One document matched: draft-carpenter-shanti-00.txt
Network Working Group B. Carpenter
Internet-Draft Univ. of Auckland
Intended status: Experimental October 28, 2007
Expires: April 30, 2008
Shimmed IPv4/IPv6 Address Network Translation Interface (SHANTI)
draft-carpenter-shanti-00
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
There is a pragmatic need for a packet-level translation mechanism
between IPv4 and IPv6, for scenarios where no other mode of IPv4 to
IPv6 interworking is possible. The mechanism defined here uses a
shim in both the translator and the IPv6 host to mitigate the
problems introduced by stateless translation.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 4
2. Summary of scenario . . . . . . . . . . . . . . . . . . . . . 4
3. General walkthroughs . . . . . . . . . . . . . . . . . . . . . 6
4. Placement of the shim . . . . . . . . . . . . . . . . . . . . 8
5. DNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. ICMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Unresolved issues . . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 12
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1. Introduction
There has long been a defined mechanism for stateless translation
betweeen IPv4 and IPv6 packet formats [RFC2765]. Its intended use is
any scenario whether dual stack coexistence between IPv4 and IPv6,
possibly accompanied by dual stack application level proxies, is
insufficient. In the most stringent case, this will occur when
communication is needed between unmodified ("legacy") IPv4 hosts and
IPv6-only hosts that have no IPv4 code, and no dual stack proxy is
available for the application protocol of interest.
The previously proposed solution for this scenario, NAT-PT [RFC2766]
has known issues and has been deprecated [RFC4966]. The present
proposal does not resolve all of those issues; a later section will
identify the issues believed to remain open. This proposal aims to
resolve those issues that can be handled if the IPv6 protocol stack
communicating with a translator can exchange information with the
translator that is specific to the translation process. The
objectives are to ensure that
a. from the IPv4 host's point of view, nothing is worse than in the
case of an IPv4-to-IPv4 translation
b. from the IPv6 host's point of view, no special code is generally
required in the transport layer or above. However, information
about the translation is available in the IPv6 host's network
stack, if needed. This is the crucial difference from NAT-PT.
To achieve these goals, a shim is inserted in the protocol stack at
both the IPv6 host and at the translator. Its objective is to allow
the IPv6 stack at the host to be aware of the presence of the
translator, of the addresses involved in the translation, and of any
other information known by the translator that may be of value to the
IPv6 host. A shim model is chosen, as in SHIM6
[I-D.ietf-shim6-proto], so that upper layer protocols (ULPs) have no
need to be aware of anything unusual. The mechanism is known as
SHimmed Address Network Translation Interface (SHANTI).
As in SHIM6, ULPs are presented with an upper layer identifier (ULID)
in the form of an IPv6 address which is independent of any
manipulation of addresses in the shim or translator.
The reader is assumed to have a general understanding of SHIM6.
Although this early draft does not assume that the SHIM6 mechanisms
defined in [I-D.ietf-shim6-proto] would be used unchanged, they form
a proof of concept for the type of communication required between two
network-layer shims.
It should be noted that this mechanism adds complexity to an IPv6-
only host. This has to be balanced against the complexity of a dual-
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stack host. In this model, no residual IPv4 code is needed in the
IPv6 host. The shim has to handle the rewriting of addresses and
port numbers, but nothing else.
It should also be noted that this mechanism strenuously avoids any
impact whatever on IPv6 addressing and routing "on the wire".
DISCLAIMER: This draft is incomplete. It is posted to seek comments
on plausibility; much more work is needed to make it implementable.
1.1. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Summary of scenario
Consider an IPv6-only host X and and IPv4-only host Y.
Let A(x) be an IPv6 address for X, and let a(y) be an IPv4 address
for Y. Let the port in use at X be P(x) and at Y be P(y).
We will observe later that it is irrelevant whether a(y) is
translated by an IPv4 NAT, and whether P(y) is translated by an IPv4
NAPT.
