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IETF RFC 7755
Last modified on Thursday, February 18th, 2016
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Internet Engineering Task Force (IETF) T. Anderson
Request for Comments: 7755 Redpill Linpro
Category: Informational February 2016
ISSN: 2070-1721
SIIT-DC: Stateless IP/ICMP Translation for IPv6 Data Center Environments
 Abstract
This document describes the use of the Stateless IP/ICMP Translation
Algorithm (SIIT) in an IPv6 Internet Data Center (IDC). In this
deployment model, traffic from legacy IPv4-only clients on the
Internet is translated to IPv6 upon reaching the IDC operator's
network infrastructure. From that point on, it may be treated the
same as traffic from native IPv6 end users. The IPv6 endpoints may
be numbered using arbitrary (non-IPv4-translatable) IPv6 addresses.
This facilitates a single-stack IPv6-only network infrastructure, as
well as efficient utilization of public IPv4 addresses.
The primary audience is IDC operators who are deploying IPv6, running
out of available IPv4 addresses, and/or feeling that dual stack
causes undesirable operational complexity.
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 7755.
Anderson Informational PAGE 1 
RFC 7755 SIIT-DC 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
1.1. Single-Stack IPv6 Operation . . . . . . . . . . . . . . . 3
1.2. Stateless Operation . . . . . . . . . . . . . . . . . . . 4
1.3. IPv4 Address Conservation . . . . . . . . . . . . . . . . 4
1.4. Clients' IPv4 Source Addresses Visible to Applications . 5
1.5. Compatible with Standard IPv4 and IPv6 Stacks . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Architectural Overview . . . . . . . . . . . . . . . . . . . 8
3.1. Packet Flow . . . . . . . . . . . . . . . . . . . . . . . 9
4. Deployment Considerations and Guidelines . . . . . . . . . . 10
4.1. Application/Device Support for IPv6 . . . . . . . . . . . 10
4.2. Application Support for NAT . . . . . . . . . . . . . . . 10
4.3. Application Communication Pattern . . . . . . . . . . . . 10
4.4. Choice of Translation Prefix . . . . . . . . . . . . . . 11
4.5. Routing Considerations . . . . . . . . . . . . . . . . . 12
4.6. Location of the SIIT-DC Border Relays . . . . . . . . . . 12
4.7. Migration from Dual Stack . . . . . . . . . . . . . . . . 13
4.8. Translation of ICMPv6 Errors to IPv4 . . . . . . . . . . 13
4.9. MTU and Fragmentation . . . . . . . . . . . . . . . . . . 13
4.9.1. IPv4/IPv6 Header Size Difference . . . . . . . . . . 14
4.9.2. IPv6 Atomic Fragments . . . . . . . . . . . . . . . . 14
4.9.3. Minimum Path MTU Difference between IPv4 and IPv6 . . 15
4.10. IPv4-Translatable IPv6 Service Addresses . . . . . . . . 16
5. Security Considerations . . . . . . . . . . . . . . . . . . . 17
5.1. Mistaking the Translation Prefix for a Trusted Network . 17
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.1. Normative References . . . . . . . . . . . . . . . . . . 17
6.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Complete SIIT-DC IDC Topology Example . . . . . . . 21
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 24
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 24
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RFC 7755 SIIT-DC February 2016
1. Introduction
Historically, dual stack [RFC 4213] [RFC 6883] has been the recommended
way to transition from a legacy IPv4-only environment to one capable
of serving IPv6 users. However, for IDC operators, dual-stack
operation has a number of disadvantages compared to single-stack
operation. In particular, running two protocols rather than one
results in increased complexity and operational overhead with little
return on investment for as long as large parts of the public
Internet remains predominantly IPv4 only. Furthermore, the dual-
stack approach does not in any way help with the depletion of the
IPv4 address space, which at the time of writing is a pressing
concern in most parts of the world.
Therefore, some IDC operators may instead prefer an approach in which
they only need to operate one protocol in the data center as they
prepare for the future. Stateless IP/ICMP Translation for IPv6 Data
Center Environments (SIIT-DC) is one such approach. Its design goals
include:
o Promote the deployment of native IPv6 services (cf. [RFC 6540]).
o Provide IPv4 service availability for legacy users with no loss of
performance or functionality.
o Ensure that the legacy users' IPv4 addresses remain visible to the
nodes and applications located in the IPv6 network.
o Conserve and maximize the utilization of the operator's public
IPv4 addresses.
o Avoid introducing more complexity than absolutely necessary,
especially on the nodes and applications.
o Easy to scale and deploy in a fault-tolerant manner.
The following subsections elaborate on how SIIT-DC meets these goals.
1.1. Single-Stack IPv6 Operation
SIIT-DC allows IDC operators to build their infrastructure and
applications on an IPv6-only foundation. IPv4 end-user connectivity
becomes a service provided by the network, which systems
administration and application development staff do not need to
concern themselves with. This promotes universal IPv6 deployment for
the IDC operator's services and applications.
