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IETF RFC 7756
Last modified on Thursday, February 18th, 2016
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Internet Engineering Task Force (IETF) T. Anderson
Request for Comments: 7756 Redpill Linpro
Category: Informational S. Steffann
ISSN: 2070-1721 S.J.M. Steffann Consultancy
February 2016
Stateless IP/ICMP Translation for IPv6 Internet Data Center
Environments (SIIT-DC): Dual Translation Mode
 Abstract
This document describes an extension of the Stateless IP/ICMP
Translation for IPv6 Internet Data Center Environments (SIIT-DC)
architecture, which allows applications, protocols, or nodes that are
incompatible with IPv6 and/or Network Address Translation to operate
correctly with SIIT-DC. This is accomplished by introducing a new
component called an SIIT-DC Edge Relay, which reverses the
translations made by an SIIT-DC Border Relay. The application and/or
node is thus provided with seemingly native IPv4 connectivity that
provides end-to-end address transparency.
The reader is expected to be familiar with the SIIT-DC architecture
described in RFC 7755.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 7756.
Anderson & Steffann Informational PAGE 1 
RFC 7756 SIIT-DC: Dual Translation Mode February 2016
Copyright Notice
Copyright (c) 2016 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
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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Edge Relay Description . . . . . . . . . . . . . . . . . . . 5
3.1. Node-Based Edge Relay . . . . . . . . . . . . . . . . . . 6
3.2. Network-Based Edge Relay . . . . . . . . . . . . . . . . 7
3.2.1. Edge Relay "on a Stick" . . . . . . . . . . . . . . . 8
3.2.2. Edge Relay That Bridges IPv6 Packets . . . . . . . . 9
4. Deployment Considerations . . . . . . . . . . . . . . . . . . 9
4.1. IPv6 Path MTU . . . . . . . . . . . . . . . . . . . . . . 9
4.2. IPv4 MTU . . . . . . . . . . . . . . . . . . . . . . . . 10
4.3. IPv4 Identification Header . . . . . . . . . . . . . . . 10
5. Intra-IDC IPv4 Communication . . . . . . . . . . . . . . . . 10
5.1. Hairpinning by the SIIT-DC Border Relay . . . . . . . . . 11
5.2. Additional EAMs Configured in Edge Relay . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. Normative References . . . . . . . . . . . . . . . . . . 14
7.2. Informative References . . . . . . . . . . . . . . . . . 14
Appendix A. Examples: Network-Based IPv4 Connectivity . . . . . 16
A.1. Subnet with IPv4 Service Addresses . . . . . . . . . . . 16
A.2. Subnet with Unrouted IPv4 Addresses . . . . . . . . . . . 16
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
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RFC 7756 SIIT-DC: Dual Translation Mode February 2016
1. Introduction
SIIT-DC [RFC 7755] describes an architecture where IPv4-only users can
access IPv6-only services through a stateless translator called an
SIIT-DC Border Relay (BR). This approach has certain limitations,
however. In particular, the following cases will work poorly or not
at all:
o Application protocols that do not support NAT (i.e., the lack of
end-to-end transparency of IP addresses).
o Nodes that cannot connect to IPv6 networks at all or that can only
connect such networks if they also provide IPv4 connectivity
(i.e., dual-stacked networks).
o Application software that makes use of legacy IPv4-only APIs or
otherwise makes assumptions that IPv4 connectivity is available.
By extending the SIIT-DC architecture with a new component called an
Edge Relay (ER), all of the above can be made to work correctly in an
otherwise IPv6-only network environment using SIIT-DC.
The purpose of the ER is to reverse the IPv4-to-IPv6 packet
translations previously done by the BR for traffic arriving from IPv4
clients and forward this as "native" IPv4 to the node or application.
In the reverse direction, IPv4 packets transmitted by the node or
application are intercepted by the ER, which translates them to IPv6
before they are forwarded to the BR, which in turn will reverse the
translations and forward them to the IPv4 client. The node or
application is thus provided with "virtual" IPv4 Internet
connectivity that retains end-to-end transparency for the IPv4
addresses.
