Recommendation for Prefix Binding in the Softwire DS-Lite Context

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Document Type Active Internet-Draft (individual)
Authors Suresh Vinapamula  , Mohamed Boucadair 
Last updated 2015-01-14 (latest revision 2014-10-27)
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Send notices to,, "Suresh Krishnan" <>
Network Working Group                                      S. Vinapamula
Internet-Draft                                          Juniper Networks
Intended status: Best Current Practice                      M. Boucadair
Expires: April 30, 2015                                   France Telecom
                                                        October 27, 2014

   Recommendation for Prefix Binding in the Softwire DS-Lite Context


   This document discusses issues induced by the change of the 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
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  The Problem . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     8.1.  Normative references  . . . . . . . . . . . . . . . . . .   5
     8.2.  Informative references  . . . . . . . . . . . . . . . . .   6

1.  Introduction

   IPv6 deployment models assumes 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], the Basic Bridging BroadBand (B4)
   element derives an IPv6 address for the softwire setup purposes.

   A B4 element might obtain a new external IPv6 address, for a variety
   of reasons including a reboot of the CPE, power outage, DHCP lease
   expiry, or other action undertaken by the Service Provider.  If this
   occurs, traffic forwarded to a B4's previous 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 PCP [RFC6887]).

   The main goal of this document is to propose recommendations to
   soften the impact of such renumbering issues.

   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].

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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3.  The Problem

   Since the network behind B4 can be overlapping across multiple CPEs,
   B4 address plays a key role in identifying associated resources
   assigned for each of the connections.  These resources maintain state
   of Endpoint-Independent Mapping (EIM), Endpoint-Independent Filtering
   (EIF), preserve external IPv4 address assigned in the AFTR, and PCP
   mappings and flows.

   However, there can be change in B4 address for any reason, may be
   because of change in CPE device or may be because of security
   extensions enabled in generating the IPv6 address.  When the address
   change, the associated mappings created in the AFTR are no more
   valid.  This may result in creation of new set of mappings.

   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 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 limit the usage of these resources on
   per subscriber basis for fairness resources usage.  To that aim , a
   subscriber is identified by the delegated IPv6 prefix and not the
   derived B4 address.  These policies are used for dimensioning
   purposes and also to ensure that AFTR resources are not exhausted.
   However when there is a change in B4 address, this policy 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.

   When services are hosted behind B4 element, and when there is a
   change in B4 address which if results in change in NAT address, these
   services have to advertise about their change, whenever there is a
   change of the B4 address.  Means to discover the change of B4 address
   and NAT address is therefore required.  Also, it doesn't address
   latency issues where a service has to advertise its newly assigned
   external IP address and port and the clients have to consume and re-
   initiate connections.

   PCP-specific failure scenarios are discussed in

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4.  Recommendations

   In order to mitigate the issues discussed in Section 3, the following
   recommendations are made:

   1.  A policy SHOULD be enforced at the AFTR level 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.

   2.  Resource contexts created at the AFTR level SHOULD be based on
       the delegated IPv6 prefix and not based on the B4 address.
       Delegated prefix may be derived from the B4 address through a
       configured subscriber-mask.  Administrators SHOULD configure per
       prefix limits of resource usage, instead of per tunnel limits.
       These resources include, number of flows, mappings including PCP,
       NAT pool resources, etc.

       1.  Subscriber-mask is an integer that indicates the length of
           significant bits to be applied on the source IPv6 address
           (internal side) to identify a subscriber.  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 prefix.  Subscriber-mask must be configurable; the
           default value is 56.

       2.  For example, suppose an IPv6 prefix 2001::/56 is delegated to
           a CPE.  Administrator should configure resource usage limits
           in AFTR based on the prefix 2001::/56 and not based on any B4
           address derived from the delegated prefix.  AFTR will derive
           the prefix from B4 address through configured subscriber-mask
           set to 56 by the administrator.

   3.  In the event a new IPv6 address is assigned to B4, the AFTR
       SHOULD migrate existing state to be bound to the new B4's IP
       address.  This ensures the traffic destined to the previous B4
       address will be redirected to the newer B4 address.  The
       destination address for tunneling return traffic SHOULD be the
       last seen as B4's address from the CPE.  Doing so avoids stale
       mappings and minimizes the risk of service disruption.

   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 IPv6 address
       and trigger updates to rendez-vous servers (e.g., dynamic DNS).

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   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 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.

5.  Security Considerations

   Security considerations related to DS-Lite are discussed in

   Enforcing the recommendations in Section 4 defends against DoS
   attacks that would result in varying the source IPv6 address to
   exhaust AFTR resources.

6.  IANA Considerations

   This document does not require any action from IANA.

7.  Acknowledgements

   G.  Krishna and C.  Jacquenet reviewed document and provided useful

   Thanks to I.  Farrer for the comments.

8.  References

8.1.  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

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8.2.  Informative references

              Boucadair, M. and R. Penno, "Analysis of Port Control
              Protocol (PCP) Failure Scenarios", draft-boucadair-pcp-
              failure-06 (work in progress), May 2013.

   [RFC6908]  Lee, Y., Maglione, R., Williams, C., Jacquenet, C., and M.
              Boucadair, "Deployment Considerations for Dual-Stack
              Lite", RFC 6908, March 2013.

Authors' Addresses

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

   Phone: +1 408 936 5441

   Mohamed Boucadair
   France Telecom
   Rennes  35000


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