INTERNET-DRAFT George Tsirtsis
NGTRANS WG BT
Expires March, 2000 Pyda Srisuresh
Consultant
October 1999
Network Address Translation - Protocol Translation (NAT-PT)
<draft-ietf-ngtrans-natpt-07.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
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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
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The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This document specifies an IPv4-to-IPv6 transition mechanism, in
addition to those already specified in [TRANS]. This solution
attempts to provide transparent routing, as defined in [NAT-TERM], to
end-nodes in V6 realm trying to communicate with end-nodes in V4
realm and vice versa. This is achieved using a combination of Network
Address Translation and Protocol Translation. The scheme described
does not mandate dual-stacks (i.e., IPv4 as well as V6 protocol
support) or special purpose routing requirements (such as requiring
tunneling support) on end nodes. This scheme is based on a
combination of address translation theme as described in [NAT-TERM]
and V6/V4 protocol translation theme as described in [SIIT].
Acknowledgements
Special thanks to Pedro Marques for reviewing an earlier version of
the draft. Also, many thanks to Alan O'Neill and Martin Tatham, as
the mechanism described in this document was initially developed
through discussions with them.
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Index
1. Introduction
2. Terminology
2.1 Network Address Translation (NAT)
2.2 NAT-PT flavors
2.2.1 Traditional-NAT-PT
2.2.2 Bi-directional-NAT-PT
2.3 Protocol Translation (PT)
2.4 Application Level Gateway (ALG)
2.5 Requirements
3. Traditional-NAT-PT operation (V6 to V4)
3.1 NAT-PT Outgoing Sessions
3.2 NAPT-PT Outgoing Sessions
4. Use of DNS-ALG for Address assignment
4.1 V4 Address Assignment for Incoming Connections (V4 to V6)
4.2 V4 Address Assignment for Outgoing Connections (V6 to V4)
5. Protocol Translation Details
5.1 Translating IPv4 Headers to IPv6 Headers
5.2 Translating IPv6 Headers to IPv4 Headers
5.3 TCP/UDP/ICMP Checksum Update
6. FTP Application Level Gateway (FTP-ALG) Support
6.1 Payload modifications for V4 originated FTP sessions
6.2 Payload modifications for V6 originated FTP sessions
6.3 Header updates for FTP control packets
7. NAT-PT Limitations and Future Work
7.1 Topology Limitations
7.2 Protocol Translation Limitations
7.3 Impact of Address Translation
7.4 Lack of End-to-End Security
7.5 DNS Translation and DNSSEC
8. Applicability Statement
9. Security Considerations
10. References
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1. Introduction
IPv6 is a new version of the IP protocol designed to modernize IPv4
which was designed in the 1970s. IPv6 has a number of advantages over
IPv4 that will allow for future Internet growth and will simplify IP
configuration and administration. IPv6 has a larger address space
than IPv4, an addressing model that promotes aggressive route
aggregation and a powerful autoconfiguration mechanism. In time, it
is expected that Internet growth and a need for a plug-and-play
solution will result in widespread adoption of IPv6.
There is expected to be a long transition period during which it will
be necessary for IPv4 and IPv6 nodes to coexist and communicate. A
strong, flexible set of IPv4-to-IPv6 transition and coexistence
mechanisms will be required during this transition period.
The SIIT proposal [SIIT] describes a protocol translation mechanism
that allows communication between IPv6-only and IPv4-only nodes via
protocol independent translation of IPv4 and IPv6 datagrams,
requiring no state information for the session. The SIIT proposal
assumes that V6 nodes are assigned a V4 address for communicating
with V4 nodes, and does not specify a mechanism for the assignment of
these addresses.
NAT-PT uses a pool of V4 addresses for assignment to V6 nodes on a
dynamic basis as sessions are initiated across V4-V6 boundaries. The
V4 addresses are assumed to be globally unique. NAT-PT with private
V4 addresses is outside the scope of this document and for further
study. NAT-PT binds addresses in V6 network with addresses in V4
network and vice versa to provide transparent routing [NAT-TERM] for
the datagrams traversing between address realms. This requires no
changes to end nodes and IP packet routing is completely transparent
[NAT-TERM] to end nodes. It does, however, require NAT-PT to track
the sessions it supports and mandates that inbound and outbound
datagrams pertaining to a session traverse the same NAT-PT router.
You will note that the topology restrictions on NAT-PT are the same
with those described for V4 NATs in [NAT-TERM]. Protocol translation
details specified in [SIIT] would be used to extend address
translation with protocol syntax/semantics translation. A detailed
applicability statement for NAT-PT may be found at the end of this
document in section 7.
By combining SIIT protocol translation with the dynamic address
translation capabilities of NAT and appropriate ALGs, NAT-PT provides
a complete solution that would allow a large number of commonly used
applications to interoperate between IPv6-only nodes and IPv4-only
nodes, without requiring any changes to these applications.