2. Terminology
This document makes use of the following terms:
SIIT-DC Border Relay (BR):
A device or a logical function that performs stateless protocol
translation between IPv4 and IPv6. It MUST do so in accordance
with [RFC 6145] and [RFC 7757].
Anderson & Steffann Informational PAGE 3
RFC 7756 SIIT-DC: Dual Translation Mode February 2016
SIIT-DC Edge Relay (ER):
A device or logical function that provides "native" IPv4
connectivity to IPv4-only devices or application software. It is
very similar in function to a BR but is typically located close to
the IPv4-only component(s) it is supporting rather than on the
outer network border of the Internet Data Center (IDC). An ER may
be either node based (Section 3.1) or network based (Section 3.2).
IPv4 Service Address:
An IPv4 address representing a node or service located in an IPv6
network. It is coupled with an IPv6 Service Address using an
Explicit Address Mapping (EAM). Packets sent to this address are
translated to IPv6 by the BR, and possibly back to IPv4 by an ER,
before reaching the node or service.
IPv6 Service Address:
An IPv6 address assigned to an application, node, or service
either directly or indirectly (through an ER). It is coupled with
an IPv4 Service Address using an EAM. IPv4-only clients
communicate with the IPv6 Service Address through SIIT-DC.
Explicit Address Mapping (EAM):
A bidirectional coupling between an IPv4 Service Address and an
IPv6 Service Address configured in a BR or ER. When translating
between IPv4 and IPv6, the BR/ER changes the address fields in the
translated packet's IP header according to any matching EAM. The
EAM algorithm is specified in [RFC 7757].
Translation Prefix:
An IPv6 prefix into which the entire IPv4 address space is mapped,
according to the algorithm in [RFC 6052]. The translation prefix
is routed to the BR's IPv6 interface. When translating between
IPv4 and IPv6, a BR/ER will insert/remove the translation prefix
into/from the address fields in the translated packet's IP header,
unless an EAM exists for the IP address that is being translated.
IPv4-Converted IPv6 Addresses:
As defined in Section 1.3 of [RFC 6052].
IDC:
Short for "Internet Data Center"; a data center whose main purpose
is to deliver services to the public Internet. SIIT-DC is
primarily targeted at being deployed in an IDC. An IDC is
typically operated by an Internet Content Provider or a Managed
Services Provider.
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RFC 7756 SIIT-DC: Dual Translation Mode February 2016
SIIT:
The Stateless IP/ICMP Translation Algorithm, as specified in
[RFC 6145].
XLAT:
Short for "Translation". Used in figures to indicate where a BR/
ER uses SIIT [RFC 6145] to translate IPv4 packets to IPv6 and vice
versa.
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].
3. Edge Relay Description
An ER is at its core an implementation of the Stateless IP/ICMP
Translation Algorithm [RFC 6145] that supports Explicit Address
Mappings [RFC 7757]. It provides virtual IPv4 connectivity for nodes
or applications that require this to operate correctly with SIIT-DC.
Packets from the IPv4 Internet destined for an IPv4 Service Address
are first translated to IPv6 by a BR. The resulting IPv6 packets are
subsequently forwarded to the ER that owns the IPv6 Service Address
the translated packets are addressed to. The ER then translates them
back to IPv4 before forwarding them to the IPv4 application or node.
In the other direction, the exact same translations happen, only in
reverse. This process provides end-to-end transparency of IPv4
addresses.
An ER may handle an arbitrary number of IPv4/IPv6 Service Addresses.
All the EAMs configured in the BR that involve the IPv4/IPv6 Service
Addresses handled by an ER MUST also be present in the ER's
configuration.
