Network Working Group M. Wasserman
Internet-Draft Painless Security
Intended status: Standards Track F. Baker
Expires: June 12, 2011 Cisco Systems
December 9, 2010
IPv6-to-IPv6 Network Prefix Translation
draft-mrw-nat66-01
Abstract
This document describes a stateless, transport-agnostic IPv6-to-IPv6
Network Prefix Translation (NPTv6) function that provides the address
independence benefit associated with IPv4-to-IPv4 NAT (NAT44), and in
addition provides a 1:1 relationship between addresses in the
"inside" and "outside" prefixes, preserving end to end reachability
at the network layer.
Requirements Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on June 12, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Wasserman & Baker Expires June 12, 2011 [Page 1]
Internet-Draft NPTv6 December 2010
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. What is Address Independence? . . . . . . . . . . . . . . 4
1.2. NPTv6 Applicability . . . . . . . . . . . . . . . . . . . 6
2. NPTv6 Overview . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1. NPTv6: the simplest case . . . . . . . . . . . . . . . . . 7
2.2. NPTv6 between peer networks . . . . . . . . . . . . . . . 8
2.3. NPTv6 redundnacy and load-sharing . . . . . . . . . . . . 9
2.4. NPTv6 multihoming . . . . . . . . . . . . . . . . . . . . 9
2.5. Mapping with No Per-Flow State . . . . . . . . . . . . . . 10
2.6. Checksum-Neutral Mapping . . . . . . . . . . . . . . . . . 10
3. NPTv6 Algorithmic Specification . . . . . . . . . . . . . . . 11
3.1. NPTv6 configuration calculations . . . . . . . . . . . . . 11
3.2. NPTv6 translation, internal network to external network . 12
3.3. NPTv6 translation, external network to internal network . 12
3.4. NPTv6 with a /48 or shorter prefix . . . . . . . . . . . . 12
3.5. NPTv6 with a /49 or longer prefix . . . . . . . . . . . . 13
3.6. /48 Prefix Mapping Example . . . . . . . . . . . . . . . . 13
3.7. Address Mapping for Longer Prefixes . . . . . . . . . . . 14
4. Implications of Network Address Translator Behavioral
Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.1. Prefix Configuration and generation . . . . . . . . . . . 15
4.2. NAT Behavioral Requirements . . . . . . . . . . . . . . . 15
5. Implications for Applications . . . . . . . . . . . . . . . . 16
6. A Note on Port Mapping . . . . . . . . . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
10. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.1. Changes Between draft-mrw-behave-nat66-00 and -01 . . . . 17
10.2. Changes between *behave-nat66-01 and -02 . . . . . . . . . 18
10.3. Changes between *behave-nat66-02 and *nat66-00 . . . . . . 18
10.4. Changes between *nat66-00 and *nat66-01 . . . . . . . . . 18
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
11.1. Normative References . . . . . . . . . . . . . . . . . . . 19
11.2. Informative References . . . . . . . . . . . . . . . . . . 20
Appendix A. Why GSE? . . . . . . . . . . . . . . . . . . . . . . 20
Wasserman & Baker Expires June 12, 2011 [Page 2]
Internet-Draft NPTv6 December 2010
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
Wasserman & Baker Expires June 12, 2011 [Page 3]
Internet-Draft NPTv6 December 2010
1. Introduction
This document describes a stateless IPv6-to-IPv6 Network Prefix
Translation (NPTv6) function, designed to provide address
independence to the edge network. It is transport-agnostic with
respect to transports that don't checksum the IP header, such as SCTP
or DCCP, and to transports that use the TCP/UDP pseudo-header and
checksum [RFC1071].
This has several ramifications:
o Any security benefit that NAT44 might offer is not present in
NPTv6, necessitating the use of a firewall to obtain those
benefits if desired. An example of such a firewall is described
in [I-D.ietf-v6ops-cpe-simple-security].
o End to end reachability is preserved, although the address used
"inside" the edge network differs from the address used "outside"
the edge network. This has implications for application referrals
and other uses of Internet layer addresses.
o If there are multiple identically-configured prefix translators
between two networks, there is no need for them to exchange
dynamic state, as there is no dynamic state - the algorithmic
translation will be identical across each of them. The network
can therefore asymmetrically route, load-share, and fail-over
among them without issue.
o Since translation is 1:1 at the network layer, there is no need to
modify port numbers or other transport parameters.
1.1. What is Address Independence?
For the purposes of this document, IPv6 Address Independence consists
of the following set of properties:
From the perspective of the edge network:
* The IPv6 addresses used inside the local network (for
interfaces, access lists, and logs) do not need to be
renumbered if the upstream network changes a site's external
prefix.
