Internet DRAFT - draft-ietf-v6ops-siit-dc
draft-ietf-v6ops-siit-dc
IPv6 Operations T. Anderson
Internet-Draft Redpill Linpro
Intended status: Informational October 12, 2015
Expires: April 14, 2016
SIIT-DC: Stateless IP/ICMP Translation for IPv6 Data Centre Environments
draft-ietf-v6ops-siit-dc-03
Abstract
This document describes the use of the Stateless IP/ICMP Translation
(SIIT) algorithm in an IPv6 Internet Data Centre (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 utilisation of public IPv4 addresses.
The primary audience is IDC operators who are deploying IPv6, running
out of available IPv4 addresses, and/or feel that dual stack causes
undesirable operational complexity.
Status of This Memo
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This Internet-Draft will expire on April 14, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
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 . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Architectural Overview . . . . . . . . . . . . . . . . . . . 7
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 . . . . . . . . . . . . . . . . 12
4.8. Translation of ICMPv6 Errors to IPv4 . . . . . . . . . . 13
4.9. MTU and Fragmentation . . . . . . . . . . . . . . . . . . 13
4.9.1. IPv4/IPv6 Header Size Difference . . . . . . . . . . 13
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. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7.1. Mistaking the Translation Prefix for a Trusted Network . 17
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1. Normative References . . . . . . . . . . . . . . . . . . 17
8.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Complete SIIT-DC IDC topology example . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
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Historically, dual stack [RFC4213] [RFC6883] 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 centre as they
prepare for the future. SIIT-DC is one such approach. Its design
goals include:
o Promote the deployment of native IPv6 services (cf. [RFC6540]).
o Provide IPv4 service availability for legacy users with no loss of
performance or functionality.
o To ensure that that the legacy users' IPv4 addresses remain
visible to the nodes and applications located in the IPv6 network.
o To conserve and maximise the utilisation of the operator's public
IPv4 addresses.
o To avoid introducing more complexity than absolutely necessary,
especially on the nodes and applications.
o To be easy to scale and deploy in a fault-tolerant manner.
The following subsections elaborates 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.
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
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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 sun-setting.
1.2. Stateless Operation
Unlike other solutions that provide either dual stack availability to
single-stack services (e.g., Stateful NAT64 [RFC6146] and Layer-4/7
proxies), or that provide conservation of IPv4 addresses (e.g.,
NAPT44 [RFC3022]), 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, it also allows for per-packet load balancing between
multiple BRs using Equal-Cost Multipath Routing [RFC2991].
Asymmetric routing is also acceptable, which makes it easy to avoid
sub-optimal 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 re-routed onto
another BR using a standard IP routing protocol. This does not
impact existing flows any more than what any other IP re-routing
event would.
1.3. IPv4 Address Conservation
In most parts of the world, it is difficult or even impossible to
obtain generously sized IPv4 delegation from the Internet Numbers
Registry System [RFC7020]. The resulting scarcity in turn impacts
individual end users and operators, which might be forced to purchase
IPv4 addresses from other operators in order to cover their needs.
This process can be risky to business continuity, in the case 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
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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 centre 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/back-end role and
only communicates 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 back-end 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 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
an IPv4 Service Address.
1.4. Clients' IPv4 Source Addresses Visible to Applications
SIIT-DC uses the [RFC6052] algorithm to map the entire end-user's
IPv4 source address into an 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 Geo-Location,
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 centre
network, and the IPv4 Internet. SIIT-DC interoperates with all
standards-compliant IPv4 or IPv6 stacks.
2. Terminology
This document makes use of the following terms:
SIIT-DC Border Relay (BR)
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A device or a logical function that performs stateless protocol
translation between IPv4 and IPv6. It MUST do so in accordance
with [RFC6145] and [I-D.ietf-v6ops-siit-eam].
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
[I-D.ietf-v6ops-siit-dc-2xlat].
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 EAM.
Packets sent to this address is 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. That this does
not necessarily have to be a "pool" per se, as it could also be
one or more host routes (whose prefix length is 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
communicates with the IPv6 Service Address through SIIT-DC.
Explicit Address Mapping (EAM)
A bi-directional 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 [I-D.ietf-v6ops-siit-eam].
Translation Prefix
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An IPv6 prefix into which the entire IPv4 address space is mapped,
according to the algorithm in [RFC6052]. The Translation Prefix
is routed to the BR's IPv6 interface. When translating between
IPv4 and IPv6, an 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 [RFC6052].
IDC
Short for "Internet Data Centre"; a data centre whose main purpose
is to deliver services to the public Internet, the use case SIIT-
DC is primarily targeted at. IDCs are typically operated by
Internet Content Providers or Managed Services Providers.
SIIT
The Stateless IP/ICMP Translation algorithm, as specified in
[RFC6145].
XLAT
Short for "Translation". Used in figures to indicate where a BR/
ER uses SIIT [RFC6145] 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 [RFC2119].