An ER may be implemented in two distinct ways: as a software-based
service residing inside an otherwise IPv6-only node or as a network-
based service that provides an isolated IPv4 network segment to which
nodes that require IPv4 can connect. In both cases, native IPv6
connectivity may be provided simultaneously with the virtual IPv4
connectivity. Thus, dual-stack connectivity is facilitated in case
the node or application supports it.
The choice between a node- or network-based ER is made on a per-
service or per-node basis. An arbitrary number of each type of ER
may co-exist in an SIIT-DC architecture.
This section describes the different approaches and discusses which
approach fits best for the various use cases.
Anderson & Steffann Informational PAGE 5
RFC 7756 SIIT-DC: Dual Translation Mode February 2016
3.1. Node-Based Edge Relay
[IPv4 Internet] [IPv6 Internet]
| |
+-----|-----+ |
| (BR/XLAT) | |
+-----|-----+ |
| | +-----<IPv6-only node/server>----------+
[IPv6-only IDC network] | +----------------+|
| | /--(ER/XLAT)--AF_INET Dual-stack ||
\-------------------------+ | application ||
| \------------AF_INET6 software ||
| +----------------+|
+--------------------------------------+
Figure 1: A Node-Based Edge Relay
A node-based ER is typically implemented as a logical software
function that runs inside the operating system of an IPv6 node. It
provides applications running on the same node with IPv4
connectivity. Its IPv4 Service Address SHOULD be considered a
regular local address that allows applications running on the same
node to use it with IPv4-only API calls, e.g., to create AF_INET
sockets that listen for and accept incoming connections to its IPv4
Service Address. An ER may accomplish this by creating a virtual
network adapter to which it assigns the IPv4 Service Address and
points a default IPv4 route. This approach is similar to the
"Bump-in-the-Stack" approach discussed in [RFC 6535]; however, it does
not include an Extension Name Resolver.
As shown in Figure 1, if the application supports dual-stack
operation, IPv6 clients will be able to communicate with it directly
using native IPv6. Neither the BR nor the ER will intercept this
communication. Support for IPv6 in the application is, however, not
a requirement; the application may opt not to establish any IPv6
sockets. Foregoing IPv6 in this manner will simply preclude
connectivity to the service from IPv6-only clients; connectivity to
the service from IPv4 clients (through the BR) will continue work in
the same way.
The ER requires a dedicated IPv6 Service Address for each IPv4
Service Address it has configured. The IPv6 network MUST forward
traffic to these IPv6 Service Addresses to the node, whose operating
system MUST in turn forward them to the ER. This document does not
attempt to fully explore the multitude of ways this could be
accomplished; however, considering that the IPv6 protocol is designed
for having multiple addresses assigned to a single node, one
particularly straight-forward way would be to assign the ER's IPv6
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RFC 7756 SIIT-DC: Dual Translation Mode February 2016
Service Addresses as secondary IPv6 addresses on the node itself so
that the upstream router learns of their location using the IPv6
Neighbor Discovery Protocol [RFC 4861].
3.2. Network-Based Edge Relay
[IPv4 Internet] [IPv6 Internet]
| |
+-----|-----+ |
| (BR/XLAT) | |
+-----|-----+ |
| |
[IPv6-only IDC network] +--<IPv4-only node/server>--+
| | +----------------+|
+-----|-----+ [v4-only] | | IPv4-only ||
| (ER/XLAT)-----[network]--------AF_INET application ||
+-----------+ [segment] | | software ||
| +----------------+|
+---------------------------+
Figure 2: A Basic Network-Based Edge Relay
A network-based ER functions the exact same way as a node-based ER
does, only that instead of assigning the IPv4 Service Addresses to an
internal-only virtual network adapter, traffic destined for them are
forwarded onto a network segment to which nodes that require IPv4
connectivity connect to. The ER also functions as the default IPv4
router for the nodes on this network segment.
Each node on the IPv4 network segment MUST acquire and assign an IPv4
Service Address to a local network interface. While this document
does not attempt to explore all the various methods by which this
could be accomplished, some examples are provided in Appendix A.