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RFC 7755 SIIT-DC February 2016
SIIT-DC requires no special support or change from the underlying
IPv6 infrastructure; it is compatible with all standard IPv6
networks. Traffic between IPv6-enabled end users and IPv6-enabled
services will always be transported native end to end; SIIT-DC does
not intercept or handle native IPv6 traffic at all.
When the day comes to discontinue all support for IPv4, no change
needs to be made to the overall architecture -- it's only a matter of
shutting off the SIIT-DC Border Relays (BRs). Operators who deploy
native IPv6 along with SIIT-DC will thus avoid requiring any future
migration or deployment projects relating to IPv6 deployment and/or
IPv4 sunsetting.
1.2. Stateless Operation
Unlike other solutions that provide either dual-stack availability to
single-stack services (e.g., Stateful Network Address and Protocol
Translation from IPv6 Clients to IPv4 Servers (NAT64) [RFC 6146] and
Layer 4/7 proxies) or conservation of IPv4 addresses (e.g., IPv4
address translation (NAPT44) [RFC 3022]), SIIT-DC does not maintain
any state associated with individual connections or flows. In this
sense, it operates exactly like a regular IP router and has similar
scaling properties -- the limiting factors are packets per second and
bandwidth. The number of concurrent flows and flow initiation rates
are irrelevant for performance.
This not only allows individual BRs to easily attain "line-rate"
performance, but it also allows for per-packet load balancing between
multiple BRs using Equal-Cost Multipath Routing [RFC 2991].
Asymmetric routing is also acceptable, which makes it easy to avoid
suboptimal traffic patterns; the prefixes involved may be anycasted
from all the BRs in the provider's network, thus ensuring that the
most optimal path through the network is used, even where the optimal
path in one direction differs from the optimal path in the opposite
direction.
Finally, stateless operation means that high availability is easily
achieved. If a BR should fail, its traffic can be rerouted onto
another BR using a standard IP routing protocol. This does not
impact existing flows any more than what any other IP rerouting event
would.
1.3. IPv4 Address Conservation
In most parts of the world, it is difficult or even impossible to
obtain generously sized IPv4 delegations from the Internet Numbers
Registry System [RFC 7020]. The resulting scarcity in turn impacts
individual end users and operators, whom might be forced to purchase
Anderson Informational PAGE 4
RFC 7755 SIIT-DC February 2016
IPv4 addresses from other operators in order to cover their needs.
This process can be risky to business continuity, in the case where
no suitable block for sale can be located, and/or turn out to be
prohibitively expensive. In spite of this, an IDC operator will find
that providing IPv4 service remains essential, as a large share of
the Internet end users still do not have IPv6 connectivity.
A key goal of SIIT-DC is to help reduce a data center operator's IPv4
address requirement to the absolute minimum by allowing the operator
to remove them entirely from nodes and applications that do not need
to communicate with endpoints in the IPv4 Internet. One example
would be servers that are operating in a supporting/backend role and
only communicating with other servers (database servers, file
servers, and so on). Another example would be the network
infrastructure itself (router-to-router links, loopback addresses,
and so on). Furthermore, as LAN prefix sizes must always be rounded
up to the nearest power of two (or larger if one reserves space for
future growth), even more IPv4 addresses will often end up being
wasted without even being used.
With SIIT-DC, the operator can remove these valuable IPv4 addresses
from his backend servers and network infrastructure and reassign them
to the SIIT-DC service as IPv4 Service Addresses. There exists no
requirement that IPv4 Service Addresses are to be assigned in an
aggregated manner, so there is nothing lost due to infrastructure
overhead; every single IPv4 address assigned to SIIT-DC can be used
as an IPv4 Service Address.
1.4. Clients' IPv4 Source Addresses Visible to Applications
SIIT-DC uses the [RFC 6052] algorithm to map the entire end-user's
IPv4 source address into a predefined IPv6 translation prefix. This
ensures that there is no loss of information; the end-user's IPv4
source address remains available to the application located in the
IPv6 network, allowing it to perform tasks like geolocation, logging,
abuse handling, and so forth.
1.5. Compatible with Standard IPv4 and IPv6 Stacks
Except for the introduction of the BRs themselves, no change to the
network, nodes, applications, or anything else is required in order
to support SIIT-DC. SIIT-DC is practically invisible from the point
of view of the IPv4 clients, the IPv6 nodes, the IPv6 data center
network, and the IPv4 Internet. SIIT-DC interoperates with all
standards-compliant IPv4 or IPv6 stacks.
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RFC 7755 SIIT-DC February 2016
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].
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
IDC's outer network border. The ER is an optional component of
SIIT-DC. It is discussed in more detail in [RFC 7756].
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.
IPv4 Service Address Pool:
One or more IPv4 prefixes routed to the BR's IPv4 interface. IPv4
Service Addresses are allocated from this pool. This does not
necessarily have to be a "pool" per se, as it could also be one or
more host routes (whose prefix lengths are equal to /32). The
purpose of using a pool rather than host routes is to facilitate
IPv4 route aggregation and ease provisioning of new IPv4 Service
Addresses.