* The IPv6 addresses used inside the edge network (for
interfaces, access lists, and logs) or within other upstream
networks (such as when multihoming) do not need to be
renumbered when a site adds, drops, or changes upstream
networks.
Wasserman & Baker Expires June 12, 2011 [Page 4]
Internet-Draft NPTv6 December 2010
* It is not necessary for an administration to convince an
upstream network to route its internal IPv6 prefixes, or for it
to advertise prefixes derived from other upstream networks into
it.
* Unless it wants to optimize routing between multiple upstream
networks in the process of multihoming, there is therefore no
need for a BGP exchange with the upstream network.
From the perspective of the upstream network:
* The IPv6 addresses used the edge network are guaranteed to use
a provider-allocated prefix, eliminating the need and concern
for BCP 38 [RFC2827] ingress filtering and the advertisement of
customer-specific prefixes.
Thus, address independence has ramifications for the edge network,
networks it directly connects with (especially its upstream
networks), and for the Internet as a whole. The desire for address
independence has been a primary driver for IPv4 NAT deployment in
medium to large-sized enterprise networks, including NAT deployments
in enterprises that have plenty of IPv4 provider-independent address
space (from IPv4 "swamp space"). It has also been a driver for edge
networks to become members of RIR communities, seeking to obtain BGP
Autonomous System Numbers and provider-independent prefixes, and as a
result has been one of the drivers of the explosion of the IPv4 route
table. Service providers have stated that the lack of address
independence from their customers has been a negative incentive to
deployment, due to the impact of customer routing expected in their
networks.
The Local Network Protection [RFC4864] document discusses a related
concept called "Address Autonomy" as a benefit of NAT44. [RFC4864]
indicates that address autonomy can be achieved by the simultaneous
use of global addresses on all nodes within a site that need external
connectivity, and Unique Local Addresses (ULAs) [RFC4193] for all
internal communication. However, this solution fails to meet the
requirement for address independence, because if an ISP renumbering
event occurs, all of the hosts, routers, DHCP servers, ACLs,
firewalls and other internal systems that are configured with global
addresses from the ISP will need to be renumbered before global
connectivity is fully restored.
The use of IPv6 Provider Independent (PI) addresses has also been
suggested as a means to fulfill the address independence requirement.
However, this solution requires that an enterprise qualify to receive
a PI assignment and persuade their ISP to install specific routes for
the enterprise's PI addresses. There are a number of practical
Wasserman & Baker Expires June 12, 2011 [Page 5]
Internet-Draft NPTv6 December 2010
issues with this approach, especially if there is a desire to route
to a number of geographically and topologically diverse set of sites,
which can sometimes involve coordinating with several ISPs to route
portions of a single PI prefix. These problems have caused numerous
enterprises with plenty of IPv4 swamp space to choose to use IPv4 NAT
for part, or substantially all, of their internal network instead of
using their provider-independent address space.
1.2. NPTv6 Applicability
NPTv6 provides a simple and compelling solution to meet the Address
Independence requirement in IPv6. The address independence benefit
stems directly from the translation function of the network address
translator. To avoid as many of the issues associated with NAT44 as
possible, NPTv6 is defined to include a two-way, checksum-neutral,
algorithmic translation function, and nothing else.
NPTv6 does not include a port mapping function, and the defined
address mapping mechanism is checksum-neutral. This avoids the need
for a NPTv6 device to re-write transport layer headers, making it
feasible to deploy new or improved transport layer protocols without
upgrading NPTv6 devices. Because NPTv6 does not involve re-writing
transport-layer headers, NPTv6 will not interfere with encryption of
the full IP payload in many cases.
The default NPTv6 address mapping mechanism is purely algorithmic, so
NPTv6 translators do not need to maintain per-node or per-connection
state, allowing deployment of more robust and adaptive networks than
can be deployed using NAT44. Since the default NPTv6 mapping can be
performed in either direction, it does not interfere with inbound
connection establishment, thus allowing internal nodes to participate
in direct Peer-to-Peer applications without the application layer
overhead one finds in many IPv4 Peer-to-Peer applications.
Although NPTv6 compares favorably to NAT44 in several ways, it does
not eliminate all of the architectural problems associated with IPv4
NAT, as described in [RFC2993]. NPTv6 involves modifying IP headers
in transit, so it is not compatible with security mechanisms, such as
the IPsec Authentication Header, that provide integrity protection
for the IP header. NPTv6 may interfere with the use of application
protocols that transmit IP addresses in the application-specific
portion of the IP packet. These applications currently require
application layer gateways (ALGs) to work correctly through NAT44
devices, and similar ALGs may be required for these applications to
work through NPTv6 devices. The use of separate internal and
external prefixes creates complexity for DNS deployment, due the
desire for internal nodes to communicate with other internal nodes
using internal addresses, while external nodes need to obtain
Wasserman & Baker Expires June 12, 2011 [Page 6]
Internet-Draft NPTv6 December 2010
external addresses to communicate with the same nodes. This
frequently results in the deployment of "split DNS", which may add
complexity to network configuration.