The basic ER illustrated in Figure 2 establishes an IPv4-only network
segment between itself and the IPv4-only nodes it serves. This is
fine if the nodes it provides IPv4 access to have no support for IPv6
whatsoever; however, if they are dual-stack capable, it would not be
ideal to take away their IPv6 connectivity in this manner. While it
is RECOMMENDED to use a node-based ER in this case, appropriate
implementations of a node-based ER might not be available for every
node. If the application protocol in question does not work
correctly in a NAT environment, standard SIIT-DC cannot be used
either, which leaves a network-based ER as the only remaining
solution. The following subsections contain examples on how the ER
could be implemented in a way that provides IPv6 connectivity for
dual-stack capable nodes.
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RFC 7756 SIIT-DC: Dual Translation Mode February 2016
3.2.1. Edge Relay "on a Stick"
[IPv4 Internet] [IPv6 Internet]
| |
+-----|-----+ |
| (BR/XLAT) | |
+-----|-----+ |
| |
[IPv6-only IDC network]
|
| +-------------+
| | _IPv6_ |
| | / \ |
+==== (ER/XLAT) |
| | \_ _/ |
| | IPv4 | +--<Dual-stack node/server>--+
| +-------------+ | +----------------+|
| | /---AF_INET Dual-stack ||
[Dual-stack network segment]----< | application ||
| \--AF_INET6 software ||
| +----------------+|
+----------------------------+
Figure 3: A Network-Based Edge Relay "on a Stick"
The ER "on a stick" approach illustrated in Figure 3 ensures that the
dual-stack capable node retains native IPv6 connectivity by
connecting the ER's IPv4 and IPv6 interfaces to the same network
segment, alternatively by using a single dual-stacked interface.
Native IPv6 traffic between the IDC network and the node bypasses the
ER entirely, while IPv4 traffic from the node will be routed directly
to the ER (because it acts as its default IPv4 router), where it is
translated to IPv6 before being transmitted to the upstream default
IPv6 router. The ER could attract inbound traffic to the IPv6
Service Addresses by responding to the upstream router's IPv6
Neighbor Discovery [RFC 4861] messages for them.
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RFC 7756 SIIT-DC: Dual Translation Mode February 2016
3.2.2. Edge Relay That Bridges IPv6 Packets
[IPv4 Internet] [IPv6 Internet]
| |
+-----|-----+ |
| (BR/XLAT) | |
+-----|-----+ |
| |
[IPv6-only IDC network]
|
+-----------|--------------+
| ____/ \_IPv6_ |
| / \ |
| (IPv6 Bridge) (ER/XLAT) |
| \____ _ _/ |
| \ / IPv4 | +--<Dual-stack node/server>--+
+-----------|--------------+ | +----------------+|
| | /---AF_INET Dual-stack ||
[Dual-stack network segment]----< | application ||
| \--AF_INET6 software ||
| +----------------+|
+----------------------------+
Figure 4: A Network-Based Edge Relay Containing an IPv6 Bridge
The ER illustrated in Figure 4 will transparently bridge IPv6 frames
between its upstream and downstream interfaces. IPv6 packets sent
from the upstream IDC network to an IPv6 Service Address are
intercepted by the ER (e.g., by responding to IPv6 Neighbor Discovery
[RFC 4861] messages for them) and routed through the translation
function before being forwarded out the ER's downstream interface as
IPv4 packets. The downstream network segment thus becomes dual
stacked.
4. Deployment Considerations
4.1. IPv6 Path MTU
The IPv6 Path MTU between the ER and the BR will typically be larger
than the default value defined in Section 4 of [RFC 6145] (1280
bytes), as it will typically be contained within a single
administrative domain. Therefore, it is RECOMMENDED that the IPv6
Path MTU configured in the ER be raised accordingly. It is
RECOMMENDED that the ER and the BR use identical configured IPv6 Path
MTU values.
