING FROM IPv4 TO IPv6............................ 9
4.1. Translating IPv4 Headers............................ 10
4.2. Translating ICMPv4.................................. 12
4.3. Translating ICMPv4 Error Messages................... 14
4.4. Knowing when to Translate........................... 15
5. TRANSLATING FROM IPv6 TO IPv4............................ 15
5.1. Translating IPv6 Headers............................ 17
5.2. Translating ICMPv6.................................. 18
5.3. Translating ICMPv6 Error Messages................... 20
5.4. Knowing when to Translate........................... 21
6. SECURITY CONSIDERATIONS.................................. 21
7. OPEN ISSUES.............................................. 21
REFERENCES................................................... 22
AUTHOR'S ADDRESS............................................. 23
APPENDIX A: CHANGES SINCE PREVIOUS DRAFT..................... 23
draft-ietf-ngtrans-header-trans-02.txt [Page 2]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
1. INTRODUCTION AND MOTIVATION
The transition mechanisms specified in [TRANS-MECH] 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.
As the pool of globally unique IPv4 addresses becomes smaller and
smaller as the Internet grows 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.
There are several different scenarios where there might be IPv6-only
hosts that need to communicate with IPv4-only hosts. These IPv6
hosts might be IPv4-capable, i.e. include an IPv4 implementation but
not be assigned an IPv4 address, or they might not even include an
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 the
routers within the new network to route IPv4 since none of the
hosts in the new network are configured with IPv4 addresses. But
these new IPv6 devices might occasionally need to communicate with
some IPv4 nodes out on the Internet.
- An existing network where a large number of IPv6 devices are
added. The IPv6 devices might have both an IPv4 and an IPv6
protocol stack but there is not enough global IPv4 address space
to give each one of them a permanent IPv4 address. In this case
it is more likely that the routers in the network already route
IPv4 and are upgraded to dual routers.
If there is no IPv4 routing inside the network i.e., the cloud that
contains the new devices, some possible solutions are to either use
the translators specified in this document at the boundary of the
cloud, or to use Application Layer Gateways (ALG) on dual nodes at
the cloud's boundary. The ALG solution is less flexible in that it
is application protocol specific and it is also less robust since a
the ALG box is likely to be a single point of failure for a
connection using that box.
If there IPv4 routing is supported inside the cloud and the
implementations support both IPv6 and IPv4 it might suffice to have a
mechanism for allocating temporary IPv4 and use IPv4 end to end when
communicating with IPv4-only nodes. However, it would seem that such
a solution would require the pool of temporary IPv4 addresses to be
partitioned across all the subnets in the cloud which would either
require a larger pool of IPv4 addresses or result in cases where
communication would fail due to no available IPv4 address for the
draft-ietf-ngtrans-header-trans-02.txt [Page 3]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
node's subnet.
This document specifies a mechanism by which IPv6-only nodes can
interoperate with IPv4-only nodes by having the IPv6-only nodes
somehow acquire a temporary IPv4 address. That IPv4 address will be
used as an IPv4-compatible IPv6 address and the packets will travel
through a stateless IP/ICMP translator that will translate the packet
headers between IPv4 and IPv6 and translate the addresses in those
headers between IPv4 addresses on one side and IPv4-compatible or
IPv4-mapped IPv6 addresses on the other side.
This specification does not cover how an IPv6 node can acquire a
temporary IPv4 address and how such a temporary address be registered
in the DNS. The DHCP protocol, perhaps with some extensions, could
probably be used to acquire temporary addresses with short leases but
that is outside the scope of this document. The mechanism for
routing this temporary IPv4 address (or the IPv4-compatible IPv6
address) in the site is currently not specified in this document.
The figures below show how the Stateless IP/ICMP Translator (SIIT)
can be used initially for small networks (e.g., a single subnet) and
later for a site which has IPv6-only hosts in a dual IPv4/IPv6
network. This use assumes a mechanism for the IPv6 nodes to acquire
an temporary address from the pool of IPv4 addresses. Note that SIIT
is not likely to be useful later during transition when most of the
Internet is IPv6 and there are only small islands of IPv4 nodes,
since such use would either require 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.)
