Internet Engineering Task Force (IETF) F. Gont
Request for Comments: 8978 SI6 Networks
Category: Informational J. Žorž
ISSN: 2070-1721 6connect
R. Patterson
Sky UK
February 2021
Reaction of IPv6 Stateless Address Autoconfiguration (SLAAC) to Flash-
Renumbering Events
Abstract
In scenarios where network configuration information related to IPv6
prefixes becomes invalid without any explicit and reliable signaling
of that condition (such as when a Customer Edge router crashes and
reboots without knowledge of the previously employed prefixes), hosts
on the local network may continue using stale prefixes for an
unacceptably long time (on the order of several days), thus resulting
in connectivity problems. This document describes this issue and
discusses operational workarounds that may help to improve network
robustness. Additionally, it highlights areas where further work may
be needed.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see 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/rfc8978.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction
2. Analysis of the Problem
2.1. Use of Dynamic Prefixes
2.2. Default PIO Lifetime Values in IPv6 Stateless Address
Autoconfiguration (SLAAC)
2.3. Recovering from Stale Network Configuration Information
2.4. Lack of Explicit Signaling about Stale Information
2.5. Interaction between DHCPv6-PD and SLAAC
3. Operational Mitigations
3.1. Stable Prefixes
3.2. SLAAC Parameter Tweaking
4. Future Work
5. IANA Considerations
6. Security Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgments
Authors' Addresses
1. Introduction
IPv6 Stateless Address Autoconfiguration (SLAAC) [RFC4862] conveys
information about prefixes to be employed for address configuration
via Prefix Information Options (PIOs) sent in Router Advertisement
(RA) messages. IPv6 largely assumes prefix stability, with network
renumbering only taking place in a planned manner: old/stale old prefixes
being are
deprecated (and eventually invalidated) via reduced prefix lifetimes
and new prefixes are introduced (with longer lifetimes) at the same
time. However, there are several scenarios that may lead to the so-called so-
called "flash-renumbering" events, where a prefix employed by a
network suddenly becomes invalid and replaced by a new prefix. In
some of these scenarios, the local router producing the network
renumbering event may try to deprecate (and eventually invalidate)
the currently employed prefix (by explicitly signaling the network
about the renumbering event), whereas in other scenarios, it may be
unable to do so.
In scenarios where network configuration information related to IPv6
prefixes becomes invalid without any explicit and reliable signaling
of that condition, hosts on the local network may continue using
stale prefixes for an unacceptably long period of time, thus
resulting in connectivity problems.
Scenarios where this problem may arise include, but are not limited
to, the following:
* The most common IPv6 deployment scenario for residential or small
office networks, where a Customer Edge (CE) router employs DHCPv6
Prefix Delegation (DHCPv6-PD) [RFC8415] to request a prefix from
an Internet Service Provider (ISP), and a sub-prefix of the leased
prefix is advertised on the LAN side of the CE router via
Stateless Address Autoconfiguration (SLAAC) [RFC4862]. In
scenarios where the CE router crashes and reboots, the CE router
may obtain (via DHCPv6-PD) a different prefix from the one
previously leased and therefore advertise (via SLAAC) a new sub-
prefix on the LAN side. Hosts will typically configure addresses
for the new sub-prefix but will also normally retain and may
actively employ the addresses configured for the previously
advertised sub-prefix, since their associated Preferred Lifetime
and Valid Lifetime allow them to do so.
* A router (e.g., Customer Edge router) advertises autoconfiguration
prefixes (corresponding to prefixes learned via DHCPv6-PD) with
constant PIO lifetimes that are not synchronized with the
DHCPv6-PD lease time (even though Section 6.3 of [RFC8415]
requires such synchronization). While this behavior violates the
aforementioned requirement from [RFC8415], it is not an unusual
behavior. For example, this is particularly true for
implementations in which DHCPv6-PD is implemented in a different
software module than SLAAC.
* A switch-port that a host is connected to is moved to another
subnet (VLAN) as a result of manual switch-port reconfiguration or
802.1x reauthentication. There has been evidence that some 802.1x
supplicants do not reset network settings after successful 802.1x
authentication. If a host fails 802.1x authentication for some
reason, it may be placed in a "quarantine" VLAN; if successfully
authenticated later on, it the host may end up having IPv6 addresses
from both the old ("quarantine") and new VLANs.
