Internet Engineering Task Force (IETF) I. Farrer

Internet Engineering Task Force (IETF) I. Farrer
Request for Comments: 8539 Deutsche Telekom AG
Updates: 7598 Q. Sun
Category: Standards Track Y. Cui
ISSN: 2070-1721 L. Sun
Tsinghua University
March 2019

Softwire Provisioning Using DHCPv4 over DHCPv6

Abstract

DHCPv4 over DHCPv6 (RFC 7341) is a mechanism for dynamically
configuring IPv4 for use as an over-the-top service in an IPv6-only
network. Softwires are an example of such a service. For DHCPv4
over DHCPv6 (DHCP 4o6) to function with some IPv4-over-IPv6 softwire
mechanisms and deployment scenarios (e.g., RFC 7596 or RFC 7597), the
operator needs to know the IPv6 address that the client will use as
the source of an IPv4-in-IPv6 softwire tunnel. This address, in
conjunction with the client's IPv4 address, and (in some deployments)
the Port Set ID are used to create a binding table entry in the
operator's softwire tunnel concentrator. This memo defines a DHCPv6
option to convey IPv6 parameters for establishing the softwire tunnel
and a DHCPv4 option (to be used only with DHCP 4o6) to communicate
the source tunnel IPv6 address between the DHCP 4o6 client and
server. It is designed to work in conjunction with the IPv4 address
allocation process.

"DHCPv6 Options for Configuration of Softwire Address and Port-Mapped
Clients" (RFC 7598) describes a deterministic DHCPv6-based mechanism
for provisioning softwires. This document updates RFC 7598, allowing
OPTION_S46_BR (90) to be enumerated in the DHCPv6 client's Option
Request Option (ORO) request and to appear directly within subsequent
messages sent by the DHCPv6 server.

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Status of This Memo

This is an Internet Standards Track document.

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). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8539.

Copyright Notice

Copyright (c) 2019 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
(https://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.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
4. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Updating RFC 7598 to Permit the Reuse of
OPTION_S46_BR (90) . . . . . . . . . . . . . . . . . . . 5
5. DHCP 4o6 IPv6/IPv4 Binding Message Flow . . . . . . . . . . . 6
6. DHCP Options . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. DHCPv6 Softwire Source Binding Prefix Hint Option . . . . 7
6.2. DHCP 4o6 Softwire Source Address Option . . . . . . . . . 8
7. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Client Initialization . . . . . . . . . . . . . . . . . . 9
7.2. Renewing or Rebinding the IPv4 Address Lease and
Softwire Source Address . . . . . . . . . . . . . . . . . 10
7.2.1. Changing the Bound IPv6 Softwire Source Address . . . 10
7.3. Releasing the IPv4 Address Lease and Softwire
Source Address . . . . . . . . . . . . . . . . . . . . . 11
7.4. OPTION_S46_BIND_IPV6_PREFIX Validation Behavior . . . . . 11
7.5. Client and Server Softwire Source Address Mismatch . . . 11
7.6. Use with Dynamic, Shared IPv4 Addresses . . . . . . . . . 12
8. Server Behavior . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Changing the Bound IPv6 Source Address . . . . . . . . . 12
8.2. Handling Conflicts between Clients' Bound IPv6 Source
Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
9. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9.1. Client Privacy Considerations . . . . . . . . . . . . . . 14
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
11.1. Normative References . . . . . . . . . . . . . . . . . . 16
11.2. Informative References . . . . . . . . . . . . . . . . . 17
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18

1. Introduction

Deterministic IPv4-over-IPv6 transition technologies require that
elements be preconfigured with binding rules for routing traffic to
clients. This places a constraint on the choice of address used as
the client's softwire source address: it must use a predetermined
prefix, which is usually configured on the home gateway device.
[RFC7598] describes a DHCPv6-based mechanism for provisioning such
deterministic softwires.

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A dynamic provisioning model, such as using DHCPv4 over DHCPv6 (DHCP
4o6) [RFC7341], allows much more flexibility in the location of the
IPv4-over-IPv6 softwire source address. In this model, the IPv6
address is dynamically communicated back to the service provider,
allowing the corresponding softwire configuration to be created in
the border relay (BR).