There are significant technical impacts associated with the
deployment of any address translation mechanism, including NPTv6, and
we strongly encourage anyone who is considering the implementation or
deployment of NPTv6 to read [RFC4864], and to carefully consider the
alternatives described in that document, some of which may cause
fewer problems than NPTv6.
2. NPTv6 Overview
NPTv6 may be implemented in an IPv6 router to map one IPv6 address
prefix to another IPv6 prefix as each IPv6 packet transits the
router. A router that implements a NPTv6 prefix translation function
is referred to as an NPTv6 Translator.
2.1. NPTv6: the simplest case
In its simplest form, a NPTv6 Translator interconnects two network
links, one of which is an "internal" network link attached to a leaf
network within a single administrative domain, and the other of which
is an "external" network with connectivity to the global Internet.
All of the hosts on the internal network will use addresses from a
single, locally-routed prefix, and those addresses will be translated
to/from addresses in a globally-routable prefix as IP packets transit
the NPTv6 Translator. The lengths of these two prefixes will be
functionally the same; if they differ, the longer of the two will
limit the ability to use subnets in the shorter.
Figure 1 shows an NPTv6 Translator attached to two networks. In this
example, the internal network uses IPv6 Unique Local Addresses (ULAs)
[RFC4193] to represent the internal IPv6 nodes, and the external
network uses globally routable IPv6 addresses to represent the same
nodes.
Wasserman & Baker Expires June 12, 2011 [Page 7]
Internet-Draft NPTv6 December 2010
External Network: Prefix = 2001:0DB8:0001:/48
--------------------------------------
|
|
+-------------+
| NPTv6 |
| Translator |
+-------------+
|
|
--------------------------------------
Internal Network: Prefix = FD01:0203:0405:/48
Figure 1: A simple translator
When a NPTv6 Translator forwards packets in the "outbound" direction,
from the internal network to the external network, NPTv6 overwrites
the IPv6 source prefix (in the IPv6 header) with a corresponding
external prefix. When packets are forwarded in the "inbound"
direction, from the external network to the internal network, the
IPv6 destination prefix is overwritten with a corresponding prefix
internal prefix. Using the prefixes shown in the diagram above, as
an IP packet passes through the NPTv6 Translator in the outbound
direction, the source prefix (FD01:0203:0405:/48) will be overwritten
with the external prefix (2001:0DB8:0001:/48). In an inbound packet,
the destination prefix (2001:0DB8:0001:/48) will be overwritten with
the internal prefix (FD01:0203:0405:/48). In both cases, it is the
local IPv6 prefix that is overwritten; the remote IPv6 prefix remains
unchanged. Nodes on the internal network are said to be "behind" the
NPTv6 Translator.
2.2. NPTv6 between peer networks
NPTv6 can also be used between two private networks. In these cases,
both networks may use ULA prefixes, with each subnet in one network
mapped into a corresponding subnet in the other network, and vice
versa. Or, each network may use ULA prefixes for internal
addressing, and global unicast addresses on the other network.
Wasserman & Baker Expires June 12, 2011 [Page 8]
Internet-Draft NPTv6 December 2010
Internal Prefix = FD01:4444:5555:/48
--------------------------------------
V | External Prefix
V | 2001:0DB8:6666:/48
V +---------+ ^
V | NPTv6 | ^
V | Device | ^
V +---------+ ^
External Prefix | ^
2001:0DB8:0001:/48 | ^
--------------------------------------
Internal Prefix = FD01:0203:0405:/48
Figure 2: Flow of Information in Translation
2.3. NPTv6 redundnacy and load-sharing
In some cases, more than one NPTv6 Translator may be attached to a
network, as show in Figure 3. In such cases, NPTv6 Translators are
configured with the same internal and external prefixes. Since there
is only one translation, even though there are multiple translators,
they map only one external address (prefix and IID) to the internal
address.
External Network: Prefix = 2001:0DB8:0001:/48
--------------------------------------
| |
| |
+-------------+ +-------------+
| NPTv6 | | NPTv6 |
| Translator | | Translator |
| #1 | | #2 |
+-------------+ +-------------+
| |
| |
--------------------------------------
Internal Network: Prefix = FD01:0203:0405:/48
Figure 3: Parallel Translators
2.4. NPTv6 multihoming
When multihoming, NPTv6 Translators are attached to an internal
network, as show in Figure 4, but connected to different external
networks. In such cases, NPTv6 Translators are configured with the
same internal prefix, but different external prefixes. Since there
are multiple translations, they map multiple external addresses
(prefix and IID) to the common internal address. A system within the
Wasserman & Baker Expires June 12, 2011 [Page 9]
Internet-Draft NPTv6 December 2010
edge network is unable to determine which external address it is
using.
