Dynamic Host Configuration Working D. Hankins
Group ISC
Internet-Draft October 26, 2009
Intended status: Standards Track
Expires: April 29, 2010
Dynamic Host Configuration Protocol Option for Dual-Stack Lite
draft-dhankins-softwire-tunnel-option-04
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Abstract
This document describes how Dual-Stack Lite configuration (the
Softwire Concentrator (SC)'s address) can be obtained by a Softwire
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Initiator (SI) via DHCPv6.
Table of Contents
1. Requirements Language . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The Dual-Stack Lite DHCPv6 Option . . . . . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
6. Normative References . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Requirements Language
In this document, the key words "MAY", "MUST, "MUST NOT", "OPTIONAL",
"RECOMMENDED", "SHOULD", and "SHOULD NOT", are to be interpreted as
described in BCP 14, RFC 2119 [RFC2119].
2. Introduction
Dual-Stack Lite [draft-ietf-softwire-dual-stack-lite-01] is a method
to extend IPv4 access to an IPv6-only addressed host. One of its key
components is an IPv4-over-IPv6 tunnel, commonly referred to as a
Softwire, but a host will not know if the network it is attached to
offers Dual-Stack Lite support, and if it did would not know the
remote end of the tunnel to establish a connection.
These are two separate pieces of information; 1) Should I shut down
my dual-stack IPv4 side, and use the softwire exclusively for IPv4
access? 2) At what IPv6 address should I establish a softwire
connection?
These two questions can be answered with one DHCPv6 [RFC3315] option.
DISCUSSION: It can be argued that if you inform a client it should
perform Dual-Stack Lite, but fail to deliver an IPv6 tunnel endpoint,
then its IPv4 access is certainly broken. If you give the client an
IPv6 tunnel endpoint but fail to inform it that it must use Dual-
Stack Lite for IPv4 access, then again its access is likely broken,
or is operating in a degraded mode of service (if an operator offers
a Dual-Stack Lite method of access, there either isn't any native
IPv4 access, or the Dual-Stack Lite method works better than native
access - if a network had better native IPv4 access than Dual-Stack
Lite access, there would be no reason to extend the service). So the
presence of a tunnel address also indicates the operator's intent for
the client to use the Softwire.
3. The Dual-Stack Lite DHCPv6 Option
The Dual-Stack Lite DHCPv6 Option is simply a list of IPv6 addresses
in binary representation.
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The Dual-Stack Lite Option Format follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_DS_LITE (TBD) | length (Nx16) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| IPv6 Address 0 |
| |
| |
+---------------------------------------------------------------+
| |
| IPv6 Address 1 |
| |
| |
+---------------------------------------------------------------+
| ... |
+---------------------------------------------------------------+
| |
| IPv6 Address N |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The code for this option is TBD. The length MUST be a multiple of
16. The IPv6 Address field is an IPv6 address.
The DS Lite option MAY appear in the root scope of a DHCPv6 packet.
It MUST NOT appear inside any IA_NA, IA_TA, IA_PD, IAADDR, or
similar.
If configured with a value, DHCPv6 servers will include the DS Lite
option if it appears on the client's Option Request Option
(OPTION_ORO). RFC 3315 Section 17.2.2 [RFC3315] describes how a
DHCPv6 client and server negotiate configuration values using the
ORO.
A client that supports DS Lite MUST include OPTION_DS_LITE on its
OPTION_ORO. There is no reasonable expectation that a server will
reply with the DS Lite option if it has not been requested.
If the client receives a DS Lite Option, it MUST verify the option
length is precisely a multiple of 16 octets, and ignore the option
otherwise. Provided it is of valid length, the client SHOULD
terminate or withdraw any DHCPv4 [RFC2131] configuration on the same
interface. If DHCPv4 configuration has concluded, the client SHOULD
perform a DHCPRELEASE as it tears down its IPv4 configuration.
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The client SHOULD establish a Softwire tunnel to the first IPv6
address ("IPv6 Address 0"), and incrementally attempt the next
address in series ("IPv6 Address 1" etc) if present until a Softwire
can be established. The specifics of timeout intervals are not
mentioned.
If at any time the Softwire is forcibly disconnected, the client MUST
restart connection at the start of the list. Note carefully that
through normal DHCPv6 Renew processes, the option's contents may be
updated, and fresh versions of the configuration should be obtained
from the DHCPv6 client in a timely manner.
DISCUSSION: The author believes that as Softwires are a "routing
layer" solution, they are likely to be managed in networks using
routing layer tools - the application of anycasting, or simply the
use of "role addresses" that can be quickly re-routed from failed
servers and applied to surviving servers. This argues a
simplification for the client to be given only one IPv6 address, as
the routing layer can ensure that address is terminated in the most
useful place. However, upon further discussion it seems to the
author that it is also necessary to provide routing protocol
operators to employ more than one route; so that faults with one
route in the table are not devastating. For this purpose, (N)
addresses are made available.
4. Security Considerations
This document does not present any new security issues, but as with
all DHCPv6-derived configuration state, it is completely possible
that the configuration is being delivered by a third party (Man In
The Middle). As such, there is no basis to trust that the access the
DS-Lite softwire connection represents can be trusted, and it should
not therefore bypass any security mechanisms such as IP firewalls.
RFC 3315 [RFC3315] discusses DHCPv6 related security issues.
draft-ietf-softwire-dual-stack-lite-01
[draft-ietf-softwire-dual-stack-lite-01] discusses DS Lite related
security issues.
5. IANA Considerations
IANA is requested to allocate one DHCPv6 Option code, referencing
this document.
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6. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[draft-ietf-softwire-dual-stack-lite-01]
Durand, A., "Dual-stack lite broadband deployments post
IPv4 exhaustion", July 2009.
Author's Address
David W. Hankins
Internet Systems Consortium, Inc.
950 Charter Street
Redwood City, CA 94063
US
Phone: +1 650 423 1307
Email: David_Hankins@isc.org
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