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RFC 7756 SIIT-DC: Dual Translation Mode February 2016
4.2. IPv4 MTU
In order to avoid IPv6 fragmentation, an ER SHOULD ensure that the
IPv4 MTU used by applications or nodes is equal to the configured
IPv6 Path MTU - 20 so that a maximum-sized IPv4 packet can fit in an
unfragmented IPv6 packet. This ensures that the application may do
its part in avoiding IP-level fragmentation from occurring, e.g., by
segmenting/fragmenting outbound packets at the application layer, and
advertising the maximum size its peer may use for inbound packets
(e.g., through the use of the TCP Maximum Segment Size (MSS) option).
A node-based ER could accomplish this by configuring this MTU value
on the virtual network adapter, while a network-based ER could do so
by advertising the MTU to its downstream nodes using the DHCPv4
Interface MTU option [RFC 2132].
4.3. IPv4 Identification Header
If the generation of IPv6 Atomic Fragments is disabled, the value of
the IPv4 Identification header will be lost during the translation.
Conversely, enabling the generation of IPv6 Atomic Fragments will
ensure that the IPv4 Identification header will be carried end to
end. Note that for this to work bidirectionally, IPv6 Atomic
Fragment generation MUST be enabled on both the BR and the ER.
Apart from certain diagnostic tools, there are few (if any)
application protocols that make use of the IPv4 Identification
header. Therefore, the loss of the IPv4 Identification value will
generally not cause any problems.
IPv6 Atomic Fragments and their impact on the IPv4 Identification
header is further discussed in Section 4.9.2 of [RFC 7755].
5. Intra-IDC IPv4 Communication
Although SIIT-DC is primarily intended to facilitate communication
between IPv4-only nodes on the Internet and services located in an
IPv6-only IDC network, an IPv4-only node or application located
behind an ER might need to communicate with other nodes or services
in the IDC. The IPv4-only node or application will need to go
through the ER, as it will typically be incapable of contacting IPv6
destinations directly. The following subsections discuss various
methods on how to facilitate such communication.
Anderson & Steffann Informational PAGE 10
RFC 7756 SIIT-DC: Dual Translation Mode February 2016
5.1. Hairpinning by the SIIT-DC Border Relay
If the BR supports hairpinning as described in Section 4.2 of
[RFC 7757], the easiest solution is to make the target service
available through SIIT-DC in the normal way; that is, by provisioning
an EAM to the BR that assigns an IPv4 Service Address with the target
service's IPv6 Service Address.
This allows the IPv4-only node or application to transmit packets
destined for the target service's IPv4 Service Address, which the ER
will then translate to a corresponding IPv4-converted IPv6 address by
inserting the translation prefix [RFC 6052]. When this IPv6 packet
reaches the BR, it will be hairpinned and transmitted back to the
target service's IPv6 Service Address (where it could possibly pass
through another ER before reaching the target service). Return
traffic from the target service will be hairpinned in the same
fashion.
+-[Pkt#1: IPv4]-+ +--[Pkt#2: IPv6]-------------+
| SRC 192.0.2.1 | (XLAT#1) | SRC 2001:db8:a:: |
| DST 192.0.2.2 |--(@ ER A)-->| DST 2001:db8:46::192.0.2.2 |---\
+---------------+ +----------------------------+ |
(XLAT#2)
+-[Pkt#4: IPv4]-+ +--[Pkt#3: IPv6]-------------+ ( @ BR )
| SRC 192.0.2.1 | (XLAT#3) | SRC 2001:db8:46::192.0.2.1 | |
| DST 192.0.2.2 |<--(@ ER B)--| DST 2001:db8:b:: |<--/
+---------------+ +----------------------------+
Figure 5: Hairpinned IPv4-IPv4 Packet Flow
Figure 5 illustrates the flow of a hairpinned packet sent from the
IPv4-only node/app behind ER A towards an IPv6-only node/app behind
ER B. ER A is configured with the EAM {192.0.2.1,2001:db8:a::} and
ER B with {192.0.2.2,2001:db8:b::}. The BR is configured with both
EAMs and supports hairpinning. Note that if the target service had
not been located behind an ER, the third and final translation
(XLAT#3) would not have happened, i.e., the target service/node would
have received and responded to packet #3 directly.