3. Architectural Overview
This section describes the basic SIIT-DC architecture.
SIIT-DC Architecture
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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 centre network)
|
+--<2001:db8:12:34::1>--[v6-only server]-+
| | |
| +-[2001:db8:12:34::1]--[v6-only app]-+ |
| | AF_INET6 socket | |
| +------------------------------------+ |
+----------------------------------------+
Figure 1
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 the EAM
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 [RFC6052].
The BR can be deployed as a separate device or as a logical function
in another multi-purpose device, such as an IP router. Any number of
BRs may exist simultaneously in the IDC's network infrastructure, as
long as they all configured with the same Translation Prefix and an
identical EAM Table.
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The IPv6 Service Address of 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 its 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 an 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 to IPv6 translation
+--[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
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 to IPv4 translation
+--[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
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 Geo-Location, 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
[I-D.ietf-v6ops-siit-dc-2xlat].
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 occur. In
general, if an application layer protocol works correctly through
standard NAT44 (see [RFC3235]), 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 [RFC2663] [RFC2993] [RFC3022].
One well-known example of such a protocol is FTP [RFC0959]. 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 [I-D.ietf-v6ops-siit-dc-2xlat].
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 an 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 [RFC7230].
It is also possible to combine SIIT-DC with DNS64 [RFC6147] in order
to allow an IPv6-only application to initiate communication with
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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
[RFC6724] 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
[I-D.ietf-v6ops-siit-dc-2xlat]. This provides the application with
seemingly native IPv4 connectivity, which it may use freely for bi-
directional 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 64:ff9b::/96
(WKP) 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 [RFC6146], 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 [RFC6052], 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
[RFC6052])), instead of being converted to hexadecimal notation.
This makes it easier to write IPv6 ACLs and similar that match
translated endpoints in the IPv4 Internet.
For the reasons discussed above, this document recommends that an NSP
with a prefix length of /96 is used. Section 3.3 of [RFC6052]
discusses the choice of 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 as 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) 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 sub-optimal 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 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 would have handled the traffic anyway,
had the topology been dual stacked. This way, a 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 of the
location could be within an 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.
4.7. Migration from Dual Stack
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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 assign 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 a 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 [RFC6791] 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
[I-D.taylor-v6ops-fragdrop]. For this reason, the recommendations in
this section seeks to minimise the use of IPv6 fragmentation.
Unless otherwise stated, the following subsections assume that the
MTU in both the IPv4 and IPv6 domains is 1500 bytes.
4.9.1. IPv4/IPv6 Header Size Difference
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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 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 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 [RFC6145], 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 Don't Fragment 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 [RFC6946].
These Atomic Fragments are generally not useful in an IDC
environment, and it is therefore recommended that this behaviour is
disabled in the BRs. To this end, Section 4 of [RFC6145] 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 IPv6 Atomic Fragments are currently being deprecated by
RFC6145bis [I-D.bao-v6ops-rfc6145bis]. As a result, a BR that
conforms to the updated standard 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
is disabled, the IPv4 Identification value will be lost during
translation to IPv6. This could potentially confuse some diagnostic
tools.
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4.9.3. Minimum Path MTU Difference Between IPv4 and IPv6
Section 5 of [RFC2460] 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 [RFC1981]. 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 [RFC0791].
Section 5.1 of [RFC6145] 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 turns 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, the last paragraph of Section 5 of
[RFC2460] gives it two choices on how to proceed:
o It may reduce its Path MTU value to the value indicated in the
Packet Too Big, i.e., limit the size of subsequent packets
transmitted to that destination to the indicated value. This
approach causes no problems for the SIIT-DC function, as it simply
allows Path MTU Discovery to work transparently across the BR.
o It may reduce its Path MTU value to exactly 1280, and in addition
include a Fragmentation header in subsequent packets sent to that
destination. In other words, the IPv6 node will start emitting
Atomic Fragments. The Fragmentation header signals to the 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, and the
operator has IPv6 nodes that implement the second option above, the
operator should consider enabling the functionality described as the
"second approach" in Section 6 of [RFC6145]. This functionality
changes the BR's behaviour as follows:
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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.
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
[RFC2460].
Note that IPv6 Atomic Fragments are currently being deprecated by
RFC6145bis [I-D.bao-v6ops-rfc6145bis]. As a result, a BR that
conforms to the updated standard 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
[I-D.ietf-v6ops-siit-eam] 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 [RFC6145] translation can be performed in a checksum-neutral
manner, cf. Section 4.1 of [RFC6052].
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
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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.
5. 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, Andrew Yourtchenko.
6. IANA Considerations
This draft makes no request of the IANA.
7. Security Considerations
7.1. 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 is 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.
8. References
8.1. Normative References
[I-D.ietf-v6ops-siit-eam]
Anderson, T. and A. Leiva, "Explicit Address Mappings for
Stateless IP/ICMP Translation", draft-ietf-v6ops-siit-
eam-01 (work in progress), June 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
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[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
DOI 10.17487/RFC6052, October 2010,
<http://www.rfc-editor.org/info/rfc6052>.
