Internet Engineering Task Force P. Savola
Internet-Draft CSC/FUNET
Expires: May 1, 2004 Nov 2003
A Simple IPv6-in-IPv4 Configured Tunnel Set-Up Procedure
draft-savola-v6ops-conftun-setup-00
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This memo describes a set of operational procedures and one
implementation mechanism to provide a very simple and straightforward
way to easily manage IPv6-over-IPv4 configured tunnels between an ISP
and a customer. The configured tunnels work even if the IPv4
addresses change dynamically, or are private addresses; the procedure
provides at least a /64 prefix per customer and requires no
administrative set-up. Support for NAT Traversal is currently out of
scope.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . 3
3. Overview of the Procedure . . . . . . . . . . . . . . . . . 3
4. Customer-side Procedures . . . . . . . . . . . . . . . . . . 4
4.1 Possible Prior Agreement with the ISP . . . . . . . . . . . 4
4.2 Learning and Configuring the Tunnel Endpoint . . . . . . . . 4
4.3 Tunnel Activation . . . . . . . . . . . . . . . . . . . . . 5
4.4 Providing Connectivity to Other Nodes . . . . . . . . . . . 5
5. ISP-side Procedures . . . . . . . . . . . . . . . . . . . . 5
5.1 Possible Prior Agreements with the Customers . . . . . . . . 6
5.2 Learning the Customers' Tunnel Endpoint Addresses . . . . . 6
5.3 Prefix Advertisement or Delegation . . . . . . . . . . . . . 7
5.4 Tunnel Activation and Maintenance . . . . . . . . . . . . . 7
5.5 Secure Operations for Tunnel Service . . . . . . . . . . . . 8
5.5.1 Sufficient Tunnel Service Provisioning . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . 9
Normative References . . . . . . . . . . . . . . . . . . . . 9
Informative References . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . 11
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1. Introduction
A need for a simple mechanism to set up IPv6-over-IPv4 configured
tunnels between a customer and the ISP seems to have been
demonstrated in 3GPP analysis [10] as well as Unmanaged [11] and ISP
analysis [12]. Most currently proposed mechanisms (like 6to4 [7] or
ISATAP [8]) appear to be unnecessarily complex or otherwise
problematic in these particular scenarios.
This memo documents a set of operational procedures which require no
additional protocol specification to provide a very simple and
suitably elegant solution to these problems.
The second section gives a brief problem statement which also
describes the applicability of the solution. The third section
explains the overview of the procedure. The fourth and the fifth
sections describe the customer- and ISP-side procedures in more
detail.
2. Problem Statement
There are ISPs which are willing to provide IPv6 connectivity to
their customers, but may not be able to do it natively due to a
number of reasons. Such ISPs want to find a method to help in
providing IPv6-over-IPv4 tunnels to the customers. The IPv4 address
of the customer may be either static or dynamic, and may be a private
address [6] as well, but not NAT'ed between the path from the
customer to the ISP. The ISP may want to offer the tunnel service
either requiring prior agreement with the user, or to every customer
who wishes to try it. The customer may have one or more nodes which
should obtain IPv6 connectivity. The solution should be as simple as
possible, requiring no new protocols or substantial modifications to
IPv6 or IPv4 implementations either at the ISP or customer side.
[XXX NOTE: a UDP encapsulation would be rather straightforward to add
if necessary for NAT traversal, might cause a problem with an IPv6
prefix size though.]
3. Overview of the Procedure
The procedure can be summarized as follows:
1. If the ISP requires prior agreement ("managed mode"), the
customer contacts the ISP off-band and registers as an IPv6 user.
2. The customer discovers (using one of a number of mechanisms) the
IPv4 tunnel end-point address of the ISP, and creates a
configured, protocol-41 tunnel [1] to the address, and sends a
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normal Neighbor Discovery [2] (ND) Route Solicitation (RS) or a
DHCPv6 [4] SOLICIT or prefix delegation request [5] message over
the tunnel.