If the IPv4-only nodes/services do not need connectivity with the
public IPv4 Internet, private IPv4 addresses [RFC 1918] could be used
as their IPv4 Service Addresses in order to conserve the IDC
operator's pool of public IPv4 addresses.
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RFC 7756 SIIT-DC: Dual Translation Mode February 2016
5.2. Additional EAMs Configured in Edge Relay
If the BR does not support hairpinning, or if the hairpinning
solution is not desired for some other reason, intra-IDC IPv4 traffic
may be facilitated by configuring additional EAMs on the ER for each
service the IPv4-only node or application needs to communicate with.
This makes the IPv6 traffic between the ER and the target service's
IPv6 Service Address follow the direct path through the IPv6 network.
The traffic does not pass the BR, which means that this solution
might yield better latency than the hairpinning approach.
The additional EAM configured in the ER consists of the target's IPv6
Service Address and an IPv4 Service Address. The IPv4-only node or
application will contact the target's assigned IPv4 Service Address
using its own IPv4 Service Address as the source. The ER will then
proceed to translate the original IPv4 packet to an IPv6 packet. The
source address of the resulting IPv6 packet will be the IPv6 Service
Address of the local node or application, while the destination
address will be the IPv6 Service Address of the target. Any replies
from the target will undergo identical translation, only in reverse.
If the target service is located behind another ER, that other ER
MUST also be provisioned with an additional EAM that contains the
IPv4 and IPv6 Service Addresses of the origin IPv4-only node or
application. Otherwise, the target service's ER will be unable to
translate the source address of the incoming packets.
+-[Pkt#1: IPv4]-+ +--[Pkt#2: IPv6]---+
| SRC 192.0.2.1 | (XLAT#1) | SRC 2001:db8:a:: |
| DST 192.0.2.2 |--(@ ER A)-->| DST 2001:db8:b:: |
+---------------+ +------------------+
|
+-[Pkt#3: IPv4]-+ |
| SRC 192.0.2.1 | (XLAT#2) |
| DST 192.0.2.2 |<-------(@ ER B)------/
+---------------+
Figure 6: Non-hairpinned IPv4-IPv4 Packet Flow
Figure 6 illustrates the flow of a packet carrying intra-IDC IPv4
traffic between two IPv4-only nodes/applications that are both
located behind ERs. Both ER A and ER B are configured with two EAMs:
{192.0.2.1,2001:db8:a::} and {192.0.2.2,2001:db8:b::}. The packet
will follow the regular routing path through the IPv6 IDC network;
the BR is not involved, and the packet will not be hairpinned.
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RFC 7756 SIIT-DC: Dual Translation Mode February 2016
The above approach is not mutually exclusive with the hairpinning
approach described in Section 5.1: If both EAMs above are also
configured on the BR, both 192.0.2.1 and 192.0.2.2 would be reachable
from other IPv4-only services/nodes using the hairpinning approach.
They would also be reachable from the IPv4 Internet.
Note that if the target service in this example was not located
behind an ER, but instead was a native IPv6 service listening on
2001:db8:b::, the second translation step in Figure 6 would not
occur; the target service would receive and respond to packet #2
directly.
As with the hairpinning approach, if the IPv4-only nodes/services do
not need connectivity to/from the public IPv4 Internet, private IPv4
addresses [RFC 1918] could be used as their IPv4 Service Addresses.
Alternatively, in the case where the target service is on native
IPv6, the target's assigned IPv4 Service Address has only local
significance behind the ER. It could therefore be assigned from the
IPv4 Service Continuity Prefix [RFC 7335].