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A fundamental assumption for NAT-PT is only to be use when no other
native IPv6 or IPv6 over IPv4 tunneled means of communication is
possible. In other words the aim is to only use translation between
IPv6 only nodes and IPv4 only nodes, while translation between IPv6
only nodes and the IPv4 part of a dual stack node should be avoided
over other alternatives.
2. Terminology
The majority of terms used in this document are borrowed almost as is
from [NAT-TERM]. The following lists terms specific to this document.
2.1 Network Address Translation (NAT)
The term NAT in this document is very similar to the IPv4 NAT
described in [NAT-TERM], but is not identical. IPv4 NAT translates
one IPv4 address into another IPv4 address. In this document, NAT
refers to translation of an IPv4 address into an IPv6 address and
vice versa.
While the V4 NAT [NAT-TERM] provides routing between private V4
and external V4 address realms, NAT in this document provides
routing between a V6 address realm and an external V4 address
realm.
2.2 NAT-PT flavors
Just as there are various flavors identified with V4 NAT in [NAT-
TERM], the following NAT-PT variations may be identified in this
document.
2.2.1 Traditional NAT-PT
Traditional-NAT-PT would allow hosts within a V6 network to
access hosts in the V4 network. In a traditional-NAT-PT,
sessions are uni-directional, outbound from the V6 network.
This is in contrast with Bi-directional-NAT-PT, which permits
sessions in both inbound and outbound directions.
Just as with V4 traditional-NAT, there are two variations to
traditional-NAT-PT, namely Basic-NAT-PT and NAPT-PT.
With Basic-NAT-PT, a block of V4 addresses are set aside for
translating addresses of V6 hosts as they originate sessions to
the V4 hosts in external domain. For packets outbound from the
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V6 domain, the source IP address and related fields such as IP,
TCP, UDP and ICMP header checksums are translated. For inbound
packets, the destination IP address and the checksums as listed
above are translated.
NAPT-PT extends the notion of translation one step further by
also translating transport identifier (e.g., TCP and UDP port
numbers, ICMP query identifiers). This allows the transport
identifiers of a number of V6 hosts to be multiplexed into the
transport identifiers of a single assigned V4 address. NAPT-PT
allows a set of V6 hosts to share a single V4 address. Note
that NAPT-PT can be combined with Basic-NAT-PT so that a pool
of external addresses are used in conjunction with port
translation.
For packets outbound from the V6 network, NAPT-PT would
translate the source IP address, source transport identifier
and related fields such as IP, TCP, UDP and ICMP header
checksums. Transport identifier can be one of TCP/UDP port or
ICMP query ID. For inbound packets, the destination IP address,
destination transport identifier and the IP and transport
header checksums are translated.
2.2.2 Bi-Directional-NAT-PT
With Bi-directional-NAT-PT, sessions can be initiated from
hosts in V4 network as well as the V6 network. V6 network
addresses are bound to V4 addresses, statically or dynamically
as connections are established in either direction. The name
space (i.e., their Fully Qualified Domain Names) between hosts
in V4 and V6 networks is assumed to be end-to-end unique.
Hosts in V4 realm access V6-realm hosts by using DNS for
address resolution. A DNS-ALG [DNS-ALG] must be employed in
conjunction with Bi-Directional-NAT-PT to facilitate name to
address mapping. Specifically, the DNS-ALG must be capable of
translating V6 addresses in DNS Queries and responses into
their V4-address bindings, and vice versa, as DNS packets
traverse between V6 and V4 realms.
2.3 Protocol Translation (PT)
PT in this document refers to the translation of an IPv4 packet
into a semantically equivalent IPv6 packet and vice versa.
Protocol translation details are described in [SIIT].
2.4 Application Level Gateway (ALG)
Application Level Gateway (ALG) [NAT-TERM] is an application
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specific agent that allows a V6 node to communicate with a V4 node
and vice versa. Some applications carry network addresses in
payloads. NAT-PT is application unaware and does not snoop the
payload. ALG could work in conjunction with NAT-PT to provide
support for many such applications.
2.5 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. Traditional-NAT-PT Operation (V6 to V4)
NAT-PT offers a straight forward solution based on transparent
routing [NAT-TERM] and address/protocol translation, allowing a large
number of applications in V6 and V4 realms to inter-operate without
requiring any changes to these applications.