3. The ISP's tunnel router sets up a configured tunnel towards the
customer's IPv4 address; the address may be obtained using a
number of mechanisms, or created ad-hoc ("ad-hoc mode") when
tunnel packets arrive. In the former case, the configured tunnel
interface is typically pre-configured prior to receiving any
packets from the customer.
4. The ISP's tunnel router sends a normal ND Route Advertisement
(RA) or a further DHCPv6 message over the tunnel to the customer;
the prefix advertised is obtained using one of a number of
mechanisms. The customer automatically configures the prefix and
the addresses and uses them normally.
No new protocols are needed. Both in the managed and ad-hoc modes,
the customer can learn the tunnel address off-band.
In the managed mode, the ISP has to know the IPv4 address assigned to
the customer, configure a new IPv6 tunnel interface for the customer,
and reserve the IPv6 prefix that will be assigned; these have to be
configured on the tunnel router using operator-specific management
techniques.
In the ad-hoc mode, on the other hand, the tunnel router has to
implement a simple mechanism to allocate a new configured tunnel for
tunnel packets received from different customers, and select an IPv6
prefix to be assigned to the customer.
4. Customer-side Procedures
4.1 Possible Prior Agreement with the ISP
The ISP may require prior agreement or notification before a customer
is allowed to use their tunnel service. In that case, the customer
must contact the ISP using off-band mechanisms. Even if not
required, special requirements (e.g., a static IPv6 prefix when IPv4
address is dynamic) may be easier to fulfill if the user has
contacted the ISP beforehand and the ISP has made arrangements.
4.2 Learning and Configuring the Tunnel Endpoint
To get started, the customer has to learn the IPv4 address of the
ISP's tunnel router somehow. Possibilities include, for example:
o Using off-band mechanisms, e.g., from the ISP's web page.
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o Using DNS to look up a service name by appending it to the DNS
search path (e.g. "tunnel-service.example.com").
o Using a (yet unspecified) DHCPv4 option.
o Using a pre-configured IPv4 anycast address, either in the private
address space, or a public, non-routable address.
o Using other, unspecified methods.
This memo does not (at least yet) take a stance on the selection of
the mechanism even though some are more problematic than others, but
it is assumed that the first or the second option should be enough
for everyone considering that the customer's own ISP is providing
IPv6 service.
Once the IPv4 address has been learned, it is configured as the
tunnel end-point for the configured IPv6-over-IPv4 tunnel. Note that
this configuration can even be done transparently to the user, with
very little or no configuration.
4.3 Tunnel Activation
Next, IPv6 is activated over the tunnel as normal; this could be done
either by a Neighbor Discovery RS, DHCPv6 Solicitation message,
DHCPv6 Prefix Delegation request message, or by simple manual
configuration (note: manual configuration does not work in the
"ad-hoc" operation, because there is no trigger to bring up the ISP's
interface).
The tunnel router responds to this query as normal by sending a Route
Advertisement or continuing with DHCP message exchanges.
4.4 Providing Connectivity to Other Nodes
If the customer has multiple nodes, they can each obtain their own
tunnel in the same manner. However, this is unoptimal especially if
such nodes have internal communications.
Instead, the customer may want to set up one node to as a Neighbor
Discovery proxy [9] for the /64 route advertisement received, or if a
less specific prefix (e.g., a /48) is being used, as a router for the
internal network(s). This does not need to be any more complicated
than just setting up the tunnel on one node.
5. ISP-side Procedures
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5.1 Possible Prior Agreements with the Customers
The ISP may operate in either or both "managed" and "ad-hoc" modes.
In only the managed mode, a prior agreement with the customer is
needed to allow the customer to use IPv6 using this procedure. In
only the ad-hoc mode, no agreements with the customers are needed. In
both modes, "basic service" can be offered in ad-hoc mode, but more
advanced services (e.g., prefix delegation or a static prefix) are
offered to those with a prior agreement.
ISPs which operate in managed mode must configure (e.g., manually or
using a script or configuration tool) the configured tunnels on the
tunnel router; also, they may want to create a link between the
stable customer identification and their IPv6 properties (e.g., a
prefix) especially if the IPv4 address is dynamic, to maintain the
stability of IPv6 properties even when the IPv4 address may change.