[RFC6145] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
Algorithm", RFC 6145, DOI 10.17487/RFC6145, April 2011,
<http://www.rfc-editor.org/info/rfc6145>.
[RFC6791] Li, X., Bao, C., Wing, D., Vaithianathan, R., and G.
Huston, "Stateless Source Address Mapping for ICMPv6
Packets", RFC 6791, DOI 10.17487/RFC6791, November 2012,
<http://www.rfc-editor.org/info/rfc6791>.
8.2. Informative References
[I-D.bao-v6ops-rfc6145bis]
Bao, C., Li, X., Baker, F., Anderson, T., and F. Gont, "IP
/ICMP Translation Algorithm (rfc6145bis)", draft-bao-
v6ops-rfc6145bis-02 (work in progress), October 2015.
[I-D.ietf-v6ops-siit-dc-2xlat]
Anderson, T. and S. Steffann, "SIIT-DC: Dual Translation
Mode", draft-ietf-v6ops-siit-dc-2xlat-01 (work in
progress), June 2015.
[I-D.taylor-v6ops-fragdrop]
Jaeggli, J., Colitti, L., Kumari, W., Vyncke, E., Kaeo,
M., and T. Taylor, "Why Operators Filter Fragments and
What It Implies", draft-taylor-v6ops-fragdrop-02 (work in
progress), December 2013.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, DOI
10.17487/RFC0791, September 1981,
<http://www.rfc-editor.org/info/rfc791>.
[RFC0959] Postel, J. and J. Reynolds, "File Transfer Protocol", STD
9, RFC 959, DOI 10.17487/RFC0959, October 1985,
<http://www.rfc-editor.org/info/rfc959>.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
1996, <http://www.rfc-editor.org/info/rfc1981>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <http://www.rfc-editor.org/info/rfc2460>.
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[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", RFC
2663, DOI 10.17487/RFC2663, August 1999,
<http://www.rfc-editor.org/info/rfc2663>.
[RFC2991] Thaler, D. and C. Hopps, "Multipath Issues in Unicast and
Multicast Next-Hop Selection", RFC 2991, DOI 10.17487/
RFC2991, November 2000,
<http://www.rfc-editor.org/info/rfc2991>.
[RFC2993] Hain, T., "Architectural Implications of NAT", RFC 2993,
DOI 10.17487/RFC2993, November 2000,
<http://www.rfc-editor.org/info/rfc2993>.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, DOI
10.17487/RFC3022, January 2001,
<http://www.rfc-editor.org/info/rfc3022>.
[RFC3235] Senie, D., "Network Address Translator (NAT)-Friendly
Application Design Guidelines", RFC 3235, DOI 10.17487/
RFC3235, January 2002,
<http://www.rfc-editor.org/info/rfc3235>.
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, DOI 10.17487/
RFC4213, October 2005,
<http://www.rfc-editor.org/info/rfc4213>.
[RFC6146] 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/RFC6146,
April 2011, <http://www.rfc-editor.org/info/rfc6146>.
[RFC6147] 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/RFC6147, April 2011,
<http://www.rfc-editor.org/info/rfc6147>.
[RFC6540] George, W., Donley, C., Liljenstolpe, C., and L. Howard,
"IPv6 Support Required for All IP-Capable Nodes", BCP 177,
RFC 6540, DOI 10.17487/RFC6540, April 2012,
<http://www.rfc-editor.org/info/rfc6540>.
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[RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
<http://www.rfc-editor.org/info/rfc6724>.
[RFC6883] Carpenter, B. and S. Jiang, "IPv6 Guidance for Internet
Content Providers and Application Service Providers", RFC
6883, DOI 10.17487/RFC6883, March 2013,
<http://www.rfc-editor.org/info/rfc6883>.
[RFC6946] Gont, F., "Processing of IPv6 "Atomic" Fragments", RFC
6946, DOI 10.17487/RFC6946, May 2013,
<http://www.rfc-editor.org/info/rfc6946>.
[RFC7020] Housley, R., Curran, J., Huston, G., and D. Conrad, "The
Internet Numbers Registry System", RFC 7020, DOI 10.17487/
RFC7020, August 2013,
<http://www.rfc-editor.org/info/rfc7020>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing", RFC
7230, DOI 10.17487/RFC7230, June 2014,
<http://www.rfc-editor.org/info/rfc7230>.
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.
Example SIIT-DC IDC topology
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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 has | |
| 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
Single Stack IPv6 Operation
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As discussed in Section 1.1, SIIT-DC facilitates an IPv6-only IDC
network infrastructure. The only places where IPv4 is absolutely
required is 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
synchronisation 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 them are not
used to run 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 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
[I-D.ietf-v6ops-siit-dc-2xlat]. 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 illustrates how the ER may be deployed as a
network service. The latter approach enables SIIT-DC to support
IPv4-only nodes/devices.
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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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