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].
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RFC 7755 SIIT-DC February 2016
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-Translatable 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.
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].
Anderson Informational PAGE 7
RFC 7755 SIIT-DC February 2016
3. Architectural Overview
This section describes the basic SIIT-DC architecture.
IPv6-capable user IPv4-only user
<2001:db8::ab:cd> <203.0.113.50>
| |
(the IPv6 Internet) (the IPv4 Internet)
| |
| +-[BR]---------<192.0.2.0/24>--------------+
| | |
| | EAM #1: 192.0.2.1,2001:db8:12:34::1 |
| | EAM #2..#n: [...] |
| | XLAT Prefix: 2001:db8:46::/96 |
| | |
| +------------<2001:db8:46::/96>------------+
| |
(the IPv6-only data center network)
|
+--<2001:db8:12:34::1>--[v6-only server]-+
| | |
| +-[2001:db8:12:34::1]--[v6-only app]-+ |
| | AF_INET6 socket | |
| +------------------------------------+ |
+----------------------------------------+
Figure 1: SIIT-DC Architecture
In Figure 1, 192.0.2.0/24 is the IPv4 Service Address Pool.
Individual IPv4 Service Addresses are assigned from this prefix, and
traffic destined for it is routed to the BR's IPv4-facing network
interface. There are no restrictions on how many IPv4 Service
Address Pools are used or their prefix length, as long as they are
all routed to the BR's IPv4-facing network interface.
When translating packets between IPv4 and IPv6, the BR uses EAM #1 to
replace any occurrence of the IPv4 Service Address (192.0.2.1) with
its corresponding IPv6 Service Address (2001:db8:12:34::1).
Addresses that do not match any EAM configured in the BR are
translated by inserting or removing the translation prefix
(2001:db8:46::/96); cf. Section 2.2 of [RFC 6052].
The BR can be deployed as a separate device or as a logical function
in another multipurpose device, such as an IP router. Any number of
BRs may exist simultaneously in the IDC's network infrastructure, as
long as they are all configured with the same translation prefix and
an identical EAM Table.
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RFC 7755 SIIT-DC February 2016
The IPv6 Service Address should be registered in DNS using an "IN
AAAA" record, while its corresponding IPv4 Service Address should be
registered using an "IN A" record. This ensures that IPv6-capable
clients access the application/service directly using native IPv6 end
to end, while IP4-only clients will access it through SIIT-DC.
3.1. Packet Flow
In this example, the "IPv4-only user" from Figure 1 initiates a
connection to the application running on the IPv6-only server. After
first having looked up the "IN A" record in DNS, the user starts by
transmitting a TCP SYN packet to the IPv4 Service Address. This IPv4
packet is routed to the BR and is there translated to IPv6 as
follows:
+--[IPv4]----------+ +--[IPv6]-----------------------+
| SRC 203.0.113.50 | | SRC 2001:db8:46::203.0.113.50 |
| DST 192.0.2.1 | --> | DST 2001:db8:12:34::1 |
| TCP SYN [..] | | TCP SYN [..] |
+------------------+ +-------------------------------+
Figure 2: IPv4-to-IPv6 Translation
The resulting IPv6 packet is routed to the IPv6-only server, which
processes and responds to it as if it had been a native IPv6 packet
all along. The server's IPv6 response packet is then routed back to
the BR, where it is translated back to IPv4 as follows:
+--[IPv6]-----------------------+ +--[IPv4]----------+
| SRC 2001:db8:12:34::1 | | SRC 192.0.2.1 |
| DST 2001:db8:46::203.0.113.50 | --> | DST 203.0.113.50 |
| TCP SYN/ACK [..] | | TCP SYN/ACK [..] |
+-------------------------------+ +------------------+
Figure 3: IPv6-to-IPv4 Translation
It is important to note that neither the IPv4 client nor the IPv6
server/application need any special support to participate in
SIIT-DC. However, the application may optionally be taught to
extract the embedded IPv4 source address from incoming IPv6 packets
with source addresses within the translation prefix. This will allow
it to perform IPv4-specific tasks such as geolocation, logging, abuse
handling, and so on.
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4. Deployment Considerations and Guidelines
4.1. Application/Device Support for IPv6
SIIT-DC as described in this document requires that the application
(and/or the node the application is located on) supports IPv6
networking and that it has no dependency on local IPv4 network
connectivity.
SIIT-DC can, however, support legacy IPv4-dependent applications and
nodes through the introduction of an ER. The ER provides the legacy
application or node with seemingly native IPv4 Internet connectivity,
so that it may operate correctly in an otherwise IPv6-only network
environment. This approach is described in more detail in [RFC 7756].
4.2. Application Support for NAT
The operator should carefully examine whether or not the application
protocols he would like to use SIIT-DC with are able to operate in a
network environment where rewriting of IP addresses occurs. In
general, if an application-layer protocol works correctly through
standard NAT44 (see [RFC 3235]), it will most likely work correctly
through SIIT-DC as well.