* During a planned network renumbering event, a router is configured
to send an RA including a Prefix Information Option (PIO) for the
"old" prefix with the Preferred Lifetime set to zero and the Valid
Lifetime set to a small value, as well as a PIO for the new prefix
with default lifetimes. However, due to unsolicited RAs being
sent to a multicast destination address, and multicast being
rather unreliable on busy Wi-Fi networks, the RA might not be
received by local hosts.
* An automated device config management system performs periodic
config pushes to network devices. In these scenarios, network
devices may simply immediately forget their previous
configuration, rather than withdraw it gracefully. If such a push
results in changing the prefix configured on a particular subnet,
hosts attached to that subnet might not get notified about the
prefix change, and their addresses from the "old" prefix might not
be deprecated (and eventually invalidated) in a timely manner. A
related scenario is an incorrect network renumbering event, where
a network administrator renumbers a network by simply removing the
"old" prefix from the configuration and configuring a new prefix
instead.
Lacking any explicit and reliable signaling to deprecate (and
eventually invalidate) the stale prefixes, hosts may continue to
employ the previously configured addresses, which will typically
result in packets being filtered or blackholed either on the CE
router or within the ISP network.
The default values for the Preferred Lifetime and Valid Lifetime of
PIOs specified in [RFC4861] mean that, in the aforementioned
scenarios, the stale addresses would be retained and could be
actively employed for new communication instances for an unacceptably
long period of time (one month and one week, respectively). This
could lead to interoperability problems, instead of hosts
transitioning to the newly advertised prefix(es) in a more timely
manner.
Some devices have implemented ad hoc mechanisms to address this
problem, such as sending RAs to deprecate (and eventually invalidate)
apparently stale prefixes when the device receives any packets
employing a source address from a prefix not currently advertised for
address configuration on the local network [FRITZ]. However, this
may introduce other interoperability problems, particularly in
multihomed/multi-prefix scenarios. This is a clear indication that
advice in this area is warranted.
Unresponsiveness to these flash-renumbering events is caused by the
inability of the network to deprecate (and eventually invalidate)
stale information as well as by the inability of hosts to react to
network configuration changes in a more timely manner. Clearly, it
would be desirable that these flash-renumbering events do not occur
and that, when they do occur, hosts are explicitly and reliably
notified of their occurrence. However, for robustness reasons, it is
paramount for hosts to be able to recover from stale configuration
information even when these flash-renumbering events occur and the
network is unable to explicitly and reliably notify hosts about such
conditions.
Section 2 analyzes this problem in more detail. Section 3 describes
possible operational mitigations. Section 4 describes possible
future work to mitigate the aforementioned problem.
2. Analysis of the Problem
As noted in Section 1, the problem discussed in this document is
exacerbated by the default values of some protocol parameters and
other factors. The following sections analyze each of them in
detail.
2.1. Use of Dynamic Prefixes
In network scenarios where dynamic prefixes are employed, renumbering
events lead to updated network configuration information being
propagated through the network, such that the renumbering events are
gracefully handled. However, if the renumbering event happens along
with, e.g., loss of configuration state by some of the devices
involved in the renumbering procedure (e.g., a router crashes,
reboots, and gets leased a new prefix), this may result in a flash-
renumbering event, where new prefixes are introduced without properly
phasing out the old ones.
In simple residential or small office scenarios, the problem
discussed in this document would be avoided if DHCPv6-PD leased
"stable" prefixes. However, a recent survey [UK-NOF] indicates that
37% of the responding ISPs employ dynamic IPv6 prefixes. That is,
dynamic IPv6 prefixes are an operational reality.
Deployment reality aside, there are a number of possible issues
associated with stable prefixes:
* Provisioning systems may be unable to deliver stable IPv6
prefixes.
* While an ISP might lease stable prefixes to the home or small
office, the Customer Edge router might in turn lease sub-prefixes
of these prefixes to other internal network devices. Unless the
associated lease databases are stored on non-volatile memory,
these internal devices might get leased dynamic sub-prefixes of
the stable prefix leased by the ISP. In other words, every time a
prefix is leased, there is the potential for the resulting
prefixes to become dynamic, even if the device leasing sub-
prefixes has been leased a stable prefix by its upstream router.