In the following paragraphs we describe the operation of traditional-
NAT-PT and the way that connections can be initiated from a host in
IPv6 domain to a host in IPv4 domain through a traditional-NAT-PT
3.1 Basic-NAT-PT Operation
[IPv6-B]-+
| +==============+
[IPv6-A]-+-[NAT-PT]---------| IPv4 network |--[IPv4-C]
| +==============+
(pool of v4 addresses)
Figure 1: IPv6 to IPv4 communication
Node IPv6-A has an IPv6 address -> FEDC:BA98::7654:3210
Node IPv6-B has an IPv6 address -> FEDC:BA98::7654:3211
Node IPv4-C has an IPv4 address -> 132.146.243.30
NAT-PT has a pool of addresses including the IPv4 subnet
120.130.26/24
The V4 addresses in the address pool could be allocated one-to-one
to the V6 addresses of the V6 end nodes in which case one needs as
many V4 addresses as V6 end points. In this document we assume
that the V6 network has less V4 addresses than V6 end nodes and
thus dynamic address allocation is required for at least some of
them.
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Say the IPv6 Node A wants to communicate with the IPv4 Node C.
Node A creates a packet with:
Source Address, SA=FEDC:BA98::7654:3210 and Destination
Address, DA = PREFIX::132.146.243.30
NOTE: The prefix PREFIX::/96 is advertised in the stub domain by
the NAT-PT, and packets addressed to this PREFIX will be routed to
the NAT-PT. The pre-configured PREFIX only needs to be routable
within the IPv6 stub domain and as such it can be any routable
prefix that the network administrator chooses.
The packet is routed via the NAT-PT gateway, where it is
translated to IPv4.
If the outgoing packet is not a session initialisation packet, the
NAT-PT SHOULD already have stored some state about the related
session, including assigned IPv4 address and other parameters for
the translation. If this state does not exist, the packet SHOULD
be silently discarded.
If the packet is a session initialisation packet, the NAT-PT
locally allocates an address (e.g: 120.130.26.10) from its pool
of addresses and the packet is translated to IPv4. The translation
parameters are cached for the duration of the session and the IPv6
to IPv4 mapping is retained by NAT-PT.
The resulting IPv4 packet has SA=120.130.26.10 and
DA=132.146.243.30. Any returning traffic will be recognised as
belonging to the same session by NAT-PT. NAT-PT will use the state
information to translate the packet, and the resulting addresses
will be SA=PREFIX::132.146.243.30, DA=FEDC:BA98::7654:3210. Note
that this packet can now be routed inside the IPv6-only stub
network as normal.
3.2 NAPT-PT Operation
NAPT-PT, which stands for "Network Address Port Translation +
Protocol Translation", would allow V6 nodes to communicate with
the V4 nodes transparently using a single V4 address. The TCP/UDP
ports of the V6 nodes are translated into TCP/UDP ports of the
registered V4 address.
While NAT-PT support is limited to TCP, UDP and other port
multiplexing type of applications, NAPT-PT solves a problem that
is inherent with NAT-PT. That is, NAT-PT would fall flat when the
pool of V4 addresses assigned for translation purposes is
exhausted. Once the address pool is exhausted, newer V6 nodes
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cannot establish sessions with the outside world anymore. NAPT-PT,
on the other hand, will allow for a maximum of 63K TCP and 63K UDP
sessions per IPv4 address before having no TCP and UDP ports left
to assign.
To modify the example sited in figure 1, we could have NAPT-PT on
the border router (instead of NAT-PT) and all V6 addresses could
be mapped to a single v4 address 120.130.26.10.
IPv6 Node A would establish a TCP session with the IPv4 Node C as
follows:
Node A creates a packet with:
Source Address, SA=FEDC:BA98::7654:3210 , source TCP port = 3017
and Destination Address, DA = PREFIX::132.146.243.30, destination
TCP port = 23.
When the packet reaches the NAPT-PT box, NAPT-PT would assign one
of the TCP ports from the assigned V4 address to translate the
tuple of (Source Address, Source TCP port) as follows:
SA=120.130.26.10, source TCP port = 1025 and
DA=132.146.243.30, destination TCP port = 23.
The returning traffic from 132.146.243.30, TCP port 23 will be
recognised as belonging to the same session and will be translated
back to V6 as follows:
SA = PREFIX::132.146.243.30, source TCP port = 23;
DA = FEDC:BA98::7654:3210 , destination TCP port = 3017
Inbound NAPT-PT sessions are restricted to one server per service,
assigned via static TCP/UDP port mapping. For example, the Node
[IPv6-A] in figure 1 may be the only HTTP server (port 80) in the
V6 domain. Node [IPv4-C] sends a packet:
SA=132.146.243.30, source TCP port = 1025 and
DA=120.130.26.10, destination TCP port = 80
NAPT-PT will translate this packet to:
SA=PREFIX::132.146.243.30, source TCP port = 1025
DA=FEDC:BA98::7654:3210, destination TCP port = 80
In the above example, note that all sessions which reach NAPT-PT
with a destination port of 80 will be redirected to the same node
[IPv6-A].
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4. Use of DNS-ALG for Address Assignment
An IPv4 address is assigned by NAT-PT to a V6 node when NAT-PT
identifies the start of session, inbound or outbound. Identification
of the start of a new inbound session is performed differently than
for outbound sessions. However, the same V4 address pool is used for
assignment to V6 nodes, irrespective of whether a session is
initiated outbound from a V6 node or initiated inbound from a V4
node.