___________
/ \
[IPv6 Host]---[SIIT]---------< IPv4 network>--[IPv4 Host]
| \___________/
(pool of v4 addresses)
Figure 1. Using SIIT for a single IPv6-only subnet.
draft-ietf-ngtrans-header-trans-02.txt [Page 4]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
___________ ___________
/ \ / \
[IPv6 Host]--< Dual network>--[SIIT]--< IPv4 network>--[IPv4 Host]
\___________/ | \___________/
(pool of v4 addresses)
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.
1.1. Applicability and Limitations
The IPv6 protocol [IPv6] has been designed so that the transport
pseudo-header checksums are not affected by such a translation thus
the translator does not need to modify TCP and UDP headers. However,
ICMPv6 include a pseudo-header checksum but it is not present in
ICMPv4 thus the checksum in ICMP messages need to be modified by the
translator. In addition, ICMP error messages contain an IP header as
part of the payload thus the translator need to rewrite those parts
of the packets to make the receiver be able to understand the
included IP header. However, all of the translators operations,
including path MTU discovery, are stateless in the sense that the
translator operates independently of each packet and does not retain
any state from one packet to another. This allows redundant
translator boxes without any coordination and a given TCP connection
can have the two directions of packets go through different
translator boxes.
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, or destination options
headers. It could be possible to define a translation between source
routing in IPv4 and IPv6. However such a translation would not be
semantically correct since the IPv4 source routing option performs a
"record route" function as the nodes listed in the source route are
traversed and the IPv6 routing header does not include the record
route aspect. Also, the usefulness of source routing when going
through a header translator might be limited since all the routers
would need to have an IPv4 address (or an IPv4-compatible IPv6
address) since the IPv4-only node will send a source option
containing 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 the
logical IP layer (IP headers, IPv6 fragment headers, and ICMP
messages) packets encrypted using ESP in Transport-mode can be
draft-ietf-ngtrans-header-trans-02.txt [Page 5]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
carried through the translator. [Note that this assumes that the key
management can operate between the IPv6-only and the IPv4-only node.]
The use of AH headers is more complex since the AH computation covers
most of the fields in the IP header. Should it be possible for the
IPv6 node to predict the value of all the IPv4 header fields on the
other side of the translator then the IPv6 node could calculate the
authentication data using an IPv4 header instead of the IPv6 header
even though it is sending and receiving IPv6 packets. [Currently
this is not possible since the IP fragment identification field is
not carried end-to-end through the translator in all cases. This
could be resolved by changing AH to not include the fragment
identification field in the AH computation for either IPv4 or IPv6.]
For ESP Tunnel-mode the IPv6 node would have to be able to parse and
generate "inner" IPv4 headers since the inner IP will be encrypted
together with the transport protocol.
IPv4 multicast addresses can not be mapped to IPv6 multicast
addresses. For instance, ::ffff:224.1.2.3 is an IPv4 mapped IPv6
address with a class D address, however it is not an IPv6 multicast
address. While the IP/ICMP translation aspect of this draft works
for multicast packets this address mapping limitation makes it hard
to the techniques in this draft for multicast traffic.
1.2. Impact Outside the Network Layer
The potential existence of stateless IP/ICMP translators is already
taken care of from a protocol perspective in [IPv6]. However, an
IPv6 node that wants to be able to use translators need some
additional logic in the network layer.
The network layer in an IPv6-only node when presented with either an
IPv4 destination address or an IPv4-mapped IPv6 destination address
by the application is likely to drop the packet and return some error
message to the application. In order to take advantage of
translators such a node should instead send an IPv6 packet where the
destination address is the IPv4-mapped address and the source address
is the nodes temporarily assigned IPv4-compatible address. If the
node does not have a temporarily assigned IPv4-compatible address it
should acquire one using mechanisms that are not discussed in this
document.
Note that the above also applies to a dual implementation node which
is not configured with any IPv4 address.