External Network #1: External Network #2:
Prefix = 2001:0DB8:0001:/48 Prefix = 2001:0DB8:5555:/48
--------------------------- --------------------------
| |
| |
+-------------+ +-------------+
| NPTv6 | | NPTv6 |
| Translator | | Translator |
| #1 | | #2 |
+-------------+ +-------------+
| |
| |
--------------------------------------
Internal Network: Prefix = FD01:0203:0405:/48
Figure 4: Parallel Translators with different upstream networks
Multihoming in this sense has one negative feature as compared with
multihoming with a provider-independent address; when routes change
between NPTv6 Translators, since the upstream network changes, the
prefix used in shifting sessions changes. This obviously causes them
to fail. This is not expected to be a major real issue, however, in
networks where routing is generally stable.
2.5. Mapping with No Per-Flow State
When NPTv6 is used as described in this document, no per-node or per-
flow state is maintained in the NPTv6 Translator. Both inbound and
outbound packets are translated algorithmically, using only
information found in the IPv6 header. Due to this property, NPTv6's
two-way, algorithmic address mapping can support both outbound and
inbound connection establishment without the need for state-priming
or rendezvous mechanisms, or the maintenance of mapping state. This
is a significant improvement over NAT44 devices, but it also has
significant security implications which are described in the Security
Considerations section.
2.6. Checksum-Neutral Mapping
When a change is made to one of the IP header fields in the IPv6
pseudo-header checksum (such as one of the IP addresses), the
checksum field in the transport layer header may become invalid.
Fortunately, an incremental change in the area covered by the
Internet standard checksum [RFC1071] will result in a well-defined
change to the checksum value [RFC1624]. So, a checksum change caused
Wasserman & Baker Expires June 12, 2011 [Page 10]
Internet-Draft NPTv6 December 2010
by modifying part of the area covered by the checksum can be
corrected by making a complementary change to a different 16-bit
field covered by the same checksum.
The NPTv6 mapping mechanisms described in this document are checksum-
neutral, which means that they result in IP headers that will
generate the same IPv6 pseudo-header checksum when the checksum is
calculated using the standard Internet checksum algorithm [RFC1071].
Any changes that are made during translation of the IPv6 prefix are
offset by changes to other parts of the IPv6 address. This results
in transport layers that use the Internet checksum (such as TCP and
UDP) calculating the same IPv6 pseudo header checksum for both the
internal and external forms of the same packet, which avoids the need
for the NPTv6 Translator to modify those transport layer headers to
correct the checksum value.
3. NPTv6 Algorithmic Specification
The [RFC4291] IPv6 Address is reproduced for clarity in Figure 5.
0 15 16 31 32 47 48 63 64 79 80 95 96 111 112 127
+-------+-------+-------+-------+-------+-------+-------+-------+
| Routing Prefix |Subnet| Interface Identifier (IID) |
+-------+-------+-------+-------+-------+-------+-------+-------+
Figure 5: Enumeration of the IPv6 Address [RFC4291]
3.1. NPTv6 configuration calculations
When an NPTv6 Translation function is configured, it is configured
with
o one or more "internal" interfaces with their "internal" routing
domain prefixes, and
o one or more "external" interfaces with their "external" routing
domain prefixes.
In the simple case, there is one of each. If a single router
provides NPTv6 translation services between a multiplicity of domains
(as might be true when multihoming), each internal/external pair must
be thought of as a separate NPTv6 Translator from the perspective of
this specification.
When an NPTv6 Translator is configured, the translation function
first ensures that the internal and external prefixes are the same
length, if necessary by extending the shorter of the two with zeroes.
Wasserman & Baker Expires June 12, 2011 [Page 11]
Internet-Draft NPTv6 December 2010
These two prefixes will be used in the prefix translation function
described in Section 3.2 and Section 3.3.
They are then zero-extended to /64, for the purposes of a
calculation. The translation function calculates the ones-complement
sum of the 16 bit words of the /64 external prefix and the /64
internal prefix. It then calculates the difference between these
values: external minus internal. This value, called the
"adjustment", is effectively constant for the lifetime of the NPTv6
Translator configuration, and used in per-packet processing.
3.2. NPTv6 translation, internal network to external network
When a datagram passes through the NPTv6 Translator from an internal
to an external network, its IPv6 Source Address is changed in two
ways:
o The internal prefix is overwritten with the external prefix, and
o A 16-bit word of the address has the "adjustment" added to it
using one's complement arithmetic. If the result is 0xFFFF, it is
overwritten as zero. The choice of word is as specified in
Section 3.4 or Section 3.5 as appropriate.