* While there is a range of information that may be employed to
correlate network activity [RFC7721], the use of stable prefixes
clearly simplifies network activity correlation and may
essentially render reduces the
effectiveness of features, such as "temporary addresses"
[RFC4941], mostly irrelevant.
[RFC4941].
* There might be existing advice for ISPs to deliver dynamic IPv6
prefixes *by default* (e.g., see [GERMAN-DP]) over privacy
concerns associated with stable prefixes.
* There might be scalability and performance drawbacks of either a
disaggregated distributed routing topology or a centralized
topology, which are often required to provide stable prefixes,
i.e., distributing more-specific routes or summarizing routes at
centralized locations.
For a number of reasons (such as the ones stated above), IPv6
deployments might employ dynamic prefixes (even at the expense of the
issues discussed in this document), and there might be scenarios in
which the dynamics of a network are such that the network exhibits
the behavior of dynamic prefixes. Rather than trying to regulate how
operators may run their networks, this document aims at improving
network robustness in the deployed Internet.
2.2. Default PIO Lifetime Values in IPv6 Stateless Address
Autoconfiguration (SLAAC)
The impact of the issue discussed in this document is a function of
the lifetime values employed for PIOs, since these values determine
for how long the corresponding addresses will be preferred and
considered valid. Thus, when the problem discussed in this document
is experienced, the longer the PIO lifetimes, the higher the impact.
[RFC4861] specifies the following default PIO lifetime values:
* Preferred Lifetime (AdvPreferredLifetime): 604800 seconds (7 days)
* Valid Lifetime (AdvValidLifetime): 2592000 seconds (30 days)
Under problematic circumstances, such as when the corresponding
network information has become stale without any explicit and
reliable signal from the network (as described in Section 1), it
could take hosts up to 7 days (one week) to deprecate the
corresponding addresses and up to 30 days (one month) to eventually
invalidate and remove any addresses configured for the stale prefix.
This means that it will typically take hosts an unacceptably long
period of time (on the order of several days) to recover from these
scenarios.
2.3. Recovering from Stale Network Configuration Information
SLAAC hosts are unable to recover from stale network configuration
information, since:
* In scenarios where SLAAC routers explicitly signal the renumbering
event, hosts will typically deprecate, but not invalidate, the
stale addresses, since item "e)" of Section 5.5.3 of [RFC4862]
specifies that an unauthenticated RA may never reduce the valid
lifetime of an address to less than two hours. Communication with
the new "users" of the stale prefix will not be possible, since
the stale prefix will still be considered "on-link" by the local
hosts.
* In the absence of explicit signaling from SLAAC routers, SLAAC
hosts will typically fail to recover from stale configuration
information in a timely manner, since hosts would need to rely on
the last Preferred Lifetime and Valid Lifetime advertised for the
stale prefix, for the corresponding addresses to become deprecated
and subsequently invalidated. Please see Section 2.2 of this
document for a discussion of the default PIO lifetime values.
2.4. Lack of Explicit Signaling about Stale Information
Whenever prefix information has changed, a SLAAC router should
advertise not only the new information but also the stale information
with appropriate lifetime values (both the Preferred Lifetime and the
Valid Lifetime set to 0). This would provide explicit signaling to
SLAAC hosts to remove the stale information (including configured
addresses and routes). However, in certain scenarios, such as when a
CE router crashes and reboots, the CE router may have no knowledge
about the previously advertised prefixes and thus might be unable to
advertise them with appropriate lifetimes (in order to deprecate and
eventually invalidate them).
In any case, we note that, as discussed in Section 2.3, PIOs with
small Valid Lifetimes in unauthenticated RAs will not lower the Valid
Lifetime to any value shorter than two hours (as per [RFC4862]).
Therefore, even if a SLAAC router tried to explicitly signal the
network about the stale configuration information via unauthenticated
RAs, implementations compliant with [RFC4862] would deprecate the
corresponding prefixes but would fail to invalidate them.
| NOTE:
|
| Some implementations have been updated to honor small PIO
| lifetimes values, as proposed in [RENUM-RXN]. For example,
| please see [Linux-update].
