Independent Submission S. Vinapamula

Independent Submission S. Vinapamula
Request for Comments: 7785 Juniper Networks
Category: Informational M. Boucadair
ISSN: 2070-1721 Orange
February 2016

Recommendations for Prefix Binding
in the Context of Softwire Dual-Stack Lite

Abstract

This document discusses issues induced by the change of the Dual-
Stack Lite (DS-Lite) Basic Bridging BroadBand (B4) IPv6 address and
sketches a set of recommendations to solve those issues.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7785.

Copyright Notice

Copyright (c) 2016 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
(http://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.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. The Problem . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Introducing Subscriber-Mask . . . . . . . . . . . . . . . . . 4
4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9

1. Introduction

IPv6 deployment models assume IPv6 prefixes are delegated by Service
Providers to the connected CPEs (Customer Premises Equipment) or
hosts, which in turn derive IPv6 addresses from that prefix. In the
case of Dual-Stack Lite (DS-Lite) [RFC6333], which is an IPv4 service
continuity mechanism over an IPv6 network, the Basic Bridging
BroadBand (B4) element derives an IPv6 address for the IPv4-in-IPv6
softwire setup purposes.

The B4 element might obtain a new IPv6 address for a variety of
reasons that include (but are not limited to) a reboot of the CPE,
power outage, DHCPv6 lease expiry, or other actions undertaken by the
Service Provider. If this occurs, traffic forwarded to a B4's
previous IPv6 address may never reach its destination or may be
delivered to another B4 that now uses the address formerly assigned
to the original B4. This situation affects all mapping types, both
implicit (e.g., by sending a TCP SYN) and explicit (e.g., using Port
Control Protocol (PCP) [RFC6887]). The problem is further elaborated
in Section 2.

This document proposes recommendations to soften the impact of such
renumbering issues (Section 4).

This document complements [RFC6908].

1.1. Requirements Language

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].

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2. The Problem

Since private IPv4 addresses assigned to hosts serviced by a B4
element overlap across multiple CPEs, the IPv6 address of a B4
element plays a key role in demultiplexing connections, enforcing
policies, and in identifying associated resources assigned for each
of the connections maintained by the Address Family Transition Router
(AFTR) [RFC6333]. For example, these resources maintain state of
Endpoint-Independent Mapping (EIM) as defined in Section 4.1 of
[RFC4787], Endpoint-Independent Filtering (EIF) as defined in
Section 5 of [RFC4787], preserve the external IPv4 address assigned
in the AFTR (i.e., "IP address pooling" behavior as defined in
Section 4.1 of [RFC4787]), PCP mappings, etc.

However, the IPv6 address used by the B4 element may change for some
reason, e.g., because of a change in the CPE itself or because of
privacy extensions enabled for generating the IPv6 address (e.g.,
[RFC7217] or [RFC4941]). Whenever the B4's IPv6 address changes, the
associated mappings created in the AFTR are no longer valid. This
may result in the creation of a new set of mappings in the AFTR.

Furthermore, a misbehaving user may be tempted to change the B4's
IPv6 address in order to "grab" more ports and resources at the AFTR
side. This behavior can be seen as a potential denial-of-service
(DoS) attack from misbehaving users. Note that this DoS attack can
be achieved whatever the port assignment policy enforced by the AFTR
may be (individual ports, port sets, randomized port bulks, etc.).

Service Providers may want to enforce policies in order to limit the
usage of the AFTR resources on a per-subscriber basis for fairness of
resource usage (see REQ-4 of [RFC6888]). These policies are used for
dimensioning purposes and also to ensure that AFTR resources are not
exhausted. If the derived B4's IPv6 address can change, resource
tracking using that address will give incomplete results. Also,
whenever the B4's IPv6 address changes, enforcing policies based on
that address doesn't resolve stale mappings hanging around in the
system, consuming not only system resources, but also reducing the
available quota of resources per subscriber. Clearing those mappings
can be envisaged, but that will cause a lot of churn in the AFTR and
could be disruptive to existing connections; this is not desirable.
More concretely, if stale mappings have not been migrated to the new
B4's IPv6 address so that packets can be forwarded to the appropriate
B4, all incoming packets that are associated with those mappings will
be rejected by the AFTR. Such behavior is not desirable because it's
detrimental to quality of experience.

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When application servers are hosted behind a B4 element, and when
there is a change of the B4's IPv6 address that results in a change
of the external IPv4 address and/or the external port number at the
AFTR side, these servers have to advertise their change (see
Section 1.1 of [RFC7393]). Some means to discover the change of B4's
IPv6 address, the external IPv4 address, and/or the external port are
therefore required. Latency issues are likely to be experienced when
an application server has to advertise its newly assigned external
IPv4 address and port, and the application clients have to discover
that newly assigned address and/or port and re-initiate connections
with the application server.