The DHCP 4o6 client and softwire client could be run on end devices
attached to a network segment using any routable IPv6 prefix
allocated to an end user, located anywhere within an arbitrary home
network topology. Dynamic allocation also helps to optimize IPv4
resource usage, because only clients that are actively renewing their
IPv4 lease hold on to the address.

This document describes a mechanism for dynamically provisioning
softwires created using DHCP 4o6, including provisioning the client
with the address of the softwire BR and informing the service
provider of a client's binding between the dynamically allocated IPv4
address and Port Set ID and the IPv6 address that the softwire
initiator will use for accessing IPv4-over-IPv6 services.

The mechanism operates alongside the DHCP 4o6 message flows to
communicate the binding information over the IPv6-only network. The
DHCP 4o6 server provides a single point in the network that holds the
current client binding information. The service provider can then
use this binding information to provision other functional elements,
such as the BR(s).

2. Applicability

The mechanism described in this document is only suitable for use for
provisioning softwire clients via DHCP 4o6. The options described
here are only applicable within the DHCP 4o6 message-exchange
process. Current softwire technologies suitable for extending to
incorporate DHCP 4o6 with dynamic IPv4 address leasing include
[RFC7597] and [RFC7596].

3. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

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4. Solution Overview

In order to provision a softwire, both IPv6 and IPv4 configurations
need to be passed to the client. To map this to the DHCP 4o6
configuration process, the IPv6 configuration is carried in DHCPv6
options [RFC8415], carried inside the DHCPv6 message DHCPV4-RESPONSE
(21) sent by the server. OPTION_S46_BR (90) is used to provision the
remote IPv6 address for the softwire BR (see Section 4.1).
OPTION_S46_BIND_IPV6_PREFIX (137) is optionally sent by the DHCP 4o6
server to indicate to the client a preferred IPv6 prefix for binding
the received IPv4 configuration and sourcing tunnel traffic. This
may be necessary if there are multiple IPv6 prefixes in use in the
customer network (e.g., Unique Local Addresses (ULAs)) or if the
specific IPv4-over-IPv6 transition mechanism requires the use of a
particular prefix for any reason.

IPv4 configuration is carried in DHCPv4 messages [RFC2131] (inside
the DHCP 4o6 option OPTION_DHCPV4_MSG (87)) using the mechanism
described in [RFC7341].

In order for the client to communicate the softwire source address, a
new DHCPv4 option OPTION_DHCP4O6_S46_SADDR (109) is defined in this
document. This is included in DHCPREQUEST messages sent by the
client and is stored by the server for the lifetime of the IPv4
address lease.

4.1. Updating RFC 7598 to Permit the Reuse of OPTION_S46_BR (90)

Section 4.2 of [RFC7598] defines option OPTION_S46_BR (90) for
communicating remote softwire BR IPv6 address(es) to a client, but it
mandates that the option can only be used when encapsulated within
one of the softwire container options: OPTION_S46_CONT_MAPE (94) or
OPTION_S46_CONT_LW (96). From Section 3 of [RFC7598]:

Softwire46 DHCPv6 clients that receive provisioning options that
are not encapsulated in container options MUST silently ignore
these options.

This document updates [RFC7598], removing this restriction for
OPTION_S46_BR (90), allowing it to be enumerated in the client's ORO
request and appear directly within subsequent messages sent by the
DHCPv6 server.

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5. DHCP 4o6 IPv6/IPv4 Binding Message Flow

Figure 1 shows the relevant extensions to the successful DHCP 4o6
IPv4 allocation client/server message flow for the softwire source
address function. The full process, including error handling, is
described in Section 7.

In each step, the DHCPv6 portion of the message and any relevant
option is shown above the arrow. The DHCP 4o6 content of the message
and its relevant options are below the arrow. All the DHCPv4
messages are encapsulated in DHCPV4-QUERY (20) or DHCPV4-RESPONSE
(21) messages. Where relevant, the necessary options and their
contents are shown.