Additionally, consider a translator T between X and Y. On the IPv6
side it has address A(t) and on the IPv4 side it has address a(t).
If port translation is in effect, P(x) will become P(tx) on the IPv4
side. We will observe later that the A(t) address can be chosen from
an address pool. We cannot assume that a(t) can be chosen from a
pool, which is why port translation will be needed.
Thus A() is always an IPv6 address and a() is always an IPv4 address.
A diagram of the solution follows:
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X T Y
___________ A(x) A(t) _______________ a(t) a(y) _______
| | | V6|P(x) P(y)| V6| | | V4|P(tx) P(y)| V4| |
| | S | | | | S | S | | | | |
| U | H | S | | S | H | I | S | | S | U |
| L | I | T |------------| T | I | I | T |-----------| T | L |
| P | M | A | | A | M | T | A | | A | P |
| | | C | | C | | | C | | C | |
| | | K | | K | | | K | | K | |
|___|___|___| |___|___|___|___| |___|___|
The address set used by the shim for X is conceptually {a(t),A(x)},
and for Y it is conceptually {a(y),A(t)}. In other words the ULP at
X sees its own ULID as a(t) and Y's ULID as a(y), both filled out
with /96 prefixes. On the wire, the IPv6 packets between X and T use
A(x) and A(t) as the actual address pair. The IPv4 packets between T
and Y use a(t) and a(y). P(y) can be used everywhere, but we must
assume that P(x) will be used on the IPv6 side and P(tx) on the IPv4
side.
If there's a NAT with routable address a(n) on the IPv4 path, it
won't know anything is special, and a(y) will be replaced by a(n).
X, Y and T won't know the NAT is there. X and T will not know if Y
has a private [RFC1918] address or if additional port translation
takes place.
T should have a pool of A(t) addresses, and should probably have a
complete /64 to itself for maximum flexibility.
When the ULP in X sends a first packet to ::FF:0:0:a(y)/128, we need
to start a SHIM6-like process. The shim in X is configured to catch
such packets, and carry out a message exchange with the shim in T to
discover the relevant a(t), A(t) and P(tx) values. It can then
rewrite the packet header and recompute a checksum as needed, and
send the packet on to A(t).
T needs to select a specific A(t) and P(tx) for each new flow, and do
SHIM6-like things to tell X what the addresses are. This should
create enough state in the shims to know what to do with outbound and
return packets. Since T has a full /64 to work with, it can create a
new A(t) for each new X or even for each new flow if that turns out
to be needed.
Note that unlike SHIM6, SHANTI must perform the shim exchange before
sending the first packet of a traffic flow. This is because the
source ULID to be used must be expanded from a(t) and is not
initially known by the source host. Also, if P(tx) is unequal to
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P(x), this must be learned by the shim in X. A consequence of this is
that the shim in X must buffer packets until it has completed its
shim exchange with T. For this solution to scale, it is important
that the SHANTI translator has adequate capacity for the number of
IPv6 hosts it serves, and adequate network connectivity to them.
OPEN QUESTION: Would it be preferable to configure X with knowledge
of a(t)? Even so, when port translation is needed, the value of
P(tx) can only be discovered dynamically.
When a data packet reaches T from X, there will already be shim state
established. The shim will pass the packet on to SIIT for
translation and IPv4 transmission.
Once the shim state is established, the ULPs in both X and Y will
work as normal. Since T uses a specific A(t) for each X, and the
shim at X is aware of that A(t), all port numbers are available in
each direction on the IPv6 side. Port mapping, if required, will
only affect the IPv4 side of T. Also, the shim in X is aware that the
ULP in Y believes it is using the address pair {a(t), a(y)} and the
ports {P(tx), P(y)}. Thus, address and port dependent fix-ups can be
performed by the shim in X. In addition to using ::FF:0:0:a(y)/128 as
the ULID for y, the IPv6 stack can use ::FF:0:0:a(t)/128 as the ULID
for X, although the IPv6 packets will be sent between A(x) and A(t).