A solution to these problems is to enforce policies based on the IPv6
prefix assigned to subscribers that have DS-Lite service instead of
based on the B4's IPv6 address. Section 3 introduces the subscriber-
mask that is meant to derive the IPv6 prefix assigned to a
subscriber's CPE from the source IPv6 address of a packet received
from a B4 element.

3. Introducing Subscriber-Mask

The subscriber-mask is defined as an integer that indicates the
length of significant bits to be applied on the source IPv6 address
(internal side) to identify unambiguously a CPE.

Subscriber-mask is an AFTR system-wide configuration parameter that
is used to enforce generic per-subscriber policies. Applying these
generic policies does not require configuring every subscriber's
prefix.

Subscriber-mask must be configurable; the default value is 56. The
default value is motivated by current practices to assign IPv6 prefix
lengths of /56 to end-sites (e.g., [RIPE], [LACNIC]).

Example: suppose the 2001:db8:100:100::/56 prefix is assigned to a
CPE that is DS-Lite enabled. Suppose also that the
2001:db8:100:100::1 address is the IPv6 address used by the B4
element that resides in that CPE. When the AFTR receives a packet
from this B4 element (i.e., the source address of the IPv4-in-IPv6
packet is 2001:db8:100:100::1), the AFTR applies the subscriber-mask
(e.g., 56) on the source IPv6 address to compute the associated
prefix for this B4 element (that is, 2001:db8:100:100::/56). Then,
the AFTR enforces policies based on that prefix
(2001:db8:100:100::/56), not on the exact source IPv6 address.

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4. Recommendations

In order to mitigate the issues discussed in Section 2, the following
recommendations are made:

1. A policy SHOULD be enforced at the AFTR to limit the number of
active DS-Lite softwires per subscriber. The default value MUST
be 1.

This policy aims to prevent a misbehaving subscriber from
mounting several DS-Lite softwires that would consume
additional AFTR resources (e.g., get more external ports if
the quota were enforced on a per-softwire basis, consume extra
processing due to a large number of active softwires).

2. Resource contexts created and maintained by the AFTR SHOULD be
based on the delegated IPv6 prefix instead of the B4's IPv6
address. The AFTR derives the delegated prefix from the B4's
IPv6 address by means of a configured subscriber-mask
(Section 3). Administrators SHOULD configure per-prefix limits
of resource usage, instead of per-tunnel limits. These resources
include the maximum number of active flows, the maximum number of
PCP-created mappings, NAT pool resources, etc.

3. In the event a new IPv6 address is assigned to the B4 element,
the AFTR SHOULD migrate existing state to be bound to the new
IPv6 address. This operation ensures that traffic destined to
the previous B4's IPv6 address will be redirected to the newer
B4's IPv6 address. The destination IPv6 address for tunneling
return traffic from the AFTR SHOULD be the last seen as the B4's
IPv6 source address from the CPE.

This recommendation avoids stale mappings at the AFTR and
minimizes the risk of service disruption for subscribers.

The AFTR uses the subscriber-mask to determine whether two
IPv6 addresses belong to the same CPE (e.g., if the
subscriber-mask is set to 56, the AFTR concludes that
2001:db8:100:100::1 and 2001:db8:100:100::2 belong to the same
CPE assigned with 2001:db8:100:100::/56).

As discussed in Section 5, changing the source B4's IPv6
address may be used as an attack vector. Packets with a new
B4's IPv6 address from the same prefix SHOULD be rate-limited.
It is RECOMMENDED to set this rate limit to 30 minutes; other
values can be set on a per-deployment basis.

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One side effect of migrating mapping state is that a server
deployed behind an AFTR does not need to update its DNS
records (if any) by means of dynamic DNS, for example. If a
dedicated mapping is instantiated, migrating the state during
its validity lifetime will ensure that the same external IP
address and port are assigned to that server.

4. In the event of change of the CPE WAN's IPv6 prefix, unsolicited
PCP ANNOUNCE messages SHOULD be sent by the B4 element to
internal hosts connected to the PCP-capable CPE so that they
update their mappings accordingly.

This allows internal PCP clients to update their mappings with
the new B4's IPv6 address and to trigger updates to rendezvous
servers (e.g., dynamic DNS). A PCP-based dynamic DNS solution
is specified in [RFC7393].

5. When a new prefix is assigned to the CPE, stale mappings may
exist in the AFTR. This will consume both implicit and explicit
resources. In order to avoid such issues, stable IPv6 prefix
assignment is RECOMMENDED.

6. If for any reason an IPv6 prefix has to be reassigned, it is
RECOMMENDED to reassign an IPv6 prefix (that was previously
assigned to a given CPE) to another CPE only when all the
resources in use associated with that prefix are cleared from the
AFTR. Doing so avoids redirecting traffic, destined to the
previous prefix owner, to the new one.

