Internet Engineering Task Force A.Durand
INTERNET-DRAFT SUN Microsystems,inc.
March, 29, 2004 F. Parent
Expires September 28, 2004 Hexago
Requirements for assisted tunneling
<draft-durand-v6ops-assisted-tunneling-requirements-00.txt>
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Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document defines requirements for a tunnel set-up protocol that
could be used by an ISP to jumpstart its IPv6 offering to its
customers by providing them IPv6 connectivity without having to
upgrade its access network.
1. Goal and Scope of the Document
The v6ops working group has worked on requirements and scenarios for
IPv6 deployment by soliciting input from network operators. This work
has identified a need for an "assisted tunneling" mechanism. For
example, an ISP starting its IPv6 offering to its customers without
upgrading its access network to support IPv6 could use a "tunnel
brokering solution" [ISP, section 5.1.] a la [3053]. What has been
identified as missing from that RFC is a tunnel set-up protocol.
In an ISP network where IPv4 is dominant, some of the initial IPv6
deployment phases consists of getting a prefix allocation from the
RIR, and peering with other IPv6 ISP (or exchange points).
Getting IPv6 connectivity to the customers involves upgrading the
access network to support IPv6, which can take a long time and/or be
costly. A tunneled infrastructure can be used as a low cost
migration path [ISP, section 5.1].
With such an infrastructure, the ISP can connect its customers to its
IPv6 network using its production IPv6 address space, thus
facilitating migration towards native IPv6 deployment. The IPv6
deployment roadmap for connecting customers becomes:
- assisted tunneling infrastructure to early adopters,
- native IPv6 to customers where economically justified,
- native IPv6 to all customers.
Note that, as the addressing space used during the transition to
native remains the same the customer routing, filtering, accounting
[ISP, section 5.] stay the same, and there is no need to maintain any
kind of relay.
"Assisted tunneling" is used in this document to described a
transition mechanism where the parameters to configure a bi-
directional tunnel between an end-node (or leaf network) and a router
in the core of an ISP are negotiated through a tunnel set-up
protocol. Although this negotiation phase can be automated, this
remains different from transition mechanisms like 6to4 that is fully
automatic or teredo/isatap which, once the IPv4 address of an initial
server/router is configured do not involve any further negotiation
phase. In particular, the 'authenticated' mode defined in section 5
enable access control to the IPv6 network where the other transion
mechanism have to rely on out of band access control.
This document analyze the requirements for such a tunnel set-up
protocol. The v6ops WG scenario and evaluation documents for
deploying IPv6 within common network environments are used as input
to this document.
2. Applicability
Assisted tunneling is applicable in different IPv6 transition
scenarios. The focus of this document is to define the requirements
to apply this mechanism in the IPv4 ISP context making the following
assumptions:
- ISP is offering IPv6 connectivity to its customers initially
using controlled tunneling infrastructure [ISP, 5.1 Steps in
Transitioning Customer Connection Networks]
- The customer configuration may be diverse, and not necessarily
predictable by the ISP. The following cases must be supported:
- a single node,
- a leaf network,
- using a globally routable IPv4 address,
- behind a NAT,
- using dynamic IPv4 address (internally or externally to the
NAT)
There are actually two cases where the IPv4 address of the customer
tunnel end point can be dynamic, and both must be supported:
- The device used as tunnel end point is using a dynamic IPv4
address provided by the ISP.
- The device used as tunnel end point is located behind a customer
owned NAT box that is also acting as a local DHCP server. In that
case, the device IPv4 address may change after a reboot.
The scenario where the ISP is providing IPv6 connectivity to non-
customers is out of scope of this document.
Althought the main focus of this document is the ISP scenario,
assisted tunneling is applicable in all the other scenarios:
unmanaged, enterprise and 3GPP.
In unmanaged networks, assisted tunneling is applicable in the case A
(a gateway which does not provide IPv6 at all) [UNMANAGED, section 3]
and C (a dual-stack gateway connected to an IPv4-only ISP)
[UNMANAGED, section 5].