6. Security Considerations
This section discusses security considerations specific to the use of
an ER. See the Security Considerations section in [RFC 7755] for
security considerations applicable to the SIIT-DC architecture in
general.
If the ER receives an IPv4 packet from the application/node from a
source address it does not have an EAM for, both the source and
destination addresses will be rewritten according to [RFC 6052].
After undergoing the reverse translation in the BR, the resulting
IPv4 packet routed to the IPv4 network will have a spoofed IPv4
source address. The ER SHOULD therefore ensure that ingress
filtering [RFC 2827] is used on the ER's IPv4 interface so that such
packets are immediately discarded.
If the ER receives an IPv6 packet with both the source and
destination address equal to one of its local IPv6 Service Addresses,
the resulting packet would appear to the IPv4-only application/node
as locally generated, as both the source address and the destination
address will be the same address. This could trick the application
into believing the packet came from a trusted source (itself). To
prevent this, the ER SHOULD discard any received IPv6 packets that
have a source address that is either 1) equal to any of its local
IPv6 Service Addresses or 2) after translation from IPv6 to IPv4,
equal to any of its local IPv4 Service Addresses.
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7. References
7.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 7755] Anderson, T., "SIIT-DC: Stateless IP/ICMP Translation for
IPv6 Data Center Environments", RFC 7755,
DOI 10.17487/RFC 7755, February 2016,
<http://www.rfc-editor.org/info/RFC 7755>.
[RFC 7757] Anderson, T. and A. Leiva, "Explicit Address Mappings for
Stateless IP/ICMP Translation", RFC 7757,
DOI 10.17487/RFC 7757, February 2016,
<http://www.rfc-editor.org/info/RFC 7757>.
7.2. Informative References
[RFC 826] Plummer, D., "Ethernet Address Resolution Protocol: Or
Converting Network Protocol Addresses to 48.bit Ethernet
Address for Transmission on Ethernet Hardware", STD 37,
RFC 826, DOI 10.17487/RFC 826, November 1982,
<http://www.rfc-editor.org/info/RFC 826>.
[RFC 1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, DOI 10.17487/RFC 1918, February 1996,
<http://www.rfc-editor.org/info/RFC 1918>.
[RFC 2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC 2131, March 1997,
<http://www.rfc-editor.org/info/RFC 2131>.
[RFC 2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, DOI 10.17487/RFC 2132, March 1997,
<http://www.rfc-editor.org/info/RFC 2132>.
[RFC 2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC 2827,
May 2000, <http://www.rfc-editor.org/info/RFC 2827>.
Anderson & Steffann Informational PAGE 14
RFC 7756 SIIT-DC: Dual Translation Mode February 2016
[RFC 4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC 4861, September 2007,
<http://www.rfc-editor.org/info/RFC 4861>.
[RFC 6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
DOI 10.17487/RFC 6052, October 2010,
<http://www.rfc-editor.org/info/RFC 6052>.
[RFC 6145] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
Algorithm", RFC 6145, DOI 10.17487/RFC 6145, April 2011,
<http://www.rfc-editor.org/info/RFC 6145>.
[RFC 6535] Huang, B., Deng, H., and T. Savolainen, "Dual-Stack Hosts
Using "Bump-in-the-Host" (BIH)", RFC 6535,
DOI 10.17487/RFC 6535, February 2012,
<http://www.rfc-editor.org/info/RFC 6535>.
[RFC 6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC 6724, September 2012,
<http://www.rfc-editor.org/info/RFC 6724>.
[RFC 6877] Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
Combination of Stateful and Stateless Translation",
RFC 6877, DOI 10.17487/RFC 6877, April 2013,
<http://www.rfc-editor.org/info/RFC 6877>.
[RFC 7335] Byrne, C., "IPv4 Service Continuity Prefix", RFC 7335,
DOI 10.17487/RFC 7335, August 2014,
<http://www.rfc-editor.org/info/RFC 7335>.