This plus the shim's knowledge of P(tx) means that TCP and UDP
checksums do not need to be fixed up by T. This has scaling
advantages compared to NAT-PT.
Additionally, with this knowledge being available in the host rather
than being hidden in the translator as in NAT-PT, it is in principle
possible for any address and port dependencies in the ULP to be fixed
up in the host itself, precluding the need for Application Level
Gateways (ALGs). Although this would introduce a layer violation, it
is in principle a more robust design than associating ALGs with a
"stateless" translator.
OPEN QUESTION: In SIIT, an "IPv4-translated" address format is
introduced to represent a synthetic IPv4 address for the IPv6 host,
with the ::FF:0:0:0/96 prefix. This format, which is not in the IPv6
address architecture [RFC4291], could be used as the ULID for X. But
since the shim has explicit knowledge of the addresses in use, is
there any reason to use this format in preference to the simpler
::FF:0:0/96 IPv4-mapped format? The latter is assumed here.
3. General walkthroughs
Consider first an IPv6 client attempting to contact an IPv4 server
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via this mechanism. The main steps that must occur are:
1. ULP in X obtains Y's IPv4-mapped address ::FF:0:0:a(y)/128. See
DNS discussion below.
2. ULP sends unsolicited packet to that address.
3. The shim in X recognises the packet as needing attention.
4. The shim creates local state for a(y), P(x), and buffers the
packet. Also, it creates a packet to send to T. This is a
packet containing nothing but a shim header indicating that a
first packet is ready from A(x):P(x) to a(y):P(y).
5. The shim at T receives this shim header and checks for existing
state for {A(x):P(x),a(y):P(y)}.
6. If no such state exists, assign an A(t) value from the pool, and
create state. Includes the ports. If P(x) is already in use by
T, assign a P(tx). Otherwise, P(tx)=P(x).
7. The shim in T creates a packet to return to X. This is a packet
containing nothing but a shim header indicating the assigned
A(t), a(t) and P(tx).
8. The shim in X records this additional state, in particular
recording ::FF:0:0:a(t)/128 as the source ULID and P(tx) as the
translated port.
9. The shim in X now applies the following process to buffered and
future packets sent from A(x), port P(x) to ::FF:0:0:a(y), port
P(y).
1. Compute checksums as for addresses DA=::FF:0:0:a(y), SA=::
FF:0:0:a(t) and ports DP=P(y), SP=P(tx).
2. Rewrite destination address as A(t).
3. Send packet to A(t). At this point its source address is
still A(x).
10. The shim in T rewrites the addresses as DA=::FF:0:0:a(y), SA=::
FF:0:0:a(t), and the source port as P(tx), and hands the packet
off to SIIT.
11. SIIT translates the packet and sends it on (destination = a(y),
source = a(t)).
12. When an IPv4 return packet comes into SIIT, SIIT translates the
packet to IPv6 and hands it to the shim in T.
13. The shim performs port demultiplexing on the destination port
(which will be P(tx)) to identify the A(x) involved.
14. The shim sends the packet on to A(x).
15. The shim at X receives the packet, rewrites the addresses to
restore the original ULIDs and P(x), and sends the packet on up
the stack.
Now consider an IPv4 client attempting to contact an IPv6 server via
T. The main steps that must occur are:
1. T must be pre-configured to admit traffic for P(x) and forward it
to A(x). This is a normal port-forwarding issue, to be solved as
for NATs or perhaps as proposed in [I-D.woodyatt-ald]. It cannot
be performed without pre-existing state. Assuming T has only one
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a(t), a given P(x) can only have one IPv6 listener.
2. ULP in Y obtains an IPv4 address for T (believing it to be the
actual server X).
3. Y sends an unsolicited packet from a(y) to a(t), port P(x).
4. T performs port demultiplexing and determines that the packet is
destined for A(x). It is therefore passed to SIIT in T,
translated to IPv6 format, and passed on to the shim in T.