5. Security Considerations

Security considerations related to DS-Lite are discussed in
[RFC6333].

Enforcing the recommendations documented in Section 4 together with
rate-limiting softwires with new source IPv6 addresses from the same
prefix defend against DoS attacks that would result in varying the
B4's IPv6 address to exhaust AFTR resources. A misbehaving CPE can
be blacklisted by enforcing appropriate policies based on the prefix
derived from the subscriber-mask.

6. Privacy Considerations

A CPE connected to a DS-Lite network is identified by a set of
information that is specific to each network domain (e.g., service
credentials, device identifiers, etc.). This document does not make
any assumption nor introduce new requirements on how such
identification is implemented network-wide.

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This document adheres to Sections 6 and 8 of [RFC6333] for handling
IPv4-in-IPv6 packets and IPv4 translation operations. In particular,
this document does not leak extra information in packets exiting a
DS-Lite network domain.

The recommendations in Section 4 (item 6, in particular) ensure that
the traffic is forwarded to a legitimate CPE. If those
recommendations are not implemented, privacy concerns may arise. For
example, if an IPv6 prefix is reassigned while mapping entries
associated with that prefix are still active in the AFTR, sensitive
data that belong to a previous prefix owner may be disclosed to the
new prefix owner.

These recommendations do not interfere with privacy extensions for
generating IPv6 addresses (e.g., [RFC7217] or [RFC4941]). These
recommendations allow a CPE to generate new IPv6 addresses with
privacy extensions without experiencing DS-Lite service degradation.
Even if activating privacy extensions makes it more difficult to
track a CPE over time when compared to using a permanent Interface
Identifier, tracking a CPE is still possible based on the first 64
bits of the IPv6 address. This is even exacerbated for deployments
relying on stable IPv6 prefixes.

This document does not nullify the privacy effects that may motivate
the use of non-stable IPv6 prefixes. Particularly, the subscriber-
mask does not enable identifying a CPE across renumbering (even
within a DS-Lite network domain). This document mitigates some of
the undesired effects of reassigning an IPv6 prefix to another CPE
(e.g., update a rendezvous service, clear stale mappings).

7. References

7.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC6333] Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
Stack Lite Broadband Deployments Following IPv4
Exhaustion", RFC 6333, DOI 10.17487/RFC6333, August 2011,
<http://www.rfc-editor.org/info/rfc6333>.

[RFC6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
DOI 10.17487/RFC6887, April 2013,
<http://www.rfc-editor.org/info/rfc6887>.

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7.2. Informative References

[LACNIC] LACNIC, "IPv6 Address Allocation and Assignment Policies",
December 2015,
<http://www.lacnic.net/en/web/lacnic/manual-4>.

[RFC4787] Audet, F., Ed. and C. Jennings, "Network Address
Translation (NAT) Behavioral Requirements for Unicast
UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
2007, <http://www.rfc-editor.org/info/rfc4787>.

[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
<http://www.rfc-editor.org/info/rfc4941>.

[RFC6888] Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa,
A., and H. Ashida, "Common Requirements for Carrier-Grade
NATs (CGNs)", BCP 127, RFC 6888, DOI 10.17487/RFC6888,
April 2013, <http://www.rfc-editor.org/info/rfc6888>.

[RFC6908] Lee, Y., Maglione, R., Williams, C., Jacquenet, C., and M.
Boucadair, "Deployment Considerations for Dual-Stack
Lite", RFC 6908, DOI 10.17487/RFC6908, March 2013,
<http://www.rfc-editor.org/info/rfc6908>.

[RFC7217] Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC7217, April 2014,
<http://www.rfc-editor.org/info/rfc7217>.

[RFC7393] Deng, X., Boucadair, M., Zhao, Q., Huang, J., and C. Zhou,
"Using the Port Control Protocol (PCP) to Update Dynamic
DNS", RFC 7393, DOI 10.17487/RFC7393, November 2014,
<http://www.rfc-editor.org/info/rfc7393>.

[RIPE] RIPE, "IPv6 Address Allocation and Assignment Policy",
August 2015, <https://www.ripe.net/publications/docs/
ripe-650>.

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Acknowledgments

G. Krishna, C. Jacquenet, I. Farrer, Y. Lee, Q. Sun, R. Weber,
T. Taylor, D. Harkins, D. Gillmor, S. Sivakumar, A. Cooper, and
B. Campbell provided useful comments. Many thanks to them.

Authors' Addresses

Suresh Vinapamula
Juniper Networks
1194 North Mathilda Avenue
Sunnyvale, CA 94089
United States

Phone: +1 408 936 5441
Email: [email protected]

Mohamed Boucadair
Orange
Rennes 35000
France

Email: [email protected]

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