Policies determining what type of sessions are allowed and in which
direction and from/to which nodes is out of the scope of this
document.
IPv4 name to address mappings are held in the DNS with "A" records.
IPv6 name to address mappings are at the moment held in the DNS with
"AAAA" records. "A6" records have also been defined but at the time
of writing they are neither fully standardized nor deployed.
In any case, the DNS-ALG's principle of operation described in this
section is the same with either "AAAA" or "A6" records. The only
difference is that a name resolution using "A6" records may require
more than one query - reply pairs. The DNS-ALG SHOULD, in that case,
track all the replies in the transaction before translating an "A6"
record to an "A" record.
One of the aims of NAT-PT design is to only use translation when
there is no other means of communication, such as native IPv6 or some
form of tunneling. For the following discussion NAT-PT, in addition
to the IPv4 connectivity that it has it may also have a native IPv6
and/or a tunneled IPv6 connection.
4.1 V4 Address assignment for incoming connections (V4 to V6)
[DNS]--+
| [DNS]------[DNS]-------[DNS]
[IPv6-B]-+ | |
| +==============+ |
[IPv6-A]-+----[NAT-PT]------| IPv4 network |--[IPv4-C]
| +==============+
(pool of v4 addresses)
Figure 2: IPv4 to IPv6 communication
Node IPv6-A has an IPv6 address -> FEDC:BA98::7654:3210
Node IPv6-B has an IPv6 address -> FEDC:BA98::7654:3211
Node IPv4-C has an IPv4 address -> 132.146.243.30
NAT-PT has a pool of addresses including the IPv4 subnet
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120.130.26/24
In figure 2 above, when Node C's name resolver sends a name look
up request for Node A, the lookup query is directed to the DNS
server on the V6 network. Considering that NAT-PT is residing on
the border router between V4 and V6 networks, this request
datagram would traverse through the NAT-PT router. The DNS-ALG on
the NAT-PT device would modify DNS Queries for A records going
into the V6 domain as follows: (Note that a TCP/UDP DNS packet is
recognised by the fact that its source or destination port number
is 53)
a) For Node Name to Node Address Query requests:
Change the Query type from "A" to "AAAA" or "A6".
b) For Node address to Node name query requests:
Replace the string "IN-ADDR.ARPA" with the string "IP6.INT".
Replace the V4 address octets (in reverse order) preceding
the string "IN-ADDR.ARPA" with the corresponding V6 address
(if there exists a map) octets in reverse order.
In the opposite direction, when a DNS response traverses from the
DNS server on the V6 network to the V4 node, the DNS-ALG once
again intercepts the DNS packet and would:
a) Translate DNS responses for "AAAA" or "A6" records into "A"
records, (only translate "A6" records when the name has
completely been resolved)
b) Replace the V6 address resolved by the V6 DNS with the V4
address internally assigned by the NAT-PT router.
If a V4 address is not previously assigned to this V6 node, NAT-PT
would assign one at this time. As an example say IPv4-C attempts
to initialise a session with node IPv6-A by making a name lookup
("A" record) for Node-A . The name query goes to the local DNS and
from there it is propagated to the DNS server of the IPv6 network.
The DNS-ALG intercepts and translates the "A" query to "AAAA" or
"A6" query and then forwards it to the DNS server in the IPv6
network which replies as follows: (The example uses AAAA records
for convenience)
Node-A AAAA FEDC:BA98::7654:3210,
this is returned by the DNS server and gets intercepted and
translated by the DNS-ALG to:
Node-A A 120.130.26.1
The DNS-ALG also holds the mapping between FEDC:BA98::7654:3210
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and 120.130.26.1 in NAT-PT. The "A" record is then returned to
Node-C. Node-C can now initiate a session as follows:
SA=132.146.243.30, source TCP port = 1025 and
DA=120.130.26.1, destination TCP port = 80
the packet will be routed to NAT-PT, which since it already holds
a mapping between FEDC:BA98::7654:3210 and 120.130.26.1 can
translate the packet to:
SA=PREFIX::132.146.243.30, source TCP port = 1025
DA=FEDC:BA98::7654:3210, destination TCP port = 80
the communication can now proceed as normal.
The TTL values on all DNS resource records (RRs) passing through
NAT-PT SHOULD be set to 0 so that DNS servers/clients do not cache
temporarily assigned RRs. Note, however, that due to some buggy
DNS client implementations a value of 1 might in some cases work
better. The TTL values should be left unchanged for statically
mapped addresses.