There are no extra changes needed to applications to operate through
a translator. The applications that have been modified to work on a
dual node already have the mechanisms to determine whether they are
draft-ietf-ngtrans-header-trans-02.txt [Page 6]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
communicating with an IPv4 or an IPv6 peer. Thus if the applications
need to modify their behavior depending on the type of the peer, such
as ftp determining which flavor of PORT command to use, they already
need to do that when running on dual nodes and the presense of
translators does not add anything. For example, when using the
socket API [RFC 2133] the applications know that the peer is IPv6 if
they get an AF_INET6 address from the name service and the address is
not an IPv4-mapped address (i.e., IN6_IS_ADDR_V4MAPPED returns
false). If this is not the case, i.e., the address is AF_INET or an
IPv4-mapped IPv6 address, the peer is IPv4.
One way of viewing the translator, which might help clarify why
applications do not need to know that a translator is used, is to
look at the information that is passed from the transport layer to
the network layer. If the transport passes down an IPv4 address
(whether or not is in the IPv4-mapped encoding) this means that at
some point there will be IPv4 packets generated. In a dual node the
generation of the IPv4 packets takes place in the sending node. In
an IPv6-only node conceptually the only difference is that the IPv4
packet is generated by the translator - all the information that the
transport layer passed to the network layer will be conveyed to the
translator in some form. That form just "happens" to be in the form
of an IPv6 header.
2. TERMINOLOGY
This documents uses the terminology defined in [IPv6] and [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 be assigned
one or more IPv4 addresses.
IPv4 enabled node: A node which has an IPv4 protocol stack
and is assigned one or more IPv4 addresses. Both
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 be assigned
one or more IPv6 addresses.
IPv6 enabled node: A node which has an IPv6 protocol stack
draft-ietf-ngtrans-header-trans-02.txt [Page 7]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
and is assigned one or more IPv6 addresses. Both
IPv6-only and IPv6/IPv4 nodes are IPv6 enabled.
2.1. Addresses
In addition to the forms of addresses defined in [ADDR-ARCH] this
document also introduces the new form of IPv4-translated address to
avoid using IPv4-compatible addresses outside the intended use of
automatic tunneling. Thus the address forms are:
IPv4-mapped:
An address of the form 0::ffff:a.b.c.d which refers
to a node that is not IPv6-capable. In addition to
its use in the API this protocol uses IPv4-mapped
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 to
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 refers
to an IPv6-enabled node. Note that the prefix
0::ffff:0:0:0/96 is chosen to checksum to zero to
avoid any changes to the transport protocol's pseudo
header checksum.
2.2. Requirements
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [KEYWORDS].
3. OVERVIEW
The protocol translators are assumed to fit around some piece of
topology that includes some IPv6-only nodes and can also include IPv4
nodes and dual nodes. There has to be a translator on each path in
and out of this cloud to ensure that the packets always get
translated. This does not require a translator at every physical
connection between the cloud and the rest of the Internet since the
routing can be used to deliver the packets to the translator.
For outbound packets i.e., packets that need to be translated from
IPv6 to IPv4, it is sufficient to have a route for the IPv4-mapped
draft-ietf-ngtrans-header-trans-02.txt [Page 8]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
address prefix (::ffff:0:0/96) injected in the internal IPv6 routing
tables. This route will deliver packets to the translator since all
IPv6 packets that need translation will have an IPv4-mapped IPv6
destination address.
Inbound IPv4 packets needing translation are likely to have some
temporary IPv4 address that is drawn from a pool of such addresses.
Thus the internal IPv4 routing tables could have one or more routes
for the whole pool that direct the packets to the translator.
3.1. Assumptions
The IPv6 nodes using the translator must have an IPv4-translated IPv6
address while it is communicating with IPv4 nodes.
4. TRANSLATING FROM IPv4 TO IPv6
When an IPv4-to-IPv6 translator receives an IPv4 datagram addressed
to a destination that lies outside of the attached IPv4 island, it
translates the IPv4 header of that packet into an IPv6 header. It
then forwards the packet based on the IPv6 destination address. The
original IPv4 header on the packet is removed and replaced by a IPv6
header. Except for ICMP packets the transport layer header and data
portion of the packet are left unchanged.