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Appendix A. Examples: Network-Based IPv4 Connectivity
A.1. Subnet with IPv4 Service Addresses
One relatively straight-forward way to provide IPv4 connectivity
between a network-based ER and the IPv4 node(s) it serves is to
ensure the IPv4 Service Address(es) can be enclosed within a larger
IPv4 prefix. The ER may then claim one address in this prefix for
itself and use it to provide an IPv4 default router address and
assign the IPv4 Service Address(es) to its downstream node(s) using
DHCPv4 [RFC 2131]. For example, if the IPv4 Service Addresses are
192.0.2.26 and 192.0.2.27, the ER would configure the address
192.0.2.25/29 on its IPv4-facing interface and would add the two IPv4
Service Addresses to its DHCPv4 pool.
One disadvantage of this method is that IPv4 communication between
the IPv4 node(s) behind the ER and other services made available
through SIIT-DC becomes impossible, if those other services are
assigned IPv4 Service Addresses that also are covered by the same
IPv4 prefix (e.g., 192.0.2.28). This happens because the IPv4 nodes
will mistakenly believe they have an on-link route to the entire
prefix and attempt to resolve the addresses using ARP [RFC 826],
instead of sending them to the ER for translation to IPv6. This
problem could, however, be overcome by avoiding assigning IPv4
Service Addresses that overlap with an IPv4 prefix handled by an ER
(at the expense of wasting some potential IPv4 Service Addresses) or
by ensuring that the overlapping IPv4 Service Addresses are only
assigned to services that do not need to communicate with the IPv4
node(s) behind the ER. A third way to avoid this problem is
discussed in Appendix A.2.
A.2. Subnet with Unrouted IPv4 Addresses
In order to avoid the problem discussed in Appendix A.1, a private
unrouted IPv4 network that does not encompass the IPv4 Service
Address(es) could be used to provide connectivity between the ER and
the IPv4-only node(s) it serves. An IPv4-only node must then assign
its IPv4 Service Address as a secondary local address, while the ER
routes each of the IPv4 Service Addresses to its assigned node using
that node's private on-link IPv4 address as the next hop. This
approach would ensure there are no overlaps with IPv4 Service
Addresses elsewhere in the infrastructure, but on the other hand, it
would preclude the use of DHCPv4 [RFC 2131] for assigning the IPv4
Service Addresses.
Anderson & Steffann Informational PAGE 16
RFC 7756 SIIT-DC: Dual Translation Mode February 2016
This approach creates a need to ensure that the IPv4 application is
selecting the IPv4 Service Address (as opposed to its private on-link
IPv4 address) as its source address when initiating outbound
connections. This could be accomplished by altering the Default
Address Selection Policy Table [RFC 6724] on the IPv4 node.
Acknowledgements
The authors would like to especially thank the authors of 464XLAT
[RFC 6877]: Masataka Mawatari, Masanobu Kawashima, and Cameron Byrne.
The architecture described by this document is merely an adaptation
of their work to a data center environment and could not have
happened without them.
The authors would like also to thank the following individuals for
their contributions, suggestions, corrections, and criticisms: Fred
Baker, Tobias Brox, Olafur Gudmundsson, Christer Holmberg, Ray
Hunter, Shucheng LIU (Will), and Andrew Yourtchenko.
Authors' Addresses
Tore Anderson
Redpill Linpro
Vitaminveien 1A
0485 Oslo
Norway
Phone: +47 959 31 212
Email: tore@redpill-linpro.com
URI: http://www.redpill-linpro.com
Sander Steffann
S.J.M. Steffann Consultancy
Tienwoningenweg 46
Apeldoorn, Gelderland 7312 DN
The Netherlands
Email: sander@steffann.nl
Anderson & Steffann Informational PAGE 17
RFC TOTAL SIZE: 39291 bytes
PUBLICATION DATE: Thursday, February 18th, 2016
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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