5. The shim inserts a shim header that will tell X the translation
in effect, translates the addresses, and sends the packet from
A(t) to A(x).
6. The shim at X receives the packet, and translates the addresses
to ::FF:0:0:a(t)/128 and ::FF:0:0:a(y)/128. This should checksum
OK.
7. The packet is delivered to the ULP, minus the shim header.
State will be created and subsequent packets will flow as in the
previous case.
4. Placement of the shim
In SHIM6 the shim is logically placed below both the transport and
IPsec layers, so that their checksums do not need recalculation. In
SHANTI, the transport layer checksum does need to be recalculated by
the shim, rather in the manner that a NAT behaves. However, this
cannot be done for cryptographic checksums for obvious reasons. The
shim should perhaps be regarded as logically below transport, but a
better implementation would be for each transport layer to invoke the
shim in-line prior to executing its checksum calculation.
5. DNS
It is required that the IPv6 hosts "behind" a SHANTI translator
either have a resolver that maps A records into AAAA records expanded
with ::FF:0:0/96, or a DNS server that actually stores such records,
or a DNS ALG that performs this transformation on the fly. On the
assumption that hosts behind a translator will need to be configured
in any case, in order to activate the shim, a mapping resolver seems
likely to be the most robust choice, applying the fate-sharing
principle. It would also work in a network with a mixture of SHANTI
and dual-stack hosts. The former would see A records mapped as AAAA,
and the latter would see native A records.
This illustrates that SHANTI is an all-or-nothing approach. It
doesn't seem plausible to activate SHANTI on a dual stack host since
DNS entries are either mapped, or they aren't. But why would it be
needed?
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"Outside" the translator, SHANTI hosts must be represented by an A
record with the address of their translator. Specifically, the
host's FQDN will have one or more AAAA records with its IPv6
address(es) and an A record with its translator's address.
6. ICMP
TBD
7. Unresolved issues
This section will be expanded to identify issues raised in [RFC4966]
that are not resolved by the present specification.
8. Security Considerations
As for NAT-PT, there is no obvious way to carry network layer IPsec
across a SHANTI translator. There seems to be no reason IKE
[RFC4306] cannot run in a SHANTI scenario, using its port agility
intended for NAT tolerance. But that in itself isn't very useful
The use of a shim layer in SHANTI will raise some of the security
issues considered for SHIM6 . More analysis of the potential threats
is needed to determine whether a cryptographic solution is needed, or
if there is a straightforward way to prevent attackers taking over a
session by impersonating the shim. It may be possible to find a
simple method of arranging a shared secret between X and T, such that
an elementary hash can be used to authenticate the shim headers.
9. IANA Considerations
This document has not yet been exhaustively checked for possible
action by the IANA.
10. Acknowledgements
Vital comments on a very primitive version of this proposal were made
by Marcelo Bagnulo Braun and Iljitsch van Beijnum. Contributions and
comments by TBD are gratefully acknowledged.
This document was produced using the xml2rfc tool [RFC2629].
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11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm
(SIIT)", RFC 2765, February 2000.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
11.2. Informative References
[I-D.ietf-shim6-proto]
Bagnulo, M. and E. Nordmark, "Shim6: Level 3 Multihoming
Shim Protocol for IPv6", draft-ietf-shim6-proto-08 (work
in progress), April 2007.
[I-D.woodyatt-ald]
Woodyatt, J., "Application Listener Discovery (ALD) for
IPv6", draft-woodyatt-ald-01 (work in progress),
June 2007.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address
Translation - Protocol Translation (NAT-PT)", RFC 2766,
February 2000.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[RFC4966] Aoun, C. and E. Davies, "Reasons to Move the Network
Address Translator - Protocol Translator (NAT-PT) to
Historic Status", RFC 4966, July 2007.
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Author's Address
Brian Carpenter
Department of Computer Science
University of Auckland
PB 92019
Auckland, 1142
New Zealand
Email: brian.e.carpenter@gmail.com
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