+-------------+ +-------------+
| IPv4 | | IPv6 |
| Header | | Header |
+-------------+ +-------------+
| Transport | | Fragment |
| Layer | ===> | Header |
| Header | |(not always) |
+-------------+ +-------------+
| | | Transport |
~ Data ~ | Layer |
| | | Header |
+-------------+ +-------------+
| |
~ Data ~
| |
+-------------+
IPv4-to-IPv6 Translation
draft-ietf-ngtrans-header-trans-02.txt [Page 9]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
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
might send back ICMP "packet too big" messages to the sender. When
these ICMP errors are sent by the IPv6 routers they will pass through
a translator which 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.
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 576 byte IPv6 packet since IPv6 guarantees that 576
byte packets never need to be fragment. 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
IPv6-to-IPv4 translator.
The above rules ensure that when packets are fragmented either by the
sender or by IPv4 routers that the low-order 16 bits of the fragment
identification is carried end-end to ensure 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.
4.1. Translating IPv4 Headers
If the DF flag is not set and the IPv4 packet will result in an IPv6
packet larger than 576 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 528 bytes (576 minus 40 for the IPv6 header and 8
draft-ietf-ngtrans-header-trans-02.txt [Page 10]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
for the Fragment header). The resulting fragments are then
translated independently using the logic described below.
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:
Version:
6
Traffic Class:
Copied from IP Type Of Service and Precedence field
(all 8 bits are copied).
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 or ICMPv6 "ttl
exceeded" error.
Source Address:
The low-order 32 bits is the IPv4 source address.
The high-order 96 bits is the IPv4-mapped prefix
(::ffff:0:0/96)
Destination Address:
The low-order 32 bits is the IPv4 destination
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 ignored
i.e., there is no attempt to translate them.
draft-ietf-ngtrans-header-trans-02.txt [Page 11]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
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:
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.
4.2. Translating ICMPv4
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 needs 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
draft-ietf-ngtrans-header-trans-02.txt [Page 12]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
ICMP checksum both take the type change into account and 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 types
Silently drop.
IGMP messages:
While there are ICMPv6 counterparts for the 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 IPv4/IPv6 translators those packets should
also be silently dropped.
ICMPv4 error messages:
Destination Unreachable (Type 3)
For all that are not explicitly listed below set the Type to
1.
Translate the code field as follows:
Code 0, 1: Set Code to 0 (no route to destination).
Code 2: Translate to an ICMPv6 Parameter Problem (Type 4,
Code 1) and make the Pointer point to the IPv6 Next Header
field.
Code 3: Set Code to 4 (port unreachable).
Code 4: Translate to an ICMPv6 Packet Too Big message
(Type 2) with code 0. The MTU field needs to be adjusted
draft-ietf-ngtrans-header-trans-02.txt [Page 13]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
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 [PMTUv4], then the
translator must use the plateau values specified in
[PMTUv4] 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: Set Code to 2 (not a neighbor).
Code 6,7: Set Code to 0 (no route to destination).
Code 8: Set Code to 0 (no route to destination).
Code 9, 10: 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.
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.
4.3. Translating ICMPv4 Error Messages
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. This translated is
likely to change the length of the datagram thus the Payload Length
field in the outer IPv6 header might need to be updated.
draft-ietf-ngtrans-header-trans-02.txt [Page 14]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
+-------------+ +-------------+
| 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 Translation
The translation of the inner IP header can be done by recursively
invoking the function that translated the outer IP headers.
4.4. Knowing when to Translate
The translator is assumed to know the pool(s) of IPv4 address that
are used to represent the internal IPv6-only nodes. Thus if the
destination address falls in these configured sets of prefixes the
packet needs to be translated to IPv6.