Higher-level protocols that embed IP addresses as part of their
payload are particularly problematic [RFC 2663] [RFC 2993] [RFC 3022].
One well-known example of such a protocol is FTP [RFC 959]. Such
protocols can be made to work with SIIT-DC through the introduction
of an ER, which provides end-to-end IPv4 address transparency by
reversing the translations performed by the BR before passing the
packets to the NAT-incompatible application. This approach is
described in more detail in [RFC 7756].
4.3. Application Communication Pattern
SIIT-DC is best suited for traditional client/server applications
where IPv4-only clients on the Internet initiate traffic towards an
IPv6-only service, which in turn is passively listening for inbound
traffic and responding as necessary. In this case, an IPv4 client
looks exactly like a native IPv6 client from the IPv6 service's point
of view and thus does not require any special treatment. One
particularly common application protocol that follows this client/
server communication pattern, and thus is ideally suited for use with
SIIT-DC, is HTTP [RFC 7230].
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RFC 7755 SIIT-DC February 2016
It is also possible to combine SIIT-DC with DNS64 [RFC 6147] in order
to allow an IPv6-only application to initiate communication with
IPv4-only nodes through SIIT-DC. However, in this case, care must be
taken so that all outgoing communication is sourced from an IPv6
Service Address that is found in an EAM configured in the BR. If
another address is used, the BR will most likely be unable to
translate it to IPv4, causing the packet to be discarded. This could
be prevented by altering the Default Address Selection Policy
Table [RFC 6724] on the IPv6 node.
An alternative approach to the above would be to place an ER in front
of the application in question, as described in [RFC 7756]. This
provides the application with seemingly native IPv4 connectivity,
which it may use freely for bidirectional communication with the IPv4
Internet. An application or node located behind an ER does not need
to worry about selecting a specific source address, as it will only
have valid options available.
4.4. Choice of Translation Prefix
Either a Network-Specific Prefix (NSP) from the provider's own IPv6
address space or the IANA-allocated Well-Known Prefix (WKP)
64:ff9b::/96 may be used. From a technical point of view, both work
equally well. However, only a single WKP exists, so if a provider
would like to deploy more than one instance of SIIT-DC in his
network, or another translation technology such as Stateful NAT64
[RFC 6146], the operator will be forced to use an NSP for all but one
of those deployments.
Another consideration is that the WKP cannot be used in inter-domain
routing. By using an NSP instead, SIIT-DC will support a deployment
where the BR and the IPv6 Service Address are located in different
Autonomous Systems.
The translation prefix may use any of the lengths described in
Section 2.2 of [RFC 6052], but /96 has two distinct advantages over
the others. First, converting it to IPv4 can be done in a single
operation by simply stripping off the first 96 bits; second, it
allows for IPv4 addresses to be embedded directly into the text
representation of an IPv6 address using the familiar dotted quad
notation, e.g., "2001:db8::198.51.100.10" (cf. Section 2.4 of
[RFC 6052]), instead of being converted to hexadecimal notation. This
makes it easier to write literal IPv6 addresses (e.g., in ACLs) that
correspond to translated endpoints in the IPv4 Internet.
For the reasons discussed above, this document recommends that an NSP
with a prefix length of /96 be used. Section 3.3 of [RFC 6052]
discusses the choice of the translation prefix in more detail.
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4.5. Routing Considerations
The prefixes that constitute the IPv4 Service Address Pool and the
IPv6 translation prefix may be routed to the BRs like any other IPv4
or IPv6 route in the provider's network. If more than one BR is
being deployed, it is recommended that a routing protocol (IGP) be
used to advertise the routes within the provider's network. This
will ensure that the traffic that is to be translated will reach the
closest BR, reducing or eliminating suboptimal traffic patterns as
well as providing high availability: should one BR fail, the IGP will
automatically redirect the traffic to the closest alternate BR.
4.6. Location of the SIIT-DC Border Relays
The goal of SIIT-DC is to facilitate a true IPv6-only application and
network architecture, with the sole exception being the IPv4
interfaces of the BRs and the network infrastructure required to
connect the BRs to the IPv4 Internet. Therefore, the BRs must be
located somewhere between the IPv4 Internet and the application
delivery stack, which includes all servers, load balancers,
firewalls, intrusion detection systems, and similar devices that are
processing traffic to a greater extent than merely forwarding it.
It is optimal to place the BRs as close as possible to the direct
path between the location of the IPv6 Service Address and the end
users. If the closest BR was located a long way from the direct
path, all packets in both directions must make a detour in order to
traverse the BR. This would increase the RTT between the service and
the end user by two times the extra latency incurred by the detour,
as well as cause unnecessary load on the network links on the detour
path.
Where possible, it is beneficial to implement the BRs as a logical
function within the routers that also handle the native IPv6 traffic
between the IPv6 Service Address and the IPv6 Internet. This way, an
SIIT-DC deployment does not require separate networks ports (which
might become saturated and impact the service quality) nor will it
require extra rack space and energy. Some particularly good choices
for the location could be within the IDC's access routers or within
the Autonomous System's border routers.