3.3. NPTv6 translation, external network to internal network
When a datagram passes through the NPTv6 Translator from an internal
to an external network, its IPv6 Destination Address is changed in
two ways:
o The external prefix is overwritten with the internal prefix, and
o A 16-bit word of the address has the "adjustment" subtracted from
it (bitwise inverted and added to it) it using one's complement
arithmetic. If the result is 0xFFFF, it is overwritten as zero.
The choice of word is as specified in Section 3.4 or Section 3.5
as appropriate.
3.4. NPTv6 with a /48 or shorter prefix
When a NPTv6 Translator is configured with internal and external
prefixes that are 48 bits in length (a /48) or shorter, the
adjustment MUST be added to or subtracted from bits 48..63 of the
address.
This mapping results in no modification of the Interface Identifier
(IID), which is held in the lower half of the IPv6 address, so it
will not interfere with future protocols that may use unique IIDs for
Wasserman & Baker Expires June 12, 2011 [Page 12]
Internet-Draft NPTv6 December 2010
node identification.
NPTv6 Translator implementations MUST implement the /48 mapping.
3.5. NPTv6 with a /49 or longer prefix
When a NPTv6 Translator is configured with internal and external
prefixes that are longer than 48 bits in length (such as a /52, /56,
or /60), the adjustment must be added to or subtracted from one of
the words in bits 64..79, 80..95, 96..111, or 112..127 of the
address. While the choice of word is immaterial as long as it is
consistent, for consistency's sake, these words MUST be inspected in
that sequence, and the first that is not initially 0xFFFF chosen.
NPTv6 Translator implementations SHOULD implement the mapping for
longer prefixes.
3.6. /48 Prefix Mapping Example
For the network shown in Figure 1, the Internal Prefix is FD01:0203:
0405:/48, and the External Prefix is 2001:0DB8:0001:/48
If a node with internal address FD01:0203:0405:0001::1234 sends an
outbound packet through the NPTv6 Translator, the resulting external
address will be 2001:0DB8:0001:D550::1234. The resulting address is
obtained by calculating the checksum of both the internal and
external 48-bit prefixes, subtracting the internal prefix from the
external prefix using one's complement arithmetic to calculate the
"adjustment", and adding the adjustment to the 16-bit subnet field
(in this case 0x0001).
To show the work:
The one's complement checksum of FD01:0203:0405 is 0xFCF5. The one's
complement checksum of 2001:0DB8:0001 is 0xD245. Using one's
complement math, 0xD245 - 0xFCF5 = 0xD54F. The subnet in the original
packet is 0x0001. Using one's complement math, 0x0001 + 0xD54F =
0xD550. Since 0xD550 != 0xFFFF, it is not changed to 0x0000.
So, the value 0xD550 is written in the 16-bit subnet area, resulting
in a mapped external address of 2001:0DB8:0001:D550::1234.
When a response packet is received, it will contain the destination
address 2001:0DB8:0001:D550::0001, which will be mapped using the
inverse mapping algorithm, back to FD01:0203:0405:0001::1234.
In this case, the difference between the two prefixes will be
calculated as follows:
Wasserman & Baker Expires June 12, 2011 [Page 13]
Internet-Draft NPTv6 December 2010
Using one's complement math, 0xFCF5 - 0xD245 = 0x2AB0. The subnet in
the original packet = 0xD550. Using one's complement math, 0xD550 +
0x2AB0 = 0x0001. Since 0x0001 != 0xFFFF, it is not changed to
0x0000.
So the value 0x0001 is written into the subnet field, and the
internal value of the subnet field is properly restored.
3.7. Address Mapping for Longer Prefixes
If the prefix being mapped is longer than 48 bits, the algorithm is
slightly more complex. A common case will be that the internal and
external prefixes are of different length. In such a case, the
shorter prefix is zero-extended to the length of the longer as
described in Section 3.1 for the purposes of overwriting the prefix.
Then, they are both zero-extended to 64 bits to facilitate one's
complement arithmetic. The "adjustment" is calculated using those 64
bit prefixes.
For example if the internal prefix is a /48 ULA and the external
prefix is a /56 provider-allocated prefix, the ULA becomes a /56 with
zeros in bits 48..55. For purposes of one's complement arithmetic,
they are then both zero-extended to 64 bits. A side-effect of this
is that a subset of the subnets possible in the shorter prefix are
untranslatable. While the security value of this is debatable, the
administration may choose to use them for subnets that it knows need
no external accessibility.
We then find the first word in the IID that does not have the value
0xFFFF, trying bits 64..79, and then 80..95, 96..111, and finally
112..127. We perform the same calculation (with the same proof of
correctness) as in Section 3.6, but applying it to that word.