DHCP 4o6 DHCP 4o6
Client Server
| |
| DHCPv6 - DHCPV4-QUERY message containing |
| OPTION_ORO (6) listing (90, 137) |
Step 1 |----------------------------------------------------->|
| DHCPv4 - DHCPDISCOVER message |
| |
| |
| DHCPv6 - DHCPV4-RESPONSE message containing |
| OPTION_S46_BR(90), OPTION_S46_BIND_IPV6_PREFIX(137) |
| (bind-ipv6-prefix with service provider's |
| preferred prefix) |
Step 2 |<-----------------------------------------------------|
| DHCPv4 - DHCPOFFER message |
| containing an available IPv4 address |
| |
| DHCPv6 - DHCPV4-QUERY message |
Step 3 |----------------------------------------------------->|
| DHCPv4 - DHCPREQUEST message containing the |
| requested IPv4 address and OPTION_DHCP4O6_S46_SADDR |
| (softwire-ipv6-src-address with client's bound |
| IPv6 softwire source address) |
| |
| |
| DHCPv6 - DHCPV4-RESPONSE message |
Step 4 |<-----------------------------------------------------|
| DHCPv4 - DHCPACK message containing |
| the leased IPv4 address and OPTION_DHCP4O6_S46_SADDR |
| (softwire-ipv6-src-address with client's bound |
| IPv6 softwire source address) |
| |

Figure 1: IPv6/IPv4 Binding Message Flow

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Step 1 The client constructs a DHCPv6 "DHCPV4-QUERY (20)" message.
This message contains two options: DHCPv6 OPTION_ORO (6) and
OPTION_DHCPV4_MSG (87). OPTION_ORO lists "90"
(OPTION_S46_BR) and "137" (OPTION_S46_BIND_IPV6_PREFIX).
OPTION_DHCPV4_MSG contains a DHCPv4 DHCPDISCOVER message.

Step 2 The server responds with a DHCPv6 "DHCPV4-RESPONSE (21)"
message. This message contains an OPTION_S46_BR (90)
containing the IPv6 address of the BR for the client's
softwire configuration. The message may also optionally
contain OPTION_S46_BIND_IPV6_PREFIX (137). OPTION_DHCPV4_MSG
contains a DHCPv4 DHCPOFFER message. The DHCPv4 message
contains an available IPv4 address.

Step 3 The client sends a DHCPv6 "DHCPV4-QUERY (20)" message
containing a DHCPv4 DHCPREQUEST message with the requested
IPv4 address and OPTION_DHCP4O6_S46_SADDR (109) with the IPv6
address that the client will use as its softwire source
address.

Step 4 The server sends a DHCPv6 "DHCPV4-RESPONSE (21)" message.
OPTION_DHCPV4_MSG contains a DHCPv4 DHCPACK message with the
allocated IPv4 address. OPTION_DHCP4O6_S46_SADDR with the
client's bound softwire source address is included.

6. DHCP Options

6.1. DHCPv6 Softwire Source Binding Prefix Hint Option

The format of the DHCPv6 source binding prefix hint option is as
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_S46_BIND_IPV6_PREFIX | option-length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|bindprefix6-len| |
+-+-+-+-+-+-+-+-+ bind-ipv6-prefix .
. (variable length) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: Format of OPTION_S46_BIND_IPV6_PREFIX

o option-code: OPTION_S46_BIND_IPV6_PREFIX (137)

o option-length: 1 + length of bind-ipv6-prefix, specified in bytes.

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o bindprefix6-len: 8-bit field expressing the bit mask length of the
IPv6 prefix specified in bind-ipv6-prefix. Valid values are 0 to
128.

o bind-ipv6-prefix: The IPv6 prefix indicating the preferred prefix
for the client to bind the received IPv4 configuration to. The
length is (bindprefix6-len + 7) / 8. The field is padded on the
right with zero bits up to the next octet boundary when
bind-ipv6-prefix is not evenly divisible by 8. These padding bits
are ignored by the receiver (see Section 7.4).

OPTION_S46_BIND_IPV6_PREFIX is a singleton. Servers MUST NOT send
more than one instance of the OPTION_S46_BIND_IPV6_PREFIX option.

6.2. DHCP 4o6 Softwire Source Address Option

The format of the DHCPv4 over DHCPv6 softwire source address option
is as follows:

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| option-code | option-length |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ softwire-ipv6-src-address +
. (128 bits) .
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

Figure 3: Format of OPTION_DHCP4O6_S46_SADDR

o option-code: OPTION_DHCP4O6_S46_SADDR (109)

o option-length: 16.

o softwire-ipv6-src-address: 16 bytes long; the IPv6 address that is
associated (either being requested for binding or currently bound)
with the client's IPv4 configuration.