Address mappings for incoming sessions, as described above, are
subject to denial of service attacks since one can make multiple
queries for nodes residing in the V6 network causing the DNS-ALG
to map all V4 addresses in NAT-PT and thus block legitimate
incoming sessions. Thus, address mappings for incoming sessions
should time out to minimise the effect of denial of service
attacks. Additionally, one IPv4 address (using NAPT-PT, see 3.2)
could be reserved for outgoing sessions only to minimise the
effect of such attacks to outgoing sessions.
4.2 V4 Address assignment for outgoing connections (V6 to V4)
V6 nodes learn the address of V4 nodes from the DNS server in the
V4 domain or from the DNS server internal to the V6 network. We
recommend that DNS servers internal to V6 domains maintain a
mapping of names to IPv6 addresses for internal nodes and possibly
cache mappings for some external nodes. In the case where the DNS
server in the v6 domain contains the mapping for external V4
nodes, the DNS queries will not cross the V6 domain and that would
obviate the need for DNS-ALG intervention. Otherwise, the queries
will cross the V6 domain and are subject to DNS-ALG intervention.
We recommend external DNS servers in the V4 domain cache name
mapping for external nodes (i.e., V4 nodes) only. Zone transfers
across IPv4 - IPv6 boundaries are strongly discouraged.
In the case of NAPT-PT, a TCP/UDP source port is assigned from the
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registered V4 address upon detection of each new outbound session.
We saw that a V6 node that needs to communicate with a V4 node
needs to use a specific prefix (PREFIX::/96) in front of the IPv4
address of the V4 node. The above technique allows the use of this
PREFIX without any configuration in the nodes.
To create another example from Figure 2 say Node-A wants to set up
a session with Node-C. For this Node-A starts by making a name
look-up ("AAAA" or "A6" record) for Node-C.
Since Node-C may also have an IPv6 address, the DNS-ALG on the
NAT-PT device just forwards the original AAAA/A6 query to the
external DNS system unchanged. If an AAAA/A6 record exists for the
destination, this will be returned to NAT-PT which will forward
it, also unchanged, to the originating host. End to end, native
or tunneled IPv6 communication can now commence.
If on the other hand there is no AAAA/A6 record for Node-C, an
error will be returned to NAT-PT. In this case NAT-PT re-issues
the query for Node C but this time looking for an A record.
Assuming there is an A record for Node-C the reply returns to the
NAT-PT The DNS-ALG translates the reply adding the appropriate
PREFIX. So, if the reply is
NodeC A 132.146.243.30, it is translated to
NodeC AAAA PREFIX::132.146.243.30 or to
NodeC A6 PREFIX::132.146.243.30
Now Node A can use this address like any other IPv6 address and
the V6 DNS server can even cache it as long as the PREFIX does not
change.
An issue here is how the V6 DNS server in the V6 stub domain talks
to the V4 domain outside the V6 stub domain. Remember that there
are no dual stack nodes here. The external V4 DNS server needs to
point to a V4 address, part of the V4 pool of addresses, available
to NAT-PT. NAT-PT keeps a one-to-one mapping between this V4
address and the V6 address of the internal V6 DNS server. In the
other direction, the V6 DNS server points to a V6 address formed
by the IPv4 address of the external V4 DNS servers and the prefix
(PREFIX::/96) that indicates non IPv6 nodes. This mechanism can
easily be extended to accommodate secondary DNS servers.
Note that the scheme described in this section impacts DNSSEC. See
section 7.5 of this document for details.
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5. Protocol Translation Details
The IPv4 and ICMPv4 headers are similar to their V6 counterparts but
a number of field are either missing, have different meaning or
different length. NAT-PT SHOULD translate all IP/ICMP headers from v4
to v6 and vice versa in order to make end-to-end IPv6 to IPv4
communication possible. Due to the address translation function and
possible port multiplexing, NAT-PT SHOULD also make appropriate
adjustments to the upper layer protocol (TCP/UDP) headers. A separate
section on FTP-ALG describes the changes FTP-ALG would make to FTP
payload as an FTP packet traverses from V4 to V6 realm or vice versa.
Protocol Translation details are described in [SIIT], but there are
some modifications required to SIIT because of the fact that NAT-PT
also performs Network Address Translation.
5.1 Translating IPv4 headers to IPv6 headers
This is done exactly the same as in SIIT apart from the following
fields:
Source Address:
The low-order 32 bits is the IPv4 source address. The high-
order 96 bits is the designated PREFIX for all v4
communications. Addresses using this PREFIX will be routed
to the NAT-PT gateway (PREFIX::/96)
Destination Address:
NAT-PT retains a mapping between the IPv4 destination
address and the IPv6 address of the destination node. The
IPv4 destination address is replaced by the IPv6 address
retained in that mapping.
5.2 Translating IPv6 headers to IPv4 headers
This is done exactly the same as in SIIT apart from the Source
Address which should be determined as follows:
Source Address:
The NAT-PT retains a mapping between the IPv6 source address
and an IPv4 address from the pool of IPv4 addresses
available. The IPv6 source address is replaced by the IPv4
address retained in that mapping.