In 3GPP networks, assisted tunneling can be used in the context of
dual stack UE connecting to IPv6 nodes through a 3GPP network that
only supports IPv4 PDP contexts [3GPP, 3.1].
3. Requirements for Simplicity
The tunnel set-up protocol must be simple to implement and easy to
deploy. In particular, it should not depend on any complex, yet to be
designed, protocols or infrastructure pieces.
This protocol is a transition mechanism, thus does not need to be
perfect. As a matter of fact, making it perfect would be counter
productive, at it would first delay its definition, then make its
deployment more cumbersome and, last but not least, diminish the
incentives to deploy native IPv6.
4. Requirements for the non-authenticated Mode (initial tryout)
Assisted tunneling can be provided in two different modes, a simple
mode, unauthenticated, essentially aimed at tryout, and a more
complete mode, authenticated, aimed at production deployment. The
tunnel set-up protocol must support both modes, however ISP deploying
it may choose to only support one mode of operation.
Assisted tunneling in the non-authenticated mode is defined for
simple "plug & play" scenarios. In this mode, the tunnel
establishment is triggered through the execution of a simple program,
without any pre-configuration or pre-registration required from the
end-user.
The tunnel established is "anonymous", the end-user does not provide
any authentication to the server. An ISP using the protocol in this
mode may be offering a free service and doesn't wish to require any
form of registration. This free service can also be used to offer
trial IPv6 service limited to the ISP customers by relying on IPv4
access control.
4.1. Address Allocation
This mode is used to provide IPv6 connectivity to a single host.
Allocation of an IPv6 address (/128) to the end-node must be
supported in this mode. This IPv6 address is "transient" and may
change, but one may implement a mechanism to provide IPv6 address
stability in this mode (e.g. cookie mechanism).
See section 6.9 for DNS considerations.
4.2. Service Discovery
In order to offer "plug & play", the non-authenticated mode needs to
discover the address of the server that will provide the tunnel
connectivity. This discovery must be automatic within an ISP network.
4.3. NAT Traversal
Tunneling through IPv4 NAT must be supported. This should be detected
during the set-up phase (section 6.)
4.4 Security Threat Analysis
The un-authenticated mode relies on out of band authentication. It
essentially offer the IPv6 service to any of its IPv4 customers.
This may be regarded as a feature, but deployment of the service with
this mode enable must be done carefully. In particular, security
considerations must be taken into account.
If ingress filtering is not in place within the access network, a
number of DoS attack can happen:
- Customer A can impersonate someone else's IPv4 address during
the set-up phase and redirect a tunnel to that IP address. A then
can, for example, start a high bandwidth multimedia flow across
that tunnel and saturate its victim's uplink.
- Customer A can impersonate a large number of IPv4 addresses and
request tunnel of their behalf. That would quickly saturate the
ISP tunnel server infrastructure.
If ingress filtering is in place in the core ISP backbone but not in
the access network, the potential victims of the above problems will
be limited to the ISP's own customers.
If specific filtering is not in place in the ISP core network,
another kind of attack can happen. Customers from another ISP could
start using its tunneling infrastructure to get free IPv6
connectivity, transforming effectively the ISP into a IPv6 transit
provider.
In this mode, IPv4 return routability checks in the set-up phase of
the tunnels seems to be a way to avoid some of these problems at the
price of an extra round trip.
5. Requirements for the Authenticated Mode
Assisted tunneling in authenticated mode offers the features listed
in the non-authenticated mode (section 4), and some extra features
documented in this section.
A particular implementation may choose to only implement a subset of
those features, but the protocol must be able to negotiate them all.
The authenticated mode is most valuable in a provider network where
deployment of IPv6 is done in a more controlled manner. In such
networks, ease of debugging, traceability, filtering and so on are
important features.