Note: The function of OPTION_DHCP4O6_S46_SADDR may seem similar to
the DHCPv4 message's "chaddr" field or the Client Identifier (61)
option in that it provides a unique lower-layer address that the
server can use for identifying the client. However, as both of these
are required to remain constant throughout the address lease
lifetime, they cannot be used with the mechanism described in this
document. This is because the client may only be able to construct
the IPv6 address to use as the source address after it has received
the first DHCPV4-RESPONSE message from the server containing
OPTION_S46_BIND_IPV6_PREFIX.

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7. Client Behavior

A client requiring dynamic softwire configuration first enables DHCP
4o6 configuration using the method described in Section 5 of
[RFC7341]. If OPTION_DHCP4_O_DHCP6_SERVER is received in the
corresponding REPLY message, the client MAY continue with the
configuration process described below.

Before the dynamic softwire configuration process can commence, the
client MUST be configured with a suitable IPv6 prefix to be used as
the local softwire endpoint. This could be obtained using DHCPv6,
Router Advertisement (RA) / Prefix Information Option (PIO), or
another mechanism.

7.1. Client Initialization

When constructing the initial DHCP 4o6 DHCPDISCOVER message, the
client includes a DHCPv6 OPTION_ORO (6) within the options field of
the DHCP-QUERY message. OPTION_ORO contains the option codes for
OPTION_S46_BR (90) and OPTION_S46_BIND_IPV6_PREFIX (137).

On receipt of the DHCP 4o6 server's reply (a DHCPV4-RESPONSE
containing a DHCPOFFER message), the client checks the contents of
the DHCPv4-RESPONSE for the presence of a valid OPTION_S46_BR option.
If this option is not present, or does not contain at least one valid
IPv6 address for a BR, then the client MUST discard the message, as
without the address of the BR the client cannot configure the
softwire and so has no interface to request IPv4 configuration for.

The DHCPV4-RESPONSE message may also include
OPTION_S46_BIND_IPV6_PREFIX, which is used by the operator to
indicate a preferred prefix that the client should bind IPv4
configuration to. If received, the client first checks the option
according to Section 7.4. If valid, the client uses this prefix as
the "IPv6 binding prefix" and follows to the process described in
Section 5.1 of [RFC7596] in order to select an active IPv6 prefix to
construct the softwire. If no match is found, or the client doesn't
receive OPTION_S46_BIND_IPV6_PREFIX, the client MAY select any valid
IPv6 prefix (of a suitable scope) to use as the tunnel source.

Once the client has selected a suitable prefix, it MAY either use an
existing IPv6 address that is already configured on an interface or
create a new address specifically for use as the softwire source
address (e.g., using an Interface Identifier constructed as per
Section 6 of [RFC7597]). If a new address is being created, the
client MUST complete configuration of the new address, performing
duplicate address detection (if required) before proceeding.

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The client then constructs a DHCPV4-QUERY message containing a DHCPv4
DHCPREQUEST message. OPTION_DHCP4O6_S46_SADDR is included in the
options field of the DHCPREQUEST message with the IPv6 address of its
softwire source address in the softwire-ipv6-src-address field.

When the client receives a DHCPv4 DHCPACK message from the server, it
checks the IPv6 address in OPTION_DHCP4O6_S46_SADDR against its
active softwire source address. If they match, the allocation
process has concluded. If there is a discrepancy, then the process
described in Section 7.5 is followed.

If the client receives a DHCPv4 DHCPNAK message from the server, then
the configuration process has been unsuccessful. The client then
restarts the process from Step 1 of Figure 1.

7.2. Renewing or Rebinding the IPv4 Address Lease and Softwire Source
Address

Whenever the client attempts to extend the lease time of the IPv4
address, OPTION_DHCP4O6_S46_SADDR with the IPv6 address of its
softwire source address in the softwire-ipv6-src-address field MUST
be included in the DHCPREQUEST message.

7.2.1. Changing the Bound IPv6 Softwire Source Address

Across the lifetime of the leased IPv4 address, it is possible that
the client's IPv6 address will change, e.g., if there is an IPv6
renumbering event.