Destination Address:
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IPv6 packets that are translated have a destination address
of the form PREFIX::IPv4/96. Thus the low-order 32 bits of
the IPv6 destination address is copied to the IPv4
destination address.
5.3 TCP/UDP/ICMP Checksum Update
NAT-PT retains mapping between IPv6 address and an IPv4 address
from the pool of IPv4 addresses available. This mapping is used in
the translation of packets that go through NAT-PT.
The following sub-sections describe TCP/UDP/ICMP checksum update
procedure in NAT-PT, as packets are translated from V4 to V6 and
vice versa.
5.3.1 TCP/UDP/ICMP Checksum Update from IPv4 to IPv6
UDP checksums, when set to a non-zero value, and TCP checksum
SHOULD be recalculated to reflect the address change from v4 to
v6. The incremental checksum adjustment algorithm may be
borrowed from [NAT]. In the case of NAPT-PT, TCP/UDP checksum
should be adjusted to account for the address and TCP/UDP port
changes, going from V4 to V6 address.
When the checksum of a V4 UDP packet is set to zero, NAT-PT
MUST evaluate the checksum in its entirety for the
V6-translated UDP packet. If a V4 UDP packet with a checksum of
zero arrives in fragments, NAT-PT MUST await all the fragments
until they can be assembled into a single non-fragmented packet
and evaluate the checksum prior to forwarding the translated V6
UDP packet.
ICMPv6, unlike ICMPv4, uses a pseudo-header, just like UDP and
TCP during checksum computation. As a result, when the ICMPv6
header checksum is computed [SIIT], the checksum needs to be
adjusted to account for the additional pseudo-header. Note,
there may also be adjustments required to the checksum due to
changes in the source and destination addresses (and changes in
TCP/UDP/ICMP identifiers in the case of NAPT-PT) of the payload
carried within ICMP.
5.3.2 TCP/UDP/ICMP Checksum Update from IPv6 to IPv4
TCP and UDP checksums SHOULD be recalculated to reflect the
address change from v6 to v4. The incremental checksum
adjustment algorithm may be borrowed from [NAT]. In the case of
NAPT-PT, TCP/UDP checksums should be adjusted to account for
the address and TCP/UDP port changes, going from V6 to V4
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addresses. For UDP packets, optionally, the checksum may simply
be changed to zero.
The checksum calculation for a V4 ICMP header needs to be
derived from the V6 ICMP header by running the checksum
adjustment algorithm [NAT] to remove the V6 pseudo header from
the computation. Note, the adjustment must additionally take
into account changes to the checksum as a result of updates to
the source and destination addresses (and transport ports in
the case of NAPT-PT) made to the payload carried within ICMP.
6. FTP Application Level Gateway (FTP-ALG) Support
Because an FTP control session carries, in its payload, the IP
address and TCP port information for the data session, an FTP-ALG is
required to provide application level transparency for this popular
Internet application.
In the FTP application running on a legacy V4 node, arguments to the
FTP PORT command and arguments in PASV response(successful) include
an IP V4 address and a TCP port, both represented in ASCII as
h1,h2,h3,h4,p1,p2. However, [FTP-IPV6] suggests EPRT and EPSV command
extensions to FTP, with an intent to eventually retire the use of
PORT and PASV commands. These extensions may be used on a V4 or V6
node. FTP-ALG, facilitating transparent FTP between V4 and V6 nodes,
works as follows.
6.1 Payload modifications for V4 originated FTP sessions
A V4 host may or may not have the EPRT and EPSV command extensions
implemented in its FTP application. If a V4 host originates the
FTP session and uses PORT or PASV command, the FTP-ALG will
translate these commands into EPRT and EPSV commands respectively
prior to forwarding to the V6 node. Likewise, EPSV response from
V6 nodes will be translated into PASV response prior to forwarding
to V4 nodes. The format of EPRT and EPSV commands and EPSV
response may be specified as follows[FTP-IPV6].
EPRT<space><d><net-prt><d><net-addr><d><tcp-port><d>
EPSV<space><net-prt>
(or)
EPSV<space>ALL
Format of EPSV response(Positive): 229 <text indicating
extended passive mode> (<d><d><d><tcp-port><d>)
PORT command from a V4 node is translated into EPRT command, by
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setting the protocol <net-prt> field to AF #2 (IPV6) and
translating the V4 host Address (represented as h1,h2,h3,h4) into
its NAT-PT assigned V6 address in string notation, as defined in
[V6ADDR] in the <net-addr> field. TCP port represented by p1,p2
in PORT command must be specified as a decimal <tcp-port> in the
EPRT command. Further, <tcp-port> translation may also be required
in the case of NAPT-PT. PASV command from a V4 node is be
translated into a EPSV command with the <net-prt> argument set to
AF #2. EPSV response from a V6 node is translated into PASV
response prior to forwarding to the target V4 host.