5. TRANSLATING FROM IPv6 TO IPv4
When an IPv6-to-IPv4 translator receives an IPv6 datagram addressed
to an IPv4-mapped IPv6 address, it translates the IPv6 header of that
packet into an IPv6 header. It then forwards the packet based on the
IPv4 destination address. The original IPv6 header on the packet is
removed and replaced by a IPv4 header. Except for ICMP packets the
transport layer header and data portion of the packet are left
unchanged.
draft-ietf-ngtrans-header-trans-02.txt [Page 15]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
+-------------+ +-------------+
| IPv6 | | IPv4 |
| Header | | Header |
+-------------+ +-------------+
| Fragment | | Transport |
| Header | ===> | Layer |
|(if present) | | Header |
+-------------+ +-------------+
| Transport | | |
| Layer | ~ Data ~
| Header | | |
+-------------+ +-------------+
| |
~ Data ~
| |
+-------------+
IPv6-to-IPv4 Translation
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 576 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 IPv6-to-IPv4 translator since the
IPv6 node might receive ICMP "packet too big" messages originated by
an IPv4 router that report an MTU less than 576. However, [IPv6]
requires IPv6 nodes to handle such ICMP "packet too big" message by
reducing the path MTU to 576 and include 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 576 bytes or greater and when
the path MTU drops below that limit IPv6 sender will originate 576
byte packets that will be fragmented by IPv4 routers along the path
after being translated to IPv4.
The only drawback with this scheme is that it is not possible to use
PMTU to do optimal UDP fragmentation at sender. The presence of an
IPv6 Fragment header is interpreted that is it OK to fragment the
packet on the IPv4 side thus if the Fragment header is present
because UDP wants to send e.g. 8k packets even though the path MTU is
smaller the path MTU discovery will not be end-to-end but only up to
and including the translator.
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.
draft-ietf-ngtrans-header-trans-02.txt [Page 16]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
5.1. Translating IPv6 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 and Precedence:
Copied from the IPv6 Traffic Class (all 8 bits).
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.
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 or ICMPv6 "ttl
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-translated or
an IPv4-mapped address then the low-order 32 bits of
the IPv6 source address is copied to the IPv4 source
draft-ietf-ngtrans-header-trans-02.txt [Page 17]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
address. Otherwise, the source address is set to
127.0.0.1.
Destination Address:
IPv6 packets that are translated have a destination
address that is either an IPv4-translated or an
IPv4-mapped address. Thus the low-order 32 bits of
the IPv6 destination address is copied to the IPv4
source address.
If any of an IPv6 hop-by-hop options header, destination options
header, or routing header 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.
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.
5.2. Translating ICMPv6
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.
draft-ietf-ngtrans-header-trans-02.txt [Page 18]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
In addition all ICMP packets needs to have the Type value translated
and for ICMP error messages the included IP header also needs
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 ICMP
checksum both take the type change into account and to
exclude the ICMPv6 pseudo-header.
Group Membership Query/Report/Reduction (Type 130, 131, 132)
Single hop message. Silently drop.
Neighbor Discover messages (Type 133 through 137)
Single hop message. Silently drop.
Unknown informational messages
Silently drop.
ICMPv6 error messages:
Destination Unreachable (Type 1)
Set the Type field to 3. Translate the code field as
follows:
Code 0: Set Code to 1 (host unreachable).
Code 1: Set Code to 10 (communication with destination
host administratively prohibited).
Code 2: Set Code to 5 (source route failed).
Code 3: Set Code to 1 (host unreachable).
Code 4: 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.
Time Exceeded (Type 3)
Set the Type to 11. The Code field is unchanged.
draft-ietf-ngtrans-header-trans-02.txt [Page 19]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
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.
Unknown error messages
Silently drop.
5.3. Translating ICMPv6 Error Messages
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. This translated is
likely to change the length of the datagram thus the Payload Length
field in the outer IPv6 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 |
+-------------+ +-------------+
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.
draft-ietf-ngtrans-header-trans-02.txt [Page 20]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
5.4. Knowing when to Translate
When the translator receives a IPv6 packet with an IPv4-mapped
destination address the packet will be translated to IPv4.
6. SECURITY CONSIDERATIONS
The use of stateless IP/ICMP translators does not introduce any new
security issues beyond the security issues that are already present
in the IPv4 and IPv6 protocols and in the routing protocols which are
used to make the packets reach the translator.