In this situation, the client MUST inform the server of the new
address. This is done by sending a DHCPREQUEST message containing
OPTION_DHCP4O6_S46_SADDR with the new IPv6 source address.

When the client receives a DHCPv4 DHCPACK message from the server, it
checks the IPv6 address in OPTION_DHCP4O6_S46_SADDR against its
active softwire source address. If they match, the allocation
process has concluded. If there is a discrepancy, then the process
described in Section 7.5 is followed.

If the client receives a DHCPv4 DHCPNAK message in response from the
server, then the change of the bound IPv6 softwire source address has
been unsuccessful. In this case, the client MUST stop using the new
IPv6 source address. The client then restarts the process from Step
1 of Figure 1.

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7.3. Releasing the IPv4 Address Lease and Softwire Source Address

When the client no longer requires the IPv4 resource, it sends a
DHCPv4 DHCPRELEASE message to the server. As the options field is
unused in this message type, OPTION_DHCP4O6_S46_SADDR is not
included.

7.4. OPTION_S46_BIND_IPV6_PREFIX Validation Behavior

On receipt of the OPTION_S46_BIND_IPV6_PREFIX option, the client
makes the following validation checks:

o The received bindprefix6-len value is not larger than 128.

o The number of bytes received in the bind-ipv6-prefix field is
consistent with the received bindprefix6-len value (calculated as
described in Section 6.1).

If either check fails, the receiver discards the invalid option and
proceeds to attempt configuration as if the option had not been
received.

The receiver MUST only use bits from the bind-ipv6-prefix field up to
the value specified in the bindprefix6-len when performing the
longest prefix match. bind-ipv6-prefix bits beyond this value MUST be
ignored.

7.5. Client and Server Softwire Source Address Mismatch

If the client receives a DHCPACK message with an
OPTION_DHCP4O6_S46_SADDR containing an IPv6 address that differs from
its active softwire source address, the client SHOULD wait for a
randomized time interval and then resend the DHCPREQUEST message with
the correct softwire source address. Section 4.1 of [RFC2131]
describes the retransmission backoff interval process.

The default minimum time for the client to attempt retransmission is
60 seconds. If, after this time has expired, the client has not
received a DHCPACK message with the correct bound IPv6 address,
client MAY send a DHCPRELEASE message and restart the process
described in Section 7. The retry interval should be configurable
and aligned with any server policy defining the minimum time interval
for client address updates as described in Section 8.1.

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7.6. Use with Dynamic, Shared IPv4 Addresses

[RFC7618] describes a mechanism for using DHCPv4 to distribute
dynamic, shared IPv4 addresses to clients. The mechanism described
in this document is compatible with IPv4 address sharing and can be
enabled by following the process described in Section 6 of [RFC7618].

8. Server Behavior

Beyond the normal DHCP 4o6 functionality defined in [RFC7341], the
server MUST also store the IPv6 softwire source address of the client
in the leasing address database, alongside the IPv4 address and
client identifier.

An OPTION_DHCP4O6_S46_SADDR containing the bound softwire source
address MUST be sent in every DHCPACK message sent by the server.

The binding entry between the client's IPv6 softwire source address
and the leased IPv4 address is valid as long as the IPv4 lease
remains valid.

8.1. Changing the Bound IPv6 Source Address

In the event that the server receives a DHCPREQUEST message for an
active IPv4 lease containing an OPTION_DHCP4O6_S46_SADDR with an IPv6
address that differs from the address that is currently stored, the
server updates the stored softwire source address with the new
address supplied by the client and sends a DHCPACK message containing
the updated softwire source address in OPTION_DHCP4O6_S46_SADDR.

The server MAY implement a policy enforcing a minimum time interval
between a client updating its softwire source IPv6 address. If a
client attempts to update the softwire source IPv6 address before the
minimum time has expired, the server can either silently drop the
client's message or send back a DHCPACK message containing the
existing IPv6 address binding in OPTION_DHCP4O6_S46_SADDR. If
implemented, the default minimum client source address update
interval is 60 seconds.

8.2. Handling Conflicts between Clients' Bound IPv6 Source Addresses

In order for traffic to be forwarded correctly, each customer edge's
(CE's) softwire IPv6 source address must be unique. To ensure this,
on receipt of every client DHCPREQUEST message containing
OPTION_DHCP4O6_S46_SADDR, the DHCP 4o6 server MUST check the received
IPv6 address against all existing CE source addresses stored for

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active client IPv4 leases. If there is a match for any active lease
other than the lease belonging to the client sending the DHCPREQUEST,
then the client's IPv6 source address MUST NOT be stored or updated.