If a V4 host originated the FTP session and was using EPRT and
EPSV commands, the FTP-ALG will simply translate the parameters to
these commands, without altering the commands themselves. The
protocol Number <net-prt> field will be translated from AF #1 to
AF #2. <net-addr> will be translated from the V4 address in ASCII
to its NAT-PT assigned V6 address in string notation as defined in
[V6ADDR]. <tcp-port> argument in EPSV response requires
translation only in the case of NAPT-PT.
6.2 Payload modifications for V6 originated FTP sessions
If a V6 host originates the FTP session, however, the FTP-ALG has
two approaches to pursue. In the first approach, the FTP-ALG will
leave the command strings "EPRT" and "EPSV" unaltered and simply
translate the <net-prt>, <net-addr> and <tcp-port> arguments from
V6 to its NAT-PT (or NAPT-PT) assigned V4 information. <tcp-port>
is translated only in the case of NAPT-PT. Same goes for EPSV
response from V4 node. This is the approach we recommend to ensure
forward support for RFC 2428. However, with this approach, the V4
hosts are mandated to have their FTP application upgraded to
support EPRT and EPSV extensions to allow access to V4 and V6
hosts, alike.
In the second approach, the FTP-ALG will translate the command
strings "EPRT" and "EPSV" and their parameters from the V6 node
into their equivalent NAT-PT assigned V4 node info and attach to
"PORT" and "PASV" commands prior to forwarding to V4 node.
Likewise, PASV response from V4 nodes is translated into EPSV
response prior to forwarding to the target V6 nodes. However, the
FTP-ALG would be unable to translate the command "EPSV<space>ALL"
issued by V6 nodes. In such a case, the V4 host, which receives
the command, may return an error code indicating unsupported
function. This error response may cause many RFC 2428 compliant
FTP applications to simply fail, because EPSV support is mandated
by RFC 2428. The benefit of this approach, however, is that is
does not impose any FTP upgrade requirements on V4 hosts.
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6.3 Header updates for FTP control packets
All the payload translations considered in the previous sections
are based on ASCII encoded data. As a result, these
translations may result in a change in the size of packet.
If the new size is the same as the previous, only the TCP
checksum needs adjustment as a result of the payload translation.
If the new size is different from the previous, TCP sequence
numbers should also be changed to reflect the change in the
length of the FTP control session payload. The IP packet length
field in the V4 header or the IP payload length field in the V6
header should also be changed to reflect the new payload size. A
table is used by the FTP- ALG to correct the TCP sequence and
acknowledgement numbers in the TCP header for control packets in
both directions.
The table entries should have the source address, source data
port, destination address and destination data port for V4 and V6
portions of the session, sequence number delta for outbound
control packets and sequence number delta for inbound control
packets.
The sequence number for an outbound control packet is increased
by the outbound sequence number delta, and the acknowledgement
number for the same outbound packet is decreased by the inbound
sequence number delta. Likewise, the sequence number for an
inbound packet is increased by the inbound sequence number delta
and the acknowledgement number for the same inbound packet is
decreased by the outbound sequence number delta.
7. NAT-PT Limitations and Future Work
All limitations associated to NAT [NAT-TERM] are also associated to NAT-
PT. Here are the most important of them in detail, as well as some
unique to NAT-PT.
7.1 Topology limitations
There are limitations to using the NAT-PT translation method. It
is mandatory that all requests and responses pertaining to a
session be routed via the same NAT-PT router. One way to guarantee
this would be to have NAT-PT based on a border router that is
unique to a stub domain, where all IP packets are either
originated from the domain or destined to the domain. This is a
generic problem with NAT and it is fully described in [NAT-TERM].
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Note, this limitation does not apply to packets originating
from or directed to dual-stack nodes that do not require packet
translation. This is because in a dual-stack set-up, IPv4
addresses implied in a V6 address can be identified from the
address format PREFIX::x.y.z.w and a dual-stack router can
accordingly route a packet between v4 and dual-stack nodes
without tracking state information.
This should also not affect IPv6 to IPv6 communication and in fact
only actually use translation when no other means of communication
is possible. For example NAT-PT may also have a native IPv6
connection and/or some kind of tunneled IPv6 connection. Both of
the above connections should be preferred over translation when
possible. The above makes sure that NAT-PT is a tool only to be
used to assist transition to native IPv6 to IPv6 communication.
7.2 Protocol Translation Limitations
A number of IPv4 fields have changed meaning in IPv6 and
translation is not straightforward. For example, the option
headers semantics and syntax have changed significantly in IPv6.
Details of IPv4 to IPv6 Protocol Translation can be found in
[SIIT].