As the Authentication Header is currently 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 predict the content of a packet at the IPv4 side
of the translator. As such it is impossible to translate packets
with AH headers.
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 preferred over ESP tunnel mode since it does not contain an
"extra" encrypted IP header which could confuse the peer.
7. OPEN ISSUES
- Can/should we make AH work through a translator e.g. by removing
the fragment ID field from the AH computation for both IPv4 and
IPv6?
- For IPv6 to IPv4 should the Identification be something other than
zero to be more friendly to header compression? The packets in
question have DF set thus it is possible to choose e.g. a local
incrementing counter without adverse effects on reassembly.
- When translation IPv6 to IPv4 when the source is pure IPv6 address
the draft specifies to use 127.0.0.1 as the source address (in
order to make an IPv4 traceroute show something). Alternatives
are to use a different address in this case (e.g. 0.0.0.0) or
simply drop the packet (which would not show such hops when using
traceroute).
- When translating IPv4 packets with IP options the draft specifies
that the IPv4 options be silently ignored. Should the packet be
dropped and an ICMP error be generated instead?
draft-ietf-ngtrans-header-trans-02.txt [Page 21]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
- When translating IPv6 packets with Hop-by-hop or destination
options extension headers this document specifies that those
headers be silently ignored. Should the packet be dropped and an
ICMP error be generated instead?
REFERENCES
[KEYWORDS] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[IPv6] S. Deering, R. Hinden, Editors, "Internet Protocol, Version
6 (IPv6) Specification", RFC 1883, January 1996.
[IPv4] J. Postel, "Internet Protocol", RFC 791, September 1981.
[ADDR-ARCH] S. Deering, R. Hinden, Editors, "IP Version 6
Addressing Architecture", RFC 1884, January 1996.
[TRANS-MECH] R. Gilligan, E. Nordmark, "Transition Mechanisms for
IPv6 Hosts and Routers", RFC 1933, April 1996.
[DISCOVERY] T. Narten, E. Nordmark, and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 1970, August 1996.
[IPv6-SA] R. Atkinson. "Security Architecture for the Internet
Protocol". RFC 1825, August 1995.
[IPv6-AUTH] R. Atkinson. "IP Authentication Header", RFC 1826,
August 1995.
[IPv6-ESP] R. Atkinson. "IP Encapsulating Security Payload (ESP)",
RFC 1827, August 1995.
[ICMPv4] J. Postel, "Internet Control Message Protocol", RFC 792,
September 1981.
[ICMPv6] A. Conta, S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6)", RFC
1885, January 1996.
[IGMP] S. Deering, "Host extensions for IP multicasting", RFC 1112,
August 1989.
draft-ietf-ngtrans-header-trans-02.txt [Page 22]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
[PMTUv4] J. Mogul, S. Deering, "Path MTU Discovery", RFC 1191,
November 1990.
[PMTUv6] J. McCann, S. Deering, J. Mogul, "Path MTU Discovery for
IP version 6", RFC 1981, August 1996.
AUTHOR'S ADDRESS
Erik Nordmark
Sun Microsystems, Inc.
901 San Antonio Road
Palo Alto, CA 94303
USA
phone: +1 650 786 5166
fax: +1 650 786 5896
email: nordmark@sun.com
APPENDIX A: CHANGES SINCE PREVIOUS DRAFT
The following changes have been made since version 01 of the draft:
o Introduced the notion of IPv4-translated addresses.
o Clarified that IPv4-mapped addresses are sent over the wire.
o Clarified generating ICMP ttl exceeded when ttl or hop limit
reaches zero.
o Added 1-1 mapping between IPv4 TOS and IPv6 Traffic Class.
The following changes have been made since version 00 of the draft:
o Added clarification about applicability for multicast.
o Clarified the terminology with IPv4 capable vs. enabled etc.
o Changed the title to provide an acronym (SIIT).
o Added some figures to explain the applicability.
draft-ietf-ngtrans-header-trans-02.txt [Page 23]
INTERNET-DRAFT Stateless IP/ICMP Translator (SIIT) August 1998
draft-ietf-ngtrans-header-trans-02.txt [Page 24]