Although any 16-bit portion of an IPv6 IID could contain 0xFFFF, an
IID of all-ones is a reserved anycast identifier that should not be
used on the network [RFC2526]. If a NPTv6 Translator discovers a
packet with an IID of all-zeros while performing address mapping,
that packet MUST be dropped, and an ICMPv6 Parameter Problem error
SHOULD be generated [RFC4443].
Note: this mechanism does involve modification of the IID; it may not
be compatible with future mechanisms that use unique IIDs for node
identification.
4. Implications of Network Address Translator Behavioral Requirements
Wasserman & Baker Expires June 12, 2011 [Page 14]
Internet-Draft NPTv6 December 2010
4.1. Prefix Configuration and generation
NPTv6 Translators MUST support manual configuration of internal and
external prefixes, and MUST NOT place any restrictions on those
prefixes except that they be valid IPv6 unicast prefixes as described
in [RFC4291].
NPTv6 Translators that do not have a manually configured internal
prefix SHOULD randomly generate a ULA prefix for the internal network
and advertise that prefix in router advertisements. NPTv6
Translators with more than one internal interface SHOULD assign a
(non-0xFFFF) subnet number to each link, and include the subnet
number in router advertisements on the corresponding link. NPTv6
Translators that generate a ULA prefix MUST generate the prefix using
a random number as described in RFC4291 [RFC4193], and SHOULD store
the randomly generated prefix in non-volatile storage for continued
use.
4.2. NAT Behavioral Requirements
NPTv6 Translators MUST support hairpinning behavior, as defined in
the NAT Behavioral Requirements for UDP document [RFC4787]. This
means that when a NPTv6 Translator receives a packet on the internal
interface that has a destination address that matches the site's
external prefix, it will translate the packet and forward it
internally. This allows internal nodes to reach other internal nodes
using their external, global addresses when necessary.
Conceptually, the datagram leaves the domain (is translated as
described in Section 3.2), and returns (is again translated as
described in Section 3.3). As a result, the datagram exchange will
be through the NPTv6 Translator in both directions for the lifetime
of the session. The alternative would be to require the NPTv6
Translator to drop the datagram, forcing the sender to use the
correct internal prefix for its peer. Performing only the external-
to-internal translation results in the datagram being sent from the
untranslated internal address of the source to the translated and
therefore internal address of its peer, which would enable the
session to bypass the NPTv6 Translator for future datagrams. It
would also mean that the original sender would be unlikely to
recognize the response when it arrived.
Because NPTv6 does not perform port mapping and uses a one-to-one,
reversible mapping algorithm, none of the other NAT behavioral
requirements apply to NPTv6.
Wasserman & Baker Expires June 12, 2011 [Page 15]
Internet-Draft NPTv6 December 2010
5. Implications for Applications
The use of NPTv6 Transition technology makes a capability available
to Applications (and the networks that contain them) that is not
readily possible in a NAT44 network. This is the ability to position
an externally-accessible application within the "internal" network.
In an IPv4 network using NAT44, externally-accessible application
must be positioned on systems with global addresses, forcing the edge
network to obtain global address allocation; if the application can
be in the translated routing domain, it automatically has an address
in each of its upstream prefixes without the edge network obtaining
such. However, there must be a means for the application to know
what addresses are usable. Reasons include at least advertisement in
DNS (which might be done statically if DNS is directly maintained by
the administration, or from the end system if Dynamic DNS is in use).
If referrals and other uses of network layer addressing do not use
names, then the application needs a means to determine what addresses
are relevant, whether from DNS or another means.
The means of address discovery is not within the scope of this
specification.
6. A Note on Port Mapping
In addition to overwriting IP addresses when packets are forwarded,
NAPT44 devices overwrite the source port number in outbound traffic,
and the destination port number in inbound traffic. This mechanism
is called "port mapping".
The major benefit of port mapping is that it allows multiple
computers to share a single IPv4 address. A large number of internal
IPv4 addresses (typically from the 10.0.0.0/8 prefix) can be mapped
into a single external, globally routable IPv4 address, with the
local port number used to identify which internal node should receive
each inbound packet. This address amplification feature should not
be needed in IPv6.
Since port mapping requires re-writing a portion of the transport
layer header, it requires NAPT44 devices to be aware of all of the
transport protocols that they forward, thus stifling the development
of new and improved transport protocols and preventing the use of
IPsec encryption. Modifying the transport layer header is
incompatible with security mechanisms that encrypt the full IP
payload, and restricts the NAPT44 to forwarding transport layers that
use weak checksum algorithms that are easily recalculated in routers.
Since there is significant detriment caused by modifying transport
Wasserman & Baker Expires June 12, 2011 [Page 16]
Internet-Draft NPTv6 December 2010
layer headers and very little, if any, benefit to the use of port
mapping in IPv6, NPTv6 Translators that comply with this
specification MUST NOT perform port mapping.