7.3 Impact of Address Translation
Since NAT-PT performs address translation, applications that
carry the IP address in the higher layers will not work. In
this case Application Layer Gateways (ALG) need to be
incorporated to provide support for those applications. This is a
generic problem with NAT and it is fully described in [NAT-TERM].
7.4 Lack of end-to-end security
One of the most important limitations of the NAT-PT proposal is
the fact that end-to-end network layer security is not possible.
Also transport and application layer security may not be possible
for applications that carry IP addresses to the application
layer. This is an inherent limitation of the Network Address
Translation function.
Independent of NAT-PT, end-to-end IPSec security is not possible
across different address realms. The two end-nodes that seek IPSec
network level security must both support one of IPv4 or IPv6.
7.5 DNS Translation and DNSSEC
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The scheme described in section 4.2 involves translation of DNS
messages. It is clear that this scheme can not be deployed in
combination with secure DNS. I.e., an authoritative DNS name
server in the V6 domain cannot sign replies to queries that
originate from the V4 world. As a result, an V4 end-node that
demands DNS replies to be signed will reject replies that have
been tampered with by NAT-PT.
The good news, however, is that only servers in V6 domain that
need to be accessible from the V4 world pay the price for the
above limitation, as V4 end-nodes may not access V6 servers due to
DNS replies not being signed.
Also note that zone transfers between DNS-SEC servers within the
same V6 network are not impacted.
Clearly, with DNS SEC deployment in DNS servers and end-host
resolvers, the scheme suggested in this document would not work.
8. Applicability Statement
NAT-PT can be a valuable transition tool at the border of a stub
network that has been deployed as an IPv6 only network when it is
connected to an Internet that is either V4-only or a combination of
V4 and V6.
NAT-PT, in its simplest form, without the support of DNS-ALG,
provides one way connectivity between an IPv6 stub domain and the
IPv4 world meaning that only sessions initialised by IPv6 nodes
internal to the IPv6 stub domain can be translated, while sessions
initiated by IPv4 nodes are dropped. This makes NAT-PT a useful
tool to IPv6 only stub networks that need to be able to maintain
connectivity with the IPv4 world without the need to deploy servers
visible to the IPv4 world.
NAT-PT combined with a DNS-ALG provides bi-directional connectivity
between the IPv6 stub domain and the IPv4 world allowing sessions to
be initialised by IPv4 nodes outside the IPv6 stub domain. This
makes NAT-PT useful for IPv6 only stub networks that need to deploy
servers visible to the IPv4 world.
Some applications count on a certain degree of address stability for
their operation. Dynamic address reuse by NAT-PT might not be
agreeable for these applications. For hosts running such address
critical applications, NAT-PT may be configured to provide static
address mapping between the host's V6 address and a specific V4
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address. This will ensure that address related changes by NAT-PT do
not become a significant source of operational failure.
9. Security Considerations
Section 7.4 of this document states that end-to-end network and
transport layer security are not possible when a session is
intercepted by a NAT-PT. Also application layer security may not be
possible for applications that carry IP addresses in the
application layer.
Section 7.5 of this document states that the DNS-ALG can not be
deployed in combination with secure DNS.
Finally, all of the security considerations described in [NAT-TERM]
are applicable to this document as well.
10. REFERENCES
[DNS-ALG] P. Srisuresh, G. Tsirtsis, P. Akkiraju, A. Heffernan, "DNS
extensions to Network Address Translators (DNS_ALG)", RFC 2694,
September 1999.
[DNSSEC] D. Eastlake, "Domain Name System Security Extensions", RFC
2065, March 1999.
[FTP-IPV6] M. Allman, S. Ostermann, C. Metz, "FTP Extensions for IPv6
and NATs", RFC 2428, September 1998.
[KEYWORDS] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[NAT] K. Egevang, P. Francis, "The IP Network Address Translator
(NAT)", RFC 1631, May 1994
[NAT-TERM] P. Srisuresh, M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations", RFC 2663, August 1999.
[SIIT] E. Nordmark, "Stateless IP/ICMP Translator (SIIT)",
<draft-ietf-ngtrans-siit-05.txt>, Work in progress, January 1999.
[TRANS] R. Gilligan, E. Nordmark, "Transition Mechanisms for IPv6 Hosts
and Routers", RFC 1933, April 1996
[V6ADDR] R. Hinden, S. Deering, "IP Version 6 Addressing Architecture",
RFC 2373, July 1998
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AUTHORS
George Tsirtsis
Internet Futures
B29 Room 129
BT Adastral Park
IPSWICH IP5 3RE
England
Phone: +44 181 8260073
Fax: +44 181 8260073
e-mail: george.tsirtsis@bt.com
e-mail (alternative): gtsirt@hotmail.com
Pyda Srisuresh
849 Erie Circle
Milpitas, CA 95035
U.S.A.
Tel: (408) 263-7527
EMail: srisuresh@yahoo.com
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