Depending on where the client and server are in the address leasing
lifecycle, the DHCP 4o6 server then takes the following action:

o If the DHCP 4o6 does not have a current, active IPv4 address lease
for the client, then the DHCP address allocation process has not
been successful. The server returns a DHCPNAK message to the
client.

o If the DHCP 4o6 does have a current, active IPv4 address lease,
then the source address update process (see Section 8.1) has not
been successful. The DHCP 4o6 server can either silently drop the
client's message or return a DHCPACK message containing the
existing IPv6 address binding in OPTION_DHCP4O6_S46_SADDR.

9. Security Considerations

Security considerations that are applicable to [RFC7341] are also
applicable here.

A rogue client could attempt to use the mechanism described in
Section 7.2.1 to redirect IPv4 traffic intended for another client to
itself. This would be performed by sending a DHCPREQUEST message for
another client's active IPv4 lease containing the attacker's softwire
IPv6 address in OPTION_DHCP4O6_S46_SADDR.

For such an attack to be effective, the attacker would need to know
both the client identifier and the active IPv4 address lease
currently in use by another client. This could be attempted in three
ways:

1. One customer learning the active IPv4 address lease and client
identifier of another customer via snooping the DHCP4o6 message
flow between the client and server. The mechanism described in
this document is intended for use in a typical ISP network
topology with a dedicated Layer 2 access network per client,
meaning that snooping of another client's traffic is not
possible. If the access network is a shared medium, then
provisioning softwire clients using dynamic DHCP4o6 as described
here is NOT RECOMMENDED.

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RFC 8539 Softwire Provisioning with DHCP 4o6 March 2019

2. Learning the active IPv4 address lease and client identifier via
snooping the DHCP4o6 message flow between the client and server
in the aggregation or core ISP network. In this case, the
attacker requires a level of access to the ISP's infrastructure
that means they can already intercept or interfere with traffic
flows to the client.

3. An attacker attempting to brute-force guess the IPv4 lease
address and client identifier tuple. The risk of this can be
reduced by using a client identifier format that is not easily
guessable, e.g., by using a random-based client identifier (see
Section 3.5 of [RFC7844]).

An attacker could attempt to redirect existing flows to a client
unable to process the traffic. This type of attack can be prevented
by implementing network ingress filtering [BCP38] in conjunction with
the BR source address validation processes described in [RFC7596]
Section 5.2 and [RFC7597] Section 8.1.

A client may attempt to overload the server by sending multiple
source address update messages (see Section 7.2.1) in a short time
frame. This risk can be reduced by implementing a server policy
enforcing a minimum time interval between client address changes, as
described in Section 8.1.

9.1. Client Privacy Considerations

[RFC7844] describes anonymity profiles for DHCP clients. These
considerations and recommendations are also applicable to clients
implementing the mechanism described in this document. As DHCP 4o6
only uses DHCPv6 as a stateless transport for DHCPv4 messages, the
"Anonymity Profile for DHCPv4" described in Section 3 is most
relevant here.

In addition to the considerations given in [RFC7844], the mechanism
that the client uses for constructing the interface identifier for
its IPv6 softwire source address (see Section 7.1) could result in
the device being trackable across different networks and sessions,
e.g., if the client's softwire Interface Identifier (IID) is
immutable.

This can be mitigated by constructing the softwire source IPv6
address as per Section 6 of [RFC7597]. Here, the address's IID
contains only the allocated IPv4 address (and port set identifier if
[RFC7618] is being used). This means no additional client
information is exposed to the DHCP 4o6 server; it also means that the
IID will change as the leased IPv4 address changes (e.g., between
sessions when Section 3.5 of [RFC7844] is implemented).