Finally, another possibility is that the IDC operator outsources the
SIIT-DC service to another entity, for example, his upstream ISP.
Doing so allows the IDC operator to build a true IPv6-only
infrastructure.
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RFC 7755 SIIT-DC February 2016
4.7. Migration from Dual Stack
While this document mainly discusses the use of IPv6-only nodes and
applications, it is important to note that SIIT-DC is fully
compatible with dual-stack infrastructures, including dual-stack
nodes and applications.
Thus, migrating a dual-stacked service to an IPv6-only one where
SIIT-DC provides the IPv4 Internet connectivity is easy. The
operator would start out by designating the service's current native
IPv6 address as the IPv6 Service Address and assigning it a
corresponding IPv4 Service Address. At this point, the service will
respond on both its old (native) IPv4 address and the SIIT-DC IPv4
Service Address. The operator may now move traffic from the former
to the latter by changing the service's "IN A" DNS record. Once all
IPv4 traffic has been successfully moved to SIIT-DC, the old IPv4
address may be reclaimed.
4.8. Translation of ICMPv6 Errors to IPv4
In response to an IPv4 packet subsequently translated to IPv6 by the
BR, an IPv6 router in the IDC network may need to transmit an ICMPv6
error back to the origin IPv4 node. By default, such an ICMPv6 error
will most likely be discarded by the BR, unless the source address of
the ICMPv6 error happens to be an IPv4-translatable IPv6 address or
covered by an EAM.
To facilitate reliable delivery of such ICMPv6 errors, an SIIT-DC
operator SHOULD implement the recommendations in [RFC 6791] in the
BRs.
4.9. MTU and Fragmentation
There are some key differences between IPv4 and IPv6 relating to
packet sizes and fragmentation that one MUST consider when deploying
SIIT-DC. They result in a few problematic corner cases, which can be
dealt with in a few different ways. The following subsections will
discuss these in detail and provide operational guidance.
In particular, the operator may find that relying on fragmentation in
the IPv6 domain is undesired or even operationally impossible
[FRAGMENTS]. For this reason, the recommendations in this section
seek to minimize the use of IPv6 fragmentation.
Unless otherwise stated, the following subsections assume that the
MTUs in both the IPv4 and IPv6 domains are 1500 bytes.
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RFC 7755 SIIT-DC February 2016
4.9.1. IPv4/IPv6 Header Size Difference
The IPv6 header is up to 20 bytes larger than the IPv4 header. This
means that a full-size 1500 bytes large IPv4 packet cannot be
translated to IPv6 without being fragmented, otherwise it would
likely have resulted in a 1520 bytes large IPv6 packet.
If the transport protocol used is TCP, this is generally not a
problem; the IPv6 node will advertise a TCP Maximum Segment Size
(MSS) of 1440 bytes during the initial TCP handshake. This causes
the IPv4 clients to never send larger packets than what can be
translated to a single full-size IPv6 packet, eliminating any need
for fragmentation.
For other transport protocols, full-size IPv4 packets with the Don't
Fragment (DF) flag cleared will need to be fragmented by the BR.
This may be avoided by increasing the Path MTU between the BR and the
IPv6 nodes to 1520 bytes or greater. If this is done, the MTU on the
IPv6 nodes themselves SHOULD NOT be increased accordingly, as doing
so would cause them to undergo Path MTU Discovery for all
destinations on the IPv6 Internet. The nodes MUST, however, be able
to accept and process incoming packets larger than their own MTU. If
the nodes' IPv6 implementation allows the initial Path MTU to be set
differently for specific destinations, it MAY be increased to 1520
for destinations within the translation prefix specifically.
4.9.2. IPv6 Atomic Fragments
In keeping with the fifth paragraph of Section 4 of [RFC 6145], a
stateless translator like a BR will by default add an IPv6
Fragmentation header to the resulting IPv6 packet when translating an
IPv4 packet with the DF flag set to 0. This happens even though the
resulting IPv6 packet isn't actually fragmented into several pieces,
resulting in an IPv6 Atomic Fragment [RFC 6946]. These Atomic
Fragments are generally not useful in an IDC environment, and it is
therefore recommended that this behavior be disabled in the BRs. To
this end, Section 4 of [RFC 6145] notes that the "translator MAY
provide a configuration function that allows the translator not to
include the Fragment Header for the non-fragmented IPv6 packets."
Note that work is currently in progress (in [RFC6145bis]) to
deprecate IPv6 Atomic Fragments. As a result, a BR that conforms to
that document is required to behave as recommended above.
In IPv6, the Identification value is located inside the Fragmentation
header. That means that if the generation of IPv6 Atomic Fragments
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is disabled, the IPv4 Identification value will be lost during
translation to IPv6. This could potentially confuse some diagnostic
tools.