7. Security Considerations
When NPTv6 is deployed using either of the two-way, algorithmic
mappings defined in the document, it allows direct inbound
connections to internal nodes. While this can be viewed as a benefit
of NPTv6 vs. NAT44, it does open internal nodes to attacks that would
be more difficult in a NAT44 network. Although this situation is not
substantially worse, from a security standpoint, than running IPv6
with no NAT, some enterprises may assume that a NPTv6 Translator will
offer similar protection to a NAT44 device. For this reason, it is
RECOMMENDED that NPTv6 Translators also implement firewall
functionality such as described in
[I-D.ietf-v6ops-cpe-simple-security], with appropriate configuration
options including turning it on or off.
8. IANA Considerations
This document has no IANA considerations.
9. Acknowledgements
The checksum-neutral algorithmic address mapping described in this
document is based on e-mail written by Iljtsch Van Beijnum.
The following people provided advice or review comments that
substantially improved this document: Jari Arrko, Iljtsch Van
Beijnum, Remi Depres, Tony Hain, Ed Jankiewicz, Dave Thaler, Mark
Townsley, and Steve Blake.
This document was written using the xml2rfc tool described in RFC
2629 [RFC2629].
10. Change Log
10.1. Changes Between draft-mrw-behave-nat66-00 and -01
There were several minor changes made between the *behave-nat66-00
and -01 versions of this draft:
Wasserman & Baker Expires June 12, 2011 [Page 17]
Internet-Draft NPTv6 December 2010
o Added Fred Baker as a co-author.
o Minor mathematical corrections.
o Added AH to paragraph on NAT security issues.
o Added additional NAT topologies to overview (diagrams TBD).
10.2. Changes between *behave-nat66-01 and -02
There were further changes made between *behave-nat66-01 and -02:
o Removed topology hiding mechanism.
o Added diagrams.
o Made minor updates based on mailing list feedback.
o Added discussion of IPv6 SAF document.
o Added applicability section.
o Added discussion of Address Independence requirement.
o Added hairpinning requirement and discussion of applicability of
other NAT behavioral requirements.
10.3. Changes between *behave-nat66-02 and *nat66-00
There were further changes made between behave-nat66-02 and nat66-02:
o Added mapping for prefixes longer than /48.
o Change draft name to remove reference to the behave WG.
o Resolved various open issues and fixed typos.
10.4. Changes between *nat66-00 and *nat66-01
o Change the acronym "NAT66" to "NPTv6", so people don't read "NAT"
and MEGO.
o Change the term used to refer to the function from "NAT66 device"
to "NPTv6 Translator". It's not a "device" function, it's a
function that is applied between two interfaces. Consider a
router with two upstreams and two legs in the local network; it
will not translate between the local legs, but will translate to
and from each upstream, and be configured differently for each of
Wasserman & Baker Expires June 12, 2011 [Page 18]
Internet-Draft NPTv6 December 2010
the two ISPs.
o Comment specifically on the security aspects.
o Comment specifically on the application issues raised on this
list.
o Comment specifically on multihoming, load-sharing, and asymmetric
routing.
o Spell out the hairpinning requirement and its implications.
o Spell out the service provider side of Address Independence.
o 00 focuses on the edge's view
o Detail the algorithm in a manner clearer to the implementor (I
think)
o Spell out the case for GSE-style DMZs between the edge and the
transit network, which is about the implications for the global
routing table.
o Refer to xref target="I-D.ietf-v6ops-cpe-simple-security"/>.
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.
[RFC2526] Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
Addresses", RFC 2526, March 1999.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
Wasserman & Baker Expires June 12, 2011 [Page 19]
Internet-Draft NPTv6 December 2010
RFC 4787, January 2007.
11.2. Informative References
[GSE] O'Dell, M., "GSE - An Alternate Addressing Architecture
for IPv6", February 1997,
<http://tools.ietf.org/id/draft-ietf-ipngwg-gseaddr>.
[I-D.ietf-v6ops-cpe-simple-security]
Woodyatt, J., "Recommended Simple Security Capabilities in
Customer Premises Equipment for Providing Residential IPv6
Internet Service", draft-ietf-v6ops-cpe-simple-security-16
(work in progress), October 2010.
[RFC1071] Braden, R., Borman, D., Partridge, C., and W. Plummer,
"Computing the Internet checksum", RFC 1071,
September 1988.
[RFC1624] Rijsinghani, A., "Computation of the Internet Checksum via
Incremental Update", RFC 1624, May 1994.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC2993] Hain, T., "Architectural Implications of NAT", RFC 2993,
November 2000.
[RFC4864] Van de Velde, G., Hain, T., Droms, R., Carpenter, B., and
E. Klein, "Local Network Protection for IPv6", RFC 4864,
May 2007.