5.1. Address and Prefix Delegation
The authenticated mode must support delegation of a single address or
a whole prefix or a combination of both. The length of the IPv6
prefix delegated must be configurable on the server. In this mode,
the protocol must be able to support servers willing to offer stable
IPv6 prefixes to the authenticated customers.
See section 6.9 for DNS considerations.
5.2. Authentication
A mechanism for easy user registration should be provided. A service
provider should be able to use its existing authentication database.
The authentication mechanism supported should be compatible with
standardized methods that are generally deployed. Hooks may be
provided to facilitate integration with the ISP management
infrastructure (e.g. RADIUS for AAA, billing).
In order to assure interoperability, at least one common
authentication method must be supported. Other authentication MAY be
supported and should be negotiated between the client and server
(e.g., SASL [2222]).
note: not clear what should be the mandatory authentication method.
Clear text password is out. Digest-MD5 [2831] seems like a good
choice.
As in sectoin 4.4, IPv4 return routability checks could help blocking
many DoS attack when IPv4 ingress filtering is not performed in the
access network.
5.3. NAT Traversal
Tunneling through IPv4 NAT must be supported. This should be detected
during the set-up phase (section 6.)
5.4. Accounting
The assisted tunneling should include tools for managing and
monitoring the provided service. Such information can be used to plan
service capacity (traffic load) or billing information.
Some useful accounting data are (not exhaustive list):
- Tunnel counters (traffic in/out)
- User utilization (tunnel uptime)
- System logging (authentication failures, resource exhaustion,
etc.)
The interface used to provide such information can be through SNMP or
an AAA protocol (e.g., RADIUS accounting).
6. General Requirements
6.1 Scalability
The tunnel set-up protocol must be scalable.
6.2 Service discovery requirements
The discovery part of the tunnel set-up protocol should be as
automatic as possible.
The discovery mechanism must be able to scale for large ISP who cover
different geographical areas and/or have a large number of customers.
Customers may very well try to use this tunnel set-up protocol even
if their ISP is not offering the service. In this case, without any
previous action taken by their ISP, the discovery part of the tunnel
set-up protocol must be able to abort immediately and display the
customers a message explaining that no service is available.
6.3 NAT Considerations
The assisted tunnel established by the protocol to be designed must
work with the existing infrastructure, in particular it must be
compatible with the various customer premise equipments available
today. This means that, in particular, the tunnels must be able to
traverse one or many NAT boxes of different kinds. There is no
requirement for any particular NAT traversal technology. However, as
NAT traversal usually requires an extra layer of encapsulation, the
tunnel set-up protocol must be able to detect automatically the
presence of one or more NAT boxes in the path.
6.4 Keep-alive
When a tunnel has to cross a NAT box, the mapping established by the
NAT must be preserved as long as the tunnel is in use. This is
usually achieved by sending keep alive messages across the tunnel.
Also, the same keep alive messages can enable the ISP tunnel end
point to perform garbage collection of its resources when tunnels are
not in use anymore. To enable those two functionalities, the tunnel
set-up protocol must include the transmission of keep-alive messages.
A client MAY choose not to send those messages (for example on ISDN
type links), but should then expect that the tunnel may be
disconnected at any time by the ISP and be prepared to restart the
set-up phase.
6.5 Latency in Set-up Phases
In certain type of networks, keeping tunnels active all the time is
not possible or simply too expensive. In those environments, the
protocol must be able to set-up tunnels on demand of the IPv6
applications requiring external connectivity.
The tunnel set-up protocol must then have a low enough latency to
enable quasi-instant configuration. Latency is usually a function of
the number of packet exchanges required, so minimizing this parameter
is important.
6.6 Security
The tunnel set-up protocol must not introduce any new vulnerability
to the network.
6.7 Traceability
In production environment, traceability is an important
consideration. The tunnel set-up protocol must be instrumentable to
enable the collection of usage data that can be used, for example,
for capacity planning.