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10. IANA Considerations

IANA has assigned the OPTION_S46_BIND_IPV6_PREFIX (137) option code
from the DHCPv6 "Option Codes" registry maintained at
<http://www.iana.org/assignments/dhcpv6-parameters> as follows:

Value: 137
Description: OPTION_S46_BIND_IPV6_PREFIX
Client ORO: Yes
Singleton Option: Yes
Reference: RFC 8539

IANA has assigned the OPTION_DHCP4O6_S46_SADDR (109) option code from
the "BOOTP Vendor Extensions and DHCP Options" registry maintained at
<http://www.iana.org/assignments/bootp-dhcp-parameters> as follows:

Tag: 109
Name: OPTION_DHCP4O6_S46_SADDR
Data Length: 16
Meaning: DHCPv4 over DHCPv6 Softwire Source Address Option
Reference: RFC 8539

IANA has updated the entry for DHCPv6 OPTION_S46_BR (90) in the
"Option Codes" registry maintained at
<https://www.iana.org/assignments/dhcpv6-parameters> as follows:

Old Entry:

Value: 90
Description: OPTION_S46_BR
Client ORO: No
Singleton Option: No
Reference: [RFC7598]

New Entry:

Value: 90
Description: OPTION_S46_BR
Client ORO: Yes
Singleton Option: No
Reference: [RFC7598], [RFC8539]

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11. References

11.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/info/rfc2131>.

[RFC7341] Sun, Q., Cui, Y., Siodelski, M., Krishnan, S., and I.
Farrer, "DHCPv4-over-DHCPv6 (DHCP 4o6) Transport",
RFC 7341, DOI 10.17487/RFC7341, August 2014,
<https://www.rfc-editor.org/info/rfc7341>.

[RFC7598] Mrugalski, T., Troan, O., Farrer, I., Perreault, S., Dec,
W., Bao, C., Yeh, L., and X. Deng, "DHCPv6 Options for
Configuration of Softwire Address and Port-Mapped
Clients", RFC 7598, DOI 10.17487/RFC7598, July 2015,
<https://www.rfc-editor.org/info/rfc7598>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
Richardson, M., Jiang, S., Lemon, T., and T. Winters,
"Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 8415, DOI 10.17487/RFC8415, November 2018,
<https://www.rfc-editor.org/info/rfc8415>.

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RFC 8539 Softwire Provisioning with DHCP 4o6 March 2019

11.2. Informative References

[BCP38] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000,
<https://www.rfc-editor.org/info/bcp38>.

[RFC7596] Cui, Y., Sun, Q., Boucadair, M., Tsou, T., Lee, Y., and I.
Farrer, "Lightweight 4over6: An Extension to the Dual-
Stack Lite Architecture", RFC 7596, DOI 10.17487/RFC7596,
July 2015, <https://www.rfc-editor.org/info/rfc7596>.

[RFC7597] Troan, O., Ed., Dec, W., Li, X., Bao, C., Matsushima, S.,
Murakami, T., and T. Taylor, Ed., "Mapping of Address and
Port with Encapsulation (MAP-E)", RFC 7597,
DOI 10.17487/RFC7597, July 2015,
<https://www.rfc-editor.org/info/rfc7597>.

[RFC7618] Cui, Y., Sun, Q., Farrer, I., Lee, Y., Sun, Q., and M.
Boucadair, "Dynamic Allocation of Shared IPv4 Addresses",
RFC 7618, DOI 10.17487/RFC7618, August 2015,
<https://www.rfc-editor.org/info/rfc7618>.

[RFC7844] Huitema, C., Mrugalski, T., and S. Krishnan, "Anonymity
Profiles for DHCP Clients", RFC 7844,
DOI 10.17487/RFC7844, May 2016,
<https://www.rfc-editor.org/info/rfc7844>.

Acknowledgements

The authors would like to thank Ted Lemon, Lishan Li, Tatuya Jinmei,
Jonas Gorski, and Razvan Becheriu for their contributions and
comments.

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RFC 8539 Softwire Provisioning with DHCP 4o6 March 2019

Authors' Addresses

Ian Farrer
Deutsche Telekom AG
Landgrabenweg 151
Bonn, NRW 53227
Germany

Email: [email protected]

Qi Sun
Tsinghua University
Beijing 100084
China

Phone: +86-10-6278-5822
Email: [email protected]

Yong Cui
Tsinghua University
Beijing 100084
China

Phone: +86-10-6260-3059
Email: [email protected]

Linhui Sun
Tsinghua University
Beijing 100084
China

Phone: +86-10-6278-5822
Email: [email protected]

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