Network Working Group S. Vinapamula
Internet-Draft Juniper Networks
Intended status: Best Current Practice M. Boucadair
Expires: October 25, 2015 France Telecom
April 23, 2015
Recommendations for Prefix Binding in the Softwire DS-Lite Context
draft-vinapamula-softwire-dslite-prefix-binding-04
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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. The Problem . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative references . . . . . . . . . . . . . . . . . . 6
7.2. Informative references . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
IPv6 deployment models assume IPv6 prefixes are delegated by Service
Providers to the connected CPEs (Customer Premise Equipments) or
hosts, which in their turn derive IPv6 addresses out of that prefix.
In the case of DS-Lite [RFC6333], that 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 external IPv6 address, for a
variety of reasons including 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 might be delivered to another B4 that now acquired that
address. This affects all mapping types, whether implicit (e.g., by
sending a TCP SYN) or 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 3).
Note that in some deployments, CPE renumbering may be required to
accommodate some privacy-related requirements to avoid the same
prefix be assigned to the same customer. It is out of scope of this
document to discuss such contexts.
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 IPv4 addresses assigned to hosts serviced by a B4 element are
overlapping across multiple CPEs, the IPv6 address of a B4 element
plays a key role in de-multiplexing connections, enforcing policies,
and in identifying associated resources assigned for each of the
connections in the Address Family Transition Router (AFTR,
[RFC6333]). For example, these resources maintain state of Endpoint-
Independent Mapping (EIM, Section 4.1 of [RFC4787]), Endpoint-
Independent Filtering (EIF, 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, there can be a change in the B4's IPv6 address for any
reason, e.g., because of a change in the CPE itself or may be because
of privacy extensions enabled in generating the IPv6 address. When
the B4's IPv6 address changes, the associated mappings created in the
AFTR are no more valid. This may result in creation of new set of
mappings in the AFTR.
Furthermore, a mis-behaving 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 mis-behaving users. Note that this DoS attack can
be achieved whatever port assignment policy configured to the AFTR
(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
resources usage (see REQ-4 of [RFC6888]). These policies are used
for dimensioning purposes and also to ensure that AFTR resources are
not exhausted. To that aim, a subscriber should be identified at the
AFTR by the delegated IPv6 prefix and not the derived B4's IPv6
address. Also, when there is a change of the B4's IPv6 address,
enforcing policies based on the B4's IPv6 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, which is not desirable.
When application servers are hosted behind a B4 element, and when
there is a change of the B4's IPv6 address which if results in a
change in the external IPv4 address and/or the external port number
at the AFTR side, these servers have to advertise about their change
(see Section 1.1 of [RFC7393]). Means to discover the change of B4's
IPv6 address, the external IPv4 address and/or the external port are
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therefore required. Latency issues are likely to be experienced
where an application server has to advertise its newly assigned
external IPv4 address and port, and the application clients have to
discover that newly assigned server and/or port and re-initiate
connections to the application server.
3. 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 softwires per-subscriber. The default value MUST be 1.
This policy aims to prevent a misbehaving subscriber to mount
several softwires to consume more resources on the AFTR side
(e.g., get more external ports if the quota was enforced on a
per-softwire basis, consume extra processing induced by a large
number of active softwires).
2. Resource contexts created at 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 through a
configured subscriber-mask. 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.
1. 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 a subscriber (or 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
to configure every subscriber's prefix.
Subscriber-mask must be configurable; the default value is
56.
2. For example, suppose 2001:db8:100:100::/56 is delegated to a
DS-Lite enabled CPE. Suppose also that 2001:db8:100:100::1
is the IPv6 address assigned to 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 derive the prefix
for this B4 element (that is 2001:db8:100:100::/56). Then,
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the AFTR enforces policies based on that derived prefix
(i.e., 2001:db8:100:100::/56), not on the exact source IPv6
address.
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
B4's IPv6 address. This operation ensures the 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 SHOULD be the last seen as B4's IPv6 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).
4. In the event of change of the CPE WAN's IPv6 prefix, unsolicited
PCP ANNOUNCE messages are to be sent by the B4 element to
internal hosts to update their mappings. This allows internal
PCP clients to update their mappings with the new B4's IPv6
address and trigger updates to rendez-vous 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. In case for any reason an IPv6 prefix has to be reassigned, it is
RECOMMENDED to reassign an IPv6 prefix to a distinct CPE only
when all the resources in use associated with that prefix are
cleared from the AFTR. Doing so avoids to redirect traffic,
destined to the previous prefix owner, to the new one.
4. Security Considerations
Security considerations related to DS-Lite are discussed in
[RFC6333].
Enforcing the recommendations in Section 3 defends against DoS
attacks that would result in varying the source IPv6 address to
exhaust AFTR resources.
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5. IANA Considerations
This document does not require any action from IANA.
6. Acknowledgements
G. Krishna, C. Jacquenet, I. Farrer, Y. Lee, and Q. Sun provided
useful comments. Many thanks to them.
7. References
7.1. Normative references
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6333] Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
Stack Lite Broadband Deployments Following IPv4
Exhaustion", RFC 6333, August 2011.
[RFC6887] Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
Selkirk, "Port Control Protocol (PCP)", RFC 6887, April
2013.
7.2. Informative references
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[RFC6888] Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A.,
and H. Ashida, "Common Requirements for Carrier-Grade NATs
(CGNs)", BCP 127, RFC 6888, April 2013.
[RFC6908] Lee, Y., Maglione, R., Williams, C., Jacquenet, C., and M.
Boucadair, "Deployment Considerations for Dual-Stack
Lite", RFC 6908, March 2013.
[RFC7393] Deng, X., Boucadair, M., Zhao, Q., Huang, J., and C. Zhou,
"Using the Port Control Protocol (PCP) to Update Dynamic
DNS", RFC 7393, November 2014.
Authors' Addresses
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Suresh Vinapamula
Juniper Networks
1194 North Mathilda Avenue
Sunnyvale, CA 94089
USA
Phone: +1 408 936 5441
EMail: sureshk@juniper.net
Mohamed Boucadair
France Telecom
Rennes 35000
France
EMail: mohamed.boucadair@orange.com
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