6.8 Phase Out
This assisted tunneling mode is only a transition mechanism to enable
ISP to jump start IPv6 service without requiring an immediate global
upgrade of access networks and instead enabling a progressive roll
out. Once IPv6 is available natively in the access network
connecting a customer, there is no reason to keep using tunnels. So
the tunnel-setup protocol must have a provision to enable the ISP to
signal the user to use native IPv6 instead.
6.9 DNS considerations
It should be possible to have the server side of the protocol
automatically register the allocated IPv6 address in the DNS system
(AAAA and PTR records) using the ISP name space. Nothing specific is
required in the protocol to support this. The details can be
implementation specific.
If stable prefix delegation is done, it is expected that the DNS
delegation of the associated reverse DNS zone will be also stable and
thus can be performed out of band, so there is no requirement to
perform this delegation at the tunnel set-up time.
7. Compatibility with other Transition Mechanisms
7.1 TSP
The tunnel set-up protocol is not required to be compatible with TSP
or any particular implementation of the tunnel broker model [3053].
Although, a great deal of experience can be drawn from the operation
of tunnel brokers currently using the TSP protocol.
7.2 TEREDO
There is a large number of Teredo clients already deployed, the
tunnel set-up protocol should explore the avenue of providing a
compatibility mode with Teredo, at least in the 'simple' mode
described in section 4. However, it may turn out that supporting a
compatibility mode with Teredo either requires to change the Teredo
specifications and/or implement Teredo on the tunnel server side. In
that case, it might be simpler to say that the compatibility mode
should be manage on the client side instead of the server side, that
is leave it up to the client to use either one of them.
7.3 ISATAP
Similar considerations as Teredo, section 7.2, applies to Isatap.
However, as Isatap can not work accross NAT, it is of much less
interest in the framework of this document.
8. Security Considerations
The establishment of a tunnel can be compared to Mobile IP
technology, where traffic can be redirected to go from one place to
another one. So similar threats exists. In particular, when a
customer is asking for the set-up of a tunnel ending at IP address X,
the ISP should check:
- the customer is allowed to set-up this tunnel, i.e. he "owns"
the IPv6 prefix.
- the customer is allowed to terminate the tunnel where he said he
would, i.e. he "owns" the IPv4 tunnel endpoint.
The first check is simply an authentication issue. The second may be
more complex, but can be omitted if strict ingress filtering is in
place in the access network, i.e. the customer is effectively
prevented from sending packet with an IPv4 source address he does not
own.
See section 4.4 for specific security consideration in the non-
authenticated mode.
9. Author Addresses
Alain Durand
SUN Microsystems, Inc
17 Network circle UMPK17-202
Menlo Park, CA, 94025
USA
Mail: Alain.Durand@sun.com
Florant Parent
Hexago
2875 boul. Laurier, bureau 300
Sainte-Foy, QC G1V 2M2
Canada
Mail: Florent.Parent@hexago.com
10. Normative References
[2119] Bradner, S., "Key Words for Use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11. Non Normative References
[2831] Leach, P. and C. Newman, "Using Digest Authentication as a SASL
Mechanism", RFC 2831, May 2000.
[2222] Myers, J., "Simple Authentication and Security Layer (SASL)",
RFC2222, October 1997.
[3053] A. Durand, P. Fasano, I. Guardini, D. Lento.,
"IPv6 Tunnel Broker", January 2001.
[ISP] Lind, M., "Scenarios and Analysis for Introducing IPv6
into ISP Networks",
draft-ietf-v6ops-isp-scenarios-analysis-01 (work in
progress), February 2004.
[UNMANAGED] Huitema, C., "Evaluation of Transition Mechanisms for
Unmanaged Networks", draft-ietf-v6ops-unmaneval-01 (work
in progress), February 2004.
[3GPP] J. Wiljakka, "Analysis on IPv6 Transition in 3GPP Networks",
draft-ietf-v6ops-3gpp-analysis-09 (work in progress), March 2004.
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