Appendix A. Why GSE?
For the purpose of this discussion, let us over-simplify the
Internet's structure by distinguishing between two broad classes of
networks: transit and edge. A "transit network", in this context, is
a network that provides connectivity services to other networks. Its
AS number may show up in a non-final position in BGP AS paths, or in
the case of mobile and residential broadband networks, it may offer
network services to smaller networks that can't justify RIR
membership. An "edge network", in contrast, is any network that is
not a transit network; it is the ultimate customer, and while it
provides internal connectivity for its own use, it is in other
Wasserman & Baker Expires June 12, 2011 [Page 20]
Internet-Draft NPTv6 December 2010
respects is a consumer of transit services. In terms of routing, a
network in the transit domain generally needs some way to make
choices about how it routes to other networks; an edge network is
generally quite satisfied with a simple default route.
The [GSE] proposal, and as a result this proposal (which is similar
to GSE in most respects and inspired by it), responds directly to
current concerns in the RIR communities. Edge networks are used to
an environment in IPv4 in which their addressing is disjoint from
that of their upstream transit networks; it is either provider
independent, or a network address translator makes their external
address distinct from their internal address, and they like the
distinction. In IPv6, there is a mantra that edge network addresses
should be derived from their upstream, and if they have multiple
upstreams, edge networks are expected to design their networks to use
all of those prefixes equivalently. They see this as unnecessary and
unwanted operational complexity, and are as a result pushing very
hard in the RIR communities for provider independent addressing.
Widespread use of provider independent addressing has a natural and
perhaps unavoidable side-effect that is likely to be very expensive
in the long term. It means that the routing table will enumerate the
networks at the edge of the transit domain, the edge networks, rather
than enumerating the transit domain. Per the CIDR Report, there are
currently more than 36,000 Autonomous Systems being advertised in
BGP, of which over 15,000 advertise only one prefix. There are in
the neighborhood of 5000 AS's that show up in a non-final position in
AS paths, and perhaps another 5000 networks whose AS numbers are
terminal in more than one AS path. In other words, we have prefixes
for some 36,000 transit and edge networks in the route table now,
many of which arguably need an Autonomous System number only for
multihoming. Current estimates suggest that we could easily see that
be on the order of 10,000,000 within fifteen years. Tens of
thousands of entries in the route table is very survivable; while our
protocols and computers will likely do quite well with tens of
millions of routes, the heat produced and power consumed by those
routers, and the inevitable impact on the cost of those routers, is
not a good outcome. To avoid having a massive and unscalable route
table, we need to find a way that is politically acceptable and
returns us to enumerating the transit domain, not the edge.
There have been a number of proposals. As described, shim6 moves the
complexity to the edge, and the edge is rebelling. Geographic
addressing in essence forces ISPs to "own" geographic territory from
a routing perspective, as otherwise there is no clue in the address
as to what network a datagram should be delivered to in order to
reach it. Metropolitan Addressing can imply regulatory authority,
and even if it is implemented using internet exchange consortia,
Wasserman & Baker Expires June 12, 2011 [Page 21]
Internet-Draft NPTv6 December 2010
visits a great deal of complexity on the transit networks that
directly serve the edge. The one that is likely to be most
acceptable is any proposal that enables an edge network to be
operationally independent of its upstreams, with no obligation to
renumber when it adds, drops, or changes ISPs, and with no additional
burden placed either on the ISP or the edge network as a result.
From an application perspective, an additional operational
requirement in the words of US NIST's Roadmap for the Smart Grid, is
that
"...the Network should enable an application in a particular
domain to communicate with an application in any other domain in
the information network, with proper management control over who
and where applications can be interconnected."
In other words, the structure of the network should allow for and
enable appropriate access control, but the structure of the network
should not inherently limit access.
The GSE model, by statelessly translating the prefix between an edge
network and its upstream transit network, accomplishes that with a
minimum of fuss and bother. Stated in the simplest terms, it enables
the edge network to behave as if it has a provider-independent prefix
from a multihoming and renumbering perspective without the overhead
of RIR membership or maintaining BGP connectivity, and it enables the
transit networks to aggressively aggregate what are from their
perspective provider-allocated customer prefixes, to maintain a
rational-sized routing table.
Authors' Addresses
Margaret Wasserman
Painless Security
North Andover, MA 01845
USA
Phone: +1 781 405 7464
Email: mrw@painless-security.com
URI: http://www.painless-secuirty.com
Wasserman & Baker Expires June 12, 2011 [Page 22]
Internet-Draft NPTv6 December 2010
Fred Baker
Cisco Systems
Santa Barbara, California 93117
USA
Phone: +1-408-526-4257
Email: fred@cisco.com
Wasserman & Baker Expires June 12, 2011 [Page 23]