4.9.3. Minimum Path MTU Difference between IPv4 and IPv6
Section 5 of [RFC 2460] specifies that the minimum IPv6 link MTU is
1280 bytes. Therefore, an IPv6 node can reasonably assume that if it
transmits an IPv6 packet that is 1280 bytes or smaller, it is
guaranteed to reach its destination without requiring fragmentation
or invoking the Path MTU Discovery algorithm [RFC 1981]. However,
this assumption might prove false if the destination is an IPv4 node
reached through a protocol translator such as a BR, as the minimum
IPv4 link MTU is 68 bytes. See Section 3.2 of [RFC 791].
Section 5.1 of [RFC 6145] specifies that a stateless translator should
set the IPv4 Don't Fragment flag to 1 when it translates a
non-fragmented IPv6 packet to IPv4. This means that when the path to
the destination IPv4 node contains an IPv4 link with an MTU smaller
than 1260 bytes (which corresponds to an IPv6 MTU smaller than 1280
bytes; cf. Section 4.9.1), the Path MTU Discovery algorithm will be
invoked, even if the original IPv6 packet was only 1280 bytes large.
This happens as a result of the IPv4 router connecting to the IPv4
link with the small MTU returning an ICMPv4 Need To Fragment error
with an MTU value smaller than 1260, which in turn is translated by
the BR to an ICMPv6 Packet Too Big error with an MTU value smaller
than 1280, which is then transmitted to the origin IPv6 node.
When an IPv6 node receives an ICMPv6 Packet Too Big error indicating
an MTU value smaller than 1280, it is not allowed to reduce its Path
MTU estimation to the indicated value. It must instead include a
Fragmentation header in subsequent packets sent on that path
[RFC 1981]. In other words, the IPv6 node will start emitting Atomic
Fragments. The Fragmentation header signals to the BR that the Don't
Fragment flag should be set to 0 in the resulting IPv4 packet, and it
also provides the Identification value.
If the use of the IPv6 Fragmentation header is problematic, the
operator should consider enabling the functionality described as the
"second approach" in Section 6 of [RFC 6145]. This functionality
changes the BR's behavior as follows:
o When translating ICMPv4 Need To Fragment to ICMPv6 Packet Too Big,
the resulting packet will never contain an MTU value lower than
1280. This prevents the IPv6 nodes from generating Atomic
Fragments.
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o When translating IPv6 packets smaller than or equal to 1280 bytes,
the Don't Fragment flag in the resulting IPv4 packet will be set
to 0. This ensures that in the eventuality that the path contains
an IPv4 link with an MTU smaller than 1260, the IPv4 router
connected to that link will have the responsibility to fragment
the packet before forwarding it towards its destination.
In summary, this approach could be seen as prompting the IPv4
protocol itself to provide the "link-specific fragmentation and
reassembly at a layer below IPv6" required for links that "cannot
convey a 1280-octet packet in one piece", to paraphrase Section 5 of
[RFC 2460].
Note that work is currently in progress (in [RFC6145bis]) to
deprecate IPv6 Atomic Fragments. As a result, a BR that conforms to
that document is required to behave as suggested above.
4.10. IPv4-Translatable IPv6 Service Addresses
SIIT-DC is designed so that the IPv6 Service Addresses are not
required to be IPv4-translatable IPv6 addresses. Section 2 of
[RFC 7757] discusses why it is desirable to avoid requiring the use of
IPv4-translatable IPv6 addresses.
It is, however, quite possible to deploy SIIT-DC in combination with
IPv4-translatable IPv6 Service Addresses. The primary benefits in
doing so are:
o The operator is not required to provision EAMs for
IPv4-translatable IPv6 Service Addresses onto the BR/ERs.
o [RFC 6145] translation can be performed in a checksum-neutral
manner; cf. Section 4.1 of [RFC 6052].
The trade-off is that the IPv4-translatable IPv6 Service Addresses
must be configured on the IPv6 nodes, and the applications must be
set up to use them -- likely in addition to their primary
(non-IPv4-translatable) IPv6 addresses. The IPv4-translatable IPv6
Service Addresses must also be routed from the BR through the IDC's
IPv6 network infrastructure to the nodes on which they are assigned.
This essentially requires the entire IPv6 infrastructure to be made
aware of and handle translated IPv4 traffic as a special case, which
significantly increases complexity. As previously described in
Section 1.1, avoiding such drawbacks is a design goal of SIIT-DC.
The use of IPv4-translatable IPv6 Service Addresses is therefore
discouraged.
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5. Security Considerations
5.1. Mistaking the Translation Prefix for a Trusted Network
If a Network-Specific Prefix from the provider's own address space is
chosen for the translation prefix, as recommended in Section 4.4,
care MUST be taken if the translation service is used in front of
services that have application-level ACLs that distinguish between
the operator's own networks and the Internet at large, as traffic
from translated IPv4 end users on the Internet might appear to be
originating from the provider's own network. It is therefore
important that the translation prefix be treated the same as the
Internet at large rather than as a trusted network.