5.2 Learning the Customers' Tunnel Endpoint Addresses
The ISP must somehow obtain the tunnel endpoint address to be
configured for a configured tunnel. Every active customer has its
own configured tunnel interface on the tunnel router.
When operating in the managed mode, this could be done from e.g.
DHCPv4 leases, RADIUS or Diameter databases, other databases or some
other means. This information will be used to update the tunnel
end-point address on the configured tunnel interface when changing or
as appropriate; the updates can be done e.g. using management tools,
scripting, etc. -- because a change of IPv4 address must be reflected
without delay to the tunnel end-point address, this configuration
update should be immediately triggered by changes in the used
database or lease.
When operating in the ad-hoc mode, the tunnel server should create a
new configured tunnel interface for each IPv6-over-IPv4 tunnel with a
different IPv4 source address. The ISP should be aware of a
potential for a resource exhaustion if the number of customers rises
too high, but actual DoS attacks are not possible if the ISP has
secured its network as described in Section 5.5. Automatical
creation of configured tunnel interfaces requires only rather trivial
implementation [XXX: does this need elaboration?]. Performing
"garbage collection" on such tunnels, e.g. in a Least-Recently-Used
(LRU) manner may be called for if the number of tunnels rises too
high. However, this should only be done after sufficiently long
period has passed, as not to disturb the existing (but maybe dormant)
IPv6 connections over the tunnels.
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5.3 Prefix Advertisement or Delegation
Each customer should be provided with at least a /64 prefix; this is
both practical (because /64 is required by Stateless Address
Autoconfiguration [3]), and architecturally correct (providing the
possibility to connect more than one node without a NAT).
In the managed mode, the ISP may advertise a static or dynamic IPv6
/64 prefix using RAs, provide a prefix delegation, or something else
(e.g., manual configuration if the IPv4 address is static).
In the ad-hoc mode, the ISP must ensure that a sufficiently large
pool of /64 prefixes are available. The prefixes can be allocated
either in a sequential fashion and advertised in RA's, or
automatically calculated, with some assumptions, from the used IPv4
addresses. For example, if the ISP uses IPv4 network 10.0.0.0/8 for
its customers, it needs 24 bits to uniquely identify each customer --
this calls for assigning an IPv6 /40 prefix to be used for
advertising /64's; in this example, a customer with address
"10.1.2.3" might get advertised an IPv6 prefix "2001:db8:FF01:0203::/
64", where "01:0203" corresponds to the client address and
"2001:db8:FF::" the /40 allocated to the ad-hoc tunneling operations
by the ISP. Mapping the most interesting bits (for the ISP) of an
IPv4 address to the IPv6 prefix allows even large ISPs to easily give
each user an algorithmically derived IPv6 prefix.
5.4 Tunnel Activation and Maintenance
When the router receives e.g. ND RS, DHCPv6 SOLICIT or prefix
delegation request from the configured tunnel, it responds normally,
as on any other interface. (When in ad-hoc mode, setting up the
tunnel from the received IPv6-over-IPv4 packet may take a while, but
the processing continues when set up.)
The ISP should avoid sending periodic messages (e.g., unsolicited
route advertisements) to the tunnel, or decrease the interval used
for sending them: if the customer disconnects for some time, and
someone else gets the same address, it might be disturbing to the
new, potentially non-IPv6 aware customer to receive "weird" protocol
41 packets meant to the previous customer. The similar effect occurs
if someone in the Internet is trying to communicate with an IPv6
user, but the user has changed its address in the meantime, and
packets may go to someone else, but this is no different to the
situation with IPv4, except that the packets are not necessarily
recognized.
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5.5 Secure Operations for Tunnel Service
The ISP should perform IPv4 ingress filtering at its borders towards
peers and upstreams, by disallowing packets with the source addresses
belonging to its own site or its customers. In particular, the ISP
must block the tunnel router's address from being used as a source
address from the outside; blocking the use of the customer prefixes
would be preferred as well.
The ISP must perform IPv4 ingress filtering towards the customers, in
particular those that use the tunnel service, so that they will not
be able to forge the IPv4 source address of the packets. In
particular, they must not be able to spoof the address of the tunnel
router to the other customers.
Both of these are very simple operations especially in the minimal
case of blocking only the abuse of the tunnel router address.
Naturally, the ISP should perform IPv6 ingress filtering as well, but
that is orthogonal to the security of this procedure.
In addition, the ISP must ensure, especially if in ad-hoc mode, that
only a selected subset of source addresses is able to communicate
with the tunnel router's designated tunnel address. For example,
creating dynamic interfaces with packets from outside of the ISP's
network could easily be used in a resource exhaustion attack. In
addition, to curtail internal resource exhaustion attacks, it makes
very much sense to ingress filter all the customers which are allowed
to use the ad-hoc tunnel service. With these precautions, resources
may only be exhausted by a real resource starvation, not through an
attack; on the other hand, if the ISP does not bother to add such
checks, it only harms itself for being susceptible to various forms
of attacks!
5.5.1 Sufficient Tunnel Service Provisioning
The ISP must naturally ensure that the tunnel router is capable to
handle the amount of users and the traffic that goes through it. It
should also be noted that all the traffic between the users of the
ISP go through the same router; "shortcuts" routes are not deemed
necessary. The increases in the latency are not significant as the
tunnel router is deployed close to the IPv4 access router (or even
co-located with it) topology-wise.
Typically, these are not believed to be a problem. If the number of
users or the amount of traffic generated increases, starting
deploying native IPv6 access instead eliminates the problem.
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6. Acknowledgements
This procedure was inspired by a need to severely simplify ISATAP
[8].
Credit available for anyone inventing a good name + acronym for this
procedure! :-)
7. Security Considerations
The requirements for reasonably secure operations within an ISP are
described in Section 5.5; with these in place, it is difficult to
imagine a case where stronger mechanisms such as IPsec for
IPv6-over-IPv4 tunnels would be needed.
A particular case occurs when an IPv4 address of the user changes,
and the user's IPv6 prefix changes as well; this may be allocated to
a different IPv6 user. However, this is no different than IPv4
address re-use threats. [XXX: can be considered more if really
needed.]
When the ISP operates in the ad-hoc mode, and there is an event where
all the IPv4 addresses change simultaneously, there may be a large
number of simultaneous updates to update the tunnel point addresses
in the tunnel router. This situation should be taken into
consideration e.g. if renumbering.
Normative References
[1] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for
IPv6 Hosts and Routers", draft-ietf-v6ops-mech-v2-01 (work in
progress), October 2003.
[2] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for
IP Version 6 (IPv6)", RFC 2461, December 1998.
[3] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[4] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M.
Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 3315, July 2003.
[5] Troan, O. and R. Droms, "IPv6 Prefix Options for DHCPv6",
draft-ietf-dhc-dhcpv6-opt-prefix-delegation-05 (work in
progress), October 2003.
Informative References
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[6] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G. and E.
Lear, "Address Allocation for Private Internets", BCP 5, RFC
1918, February 1996.
[7] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via
IPv4 Clouds", RFC 3056, February 2001.
[8] Templin, F., Gleeson, T., Talwar, M. and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)",
draft-ietf-ngtrans-isatap-16 (work in progress), October 2003.
[9] Thaler, D. and M. Talwar, "Bridge-like Neighbor Discovery
Proxies (ND Proxy)", draft-thaler-ipv6-ndproxy-01 (work in
progress), October 2003.
[10] Wiljakka, J., "Analysis on IPv6 Transition in 3GPP Networks",
draft-ietf-v6ops-3gpp-analysis-07 (work in progress), October
2003.
[11] Huitema, C., "Evaluation of Transition Mechanisms for Unmanaged
Networks", draft-ietf-v6ops-unmaneval-00 (work in progress),
June 2003.
[12] Ksinant, V., "Analysis of Transition Mechanisms for Introducing
IPv6 into ISP Networks", draft-ksinant-v6ops-isp-analysis-00
(work in progress), October 2003.
Author's Address
Pekka Savola
CSC/FUNET
Espoo
Finland
EMail: psavola@funet.fi
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