In order to alleviate this problem, the operator may opt to use a
translation prefix that is distinct from and not a subset of the IPv6
prefixes used elsewhere in the network infrastructure.
6. References
6.1. 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 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 6791] Li, X., Bao, C., Wing, D., Vaithianathan, R., and G.
Huston, "Stateless Source Address Mapping for ICMPv6
Packets", RFC 6791, DOI 10.17487/RFC 6791, November 2012,
<http://www.rfc-editor.org/info/RFC 6791>.
[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>.
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6.2. Informative References
[FRAGMENTS]
Jaeggli, J., Colitti, L., Kumari, W., Vyncke, E., Kaeo,
M., and T. Taylor, "Why Operators Filter Fragments and
What It Implies", Work in Progress, draft-taylor-v6ops-
fragdrop-02, December 2013.
[RFC 791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC 791, September 1981,
<http://www.rfc-editor.org/info/RFC 791>.
[RFC 959] Postel, J. and J. Reynolds, "File Transfer Protocol",
STD 9, RFC 959, DOI 10.17487/RFC 959, October 1985,
<http://www.rfc-editor.org/info/RFC 959>.
[RFC 1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, DOI 10.17487/RFC 1981, August
1996, <http://www.rfc-editor.org/info/RFC 1981>.
[RFC 2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC 2460,
December 1998, <http://www.rfc-editor.org/info/RFC 2460>.
[RFC 2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, DOI 10.17487/RFC 2663, August 1999,
<http://www.rfc-editor.org/info/RFC 2663>.
[RFC 2991] Thaler, D. and C. Hopps, "Multipath Issues in Unicast and
Multicast Next-Hop Selection", RFC 2991,
DOI 10.17487/RFC 2991, November 2000,
<http://www.rfc-editor.org/info/RFC 2991>.
[RFC 2993] Hain, T., "Architectural Implications of NAT", RFC 2993,
DOI 10.17487/RFC 2993, November 2000,
<http://www.rfc-editor.org/info/RFC 2993>.
[RFC 3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
DOI 10.17487/RFC 3022, January 2001,
<http://www.rfc-editor.org/info/RFC 3022>.
[RFC 3235] Senie, D., "Network Address Translator (NAT)-Friendly
Application Design Guidelines", RFC 3235,
DOI 10.17487/RFC 3235, January 2002,
<http://www.rfc-editor.org/info/RFC 3235>.
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RFC 7755 SIIT-DC February 2016
[RFC 4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213,
DOI 10.17487/RFC 4213, October 2005,
<http://www.rfc-editor.org/info/RFC 4213>.
[RFC6145bis]
Bao, C., Li, X., Baker, F., Anderson, T., and F. Gont,
"IP/ICMP Translation Algorithm (rfc6145bis)", Work in
Progress, draft-bao-v6ops-rfc6145bis-05, January 2016.
[RFC 6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC 6146,
April 2011, <http://www.rfc-editor.org/info/RFC 6146>.
[RFC 6147] Bagnulo, M., Sullivan, A., Matthews, P., and I. van
Beijnum, "DNS64: DNS Extensions for Network Address
Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
DOI 10.17487/RFC 6147, April 2011,
<http://www.rfc-editor.org/info/RFC 6147>.
[RFC 6540] George, W., Donley, C., Liljenstolpe, C., and L. Howard,
"IPv6 Support Required for All IP-Capable Nodes", BCP 177,
RFC 6540, DOI 10.17487/RFC 6540, April 2012,
<http://www.rfc-editor.org/info/RFC 6540>.
[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 6883] Carpenter, B. and S. Jiang, "IPv6 Guidance for Internet
Content Providers and Application Service Providers",
RFC 6883, DOI 10.17487/RFC 6883, March 2013,
<http://www.rfc-editor.org/info/RFC 6883>.
[RFC 6946] Gont, F., "Processing of IPv6 "Atomic" Fragments",
RFC 6946, DOI 10.17487/RFC 6946, May 2013,
<http://www.rfc-editor.org/info/RFC 6946>.
[RFC 7020] Housley, R., Curran, J., Huston, G., and D. Conrad, "The
Internet Numbers Registry System", RFC 7020,
DOI 10.17487/RFC 7020, August 2013,
<http://www.rfc-editor.org/info/RFC 7020>.
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RFC 7755 SIIT-DC February 2016
[RFC 7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC 7230, June 2014,
<http://www.rfc-editor.org/info/RFC 7230>.
[RFC 7756] Anderson, T. and S. Steffann, "Stateless IP/ICMP
Translation for IPv6 Internet Data Center Environments
(SIIT-DC): Dual Translation Mode", RFC 7756,
DOI 10.17487/RFC 7756, February 2016,
<http://www.rfc-editor.org/info/RFC 7756>.
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Appendix A. Complete SIIT-DC IDC Topology Example
Figure 4 attempts to "tie it all together" and show a more complete
SIIT-DC topology, in order to better demonstrate its advantageous
properties discussed in Section 1. These are discussed in more
detail below.
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/--------------------------------\ /---------------\
| IPv4 Internet | | IPv6 Internet |
\-+----------------------------+-/ \--------+------/
| | |
| <----------[BGP]---------> | [BGP]
| | |
+-------<192.0.2.0/24>---------+ +---<192.0.2.0/24>---+ |
| BR #1 | | BR #2 | |
| EAM Table: | | | |
| ========== | | | |
| 192.0.2.1,2001:db8:12:34::1 | | | |
| 192.0.2.2,2001:db8:12:34::2 | | Exactly the same | |
| 192.0.2.3,2001:db8:fe:dc::1 | | configuration as | |
| 192.0.2.4,2001:db8:12:34::4 | | BR #1 | |
| 192.0.2.5,2001:db8:fe:dc::e | | | |
| | | | |
| XLAT Prefix 2001:db8:46::/96 | | | |
| | | | |
+--------<2001:db8:46::/96>----+ +-<2001:db8:46::/96>-+ |
| | |
| <------[ECMP]------> | |
| | |
/-----------------+----------------------+--\ |
| IPv6 IDC network w/ OSPFv3 +------------/
\-+--------------------------------+--------/
| |
| Tenant A's server LAN | Tenant B's server LAN
| 2001:db8:12:34::/64 | 2001:db8:fe:dc::/64
| |
+-- www ::1 (IPv6+SIIT-DC) +-- www-lb ::1 (IPv6+SIIT-DC)
| |
+-- mta ::2 (IPv6+SIIT-DC) +-- web ::80:01 (IPv6 only)
| | [...]
+-- ftp ::3 (IPv6) +-- web ::80:99 (IPv6 only)
| ::4 (IPv4, via ER) |
| | +----+
+-- app01 ::a:01 (IPv6 only) \---- ::e | ER | --\
| [...] +----+ |
+- app99 ::a:99 (IPv6 only) |
| ftp 192.0.2.5 ---/
+-- db01 ::d:01 (IPv6 only)
| [..]
\-- db99 ::d:99 (IPv6 only)
Figure 4: Example SIIT-DC IDC Topology
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Single-Stack IPv6 Operation:
As discussed in Section 1.1, SIIT-DC facilitates an IPv6-only IDC
network infrastructure. The only places where IPv4 is absolutely
required are between the BRs and the IPv4 Internet and between any
ERs and the IPv4-only applications or devices they are serving
(illustrated here as the two tenants' FTP servers). The figure
also illustrates how SIIT-DC does not interfere with native IPv6;
when there is no longer a need to support IPv4 clients, the BRs
may be decommissioned without causing any impact to native IPv6
traffic.
Stateless Operation:
As discussed in Section 1.2, SIIT-DC operates in a stateless
fashion. In the illustration, both BRs are simultaneously
advertising (i.e., anycasting) the IPv4 Service Address Pool and
the IPv6 translation prefix, so incoming traffic from the IPv4
Internet may arrive at either of the BRs, while outgoing IPv6
traffic destined for IPv4 endpoints are load balanced between them
using Equal-Cost Multipath Routing. No continuous state
synchronization between the two BRs occurs. Should one of the BRs
fail, the BGP and OSPF protocols will ensure that traffic
converges on the remaining BR. Existing sessions will not be
disrupted beyond any disruption caused by the BGP/OSPF convergence
process itself.
IPv4 Address Conservation:
As discussed in Section 1.3, SIIT-DC conserves the IDC operator's
IPv4 address space. Even though the two customers in the example
above have several hundred servers, the majority of the servers
are not used for running services made available directly from the
Internet and therefore do not need to consume IPv4 addresses. The
IDC network infrastructure consumes no IPv4 addresses, either.
Finally, the IPv4 addresses that are assigned to the SIIT-DC
function as IPv4 Service Address Pools may be assigned with 100%
efficiency, one address at a time; there is no requirement to
assign multiple addresses to a single customer in a contiguous
block.
Application Support:
As discussed in Section 1.5, as long as the application protocol
is translation friendly (illustrated here with HTTP and SMTP), it
will work with SIIT-DC without requiring any special adaptation.
Furthermore, translation-unfriendly applications (illustrated here
with FTP) will also work when located behind an ER [RFC 7756].
Tenant A's FTP server illustrates how an ER may be located in the
networking stack of a node, while Tenant B's FTP server
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illustrates how the ER may be deployed as a network service. The
latter approach enables SIIT-DC to support IPv4-only
nodes/devices.
Acknowledgements
The author would like to thank the following individuals for their
contributions, suggestions, corrections, and criticisms: Fred Baker,
Cameron Byrne, Brian E. Carpenter, Ross Chandler, Tobias Gondrom,
Christer Holmberg, Dagfinn Ilmari Mannsaaker, Lars Olafsen, Stig
Sandbeck Mathisen, Knut A. Syed, Qin Wu, and Andrew Yourtchenko.
Author's Address
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
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RFC TOTAL SIZE: 54648 bytes
PUBLICATION DATE: Thursday, February 18th, 2016
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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