2.5. Interaction between DHCPv6-PD and SLAAC
While DHCPv6-PD is normally employed along with SLAAC, the
interaction between the two protocols is largely unspecified. Not
unusually, the two protocols are implemented in two different
software components, with the interface between the two implemented
by means of some sort of script that feeds the SLAAC implementation
with values learned from DHCPv6-PD.
At times, the prefix lease time is fed as a constant value to the
SLAAC router implementation, meaning that, eventually, the prefix
lifetimes advertised on the LAN side will span *past* the DHCPv6-PD
lease time. This is clearly incorrect, since the SLAAC router
implementation would be allowing the use of such prefixes for a
period of time that is longer than the one they have been leased for
via DHCPv6-PD.
3. Operational Mitigations
The following subsections discuss possible operational workarounds
for the aforementioned problems.
3.1. Stable Prefixes
As noted in Section 2.1, the use of stable prefixes would eliminate
the issue in *some* of the scenarios discussed in Section 1 of this
document, such as the typical home network deployment. However, as
noted in Section 2.1, there might be reasons for which an
administrator may want or may need to employ dynamic prefixes.
3.2. SLAAC Parameter Tweaking
An operator may wish to override some SLAAC parameters such that,
under normal circumstances, the associated timers will be refreshed/
reset, but in the presence of network faults (such as the one
discussed in this document), the associated timers go off and trigger
some fault recovering action (e.g., deprecate and eventually
invalidate stale addresses).
The following router configuration variables from [RFC4861]
(corresponding to the "lifetime" parameters of PIOs) could be
overridden as follows:
* AdvPreferredLifetime: 2700 seconds (45 minutes)
* AdvValidLifetime: 5400 seconds (90 minutes)
| NOTES:
|
| The aforementioned values for AdvPreferredLifetime and
| AdvValidLifetime are expected to be appropriate for most
| networks. In some networks, particularly those where the
| operator has complete control of prefix allocation and where
| hosts on the network may spend long periods of time sleeping
| (e.g., sensors with limited battery), longer values may be
| appropriate.
|
| A CE router advertising a sub-prefix of a prefix leased via
| DHCPv6-PD will periodically refresh the Preferred Lifetime and
| the Valid Lifetime of an advertised prefix to
| AdvPreferredLifetime and AdvValidLifetime, respectively, as
| long as the resulting lifetimes of the corresponding prefixes
| do not extend past the DHCPv6-PD lease time [RENUM-CPE].
RATIONALE:
* In the context of [RFC8028], where it is clear that use of
addresses configured for a given prefix is tied to using the next-
hop router that advertised the prefix, it does not make sense for
the Preferred Lifetime of a PIO to be larger than the Router
Lifetime (AdvDefaultLifetime) of the corresponding Router
Advertisement messages. The Valid Lifetime is set to a larger
value to cope with transient network problems.
* Lacking RAs that refresh information, addresses configured for
advertised prefixes become deprecated in a more timely manner;
therefore, Rule 3 of [RFC6724] causes other configured addresses
(if available) to be used instead.
* Reducing the Valid Lifetime of PIOs helps reduce the amount of
time a host may maintain stale information and the amount of time
an advertising router would need to advertise stale prefixes to
invalidate them. Reducing the Preferred Lifetime of PIOs helps
reduce the amount of time it takes for a host to prefer other
working prefixes (see Section 12 of [RFC4861]). However, we note
that while the values suggested in this section are an improvement
over the default values specified in [RFC4861], they represent a
trade-off among a number of factors, including responsiveness,
possible impact on the battery life of connected devices
[RFC7772], etc. Thus, they may or may not provide sufficient
mitigation to the problem discussed in this document.
4. Future Work
Improvements in Customer Edge routers [RFC7084], such that they can
signal hosts about stale prefixes to depreacte (and eventually
invalidate) them accordingly, can help mitigate the problem discussed
in this document for the "home network" scenario. Such work is
currently being pursued in [RENUM-CPE].
Improvements in the SLAAC protocol [RFC4862] and some IPv6-related
algorithms, such as "Default Address Selection for Internet Protocol
Version 6 (IPv6)" [RFC6724], would help improve network robustness.
Such work is currently being pursued in [RENUM-RXN].
The aforementioned work is considered out of the scope of this
present document, which only focuses on documenting the problem and
discussing operational mitigations.
5. IANA Considerations
This document has no IANA actions.
6. Security Considerations
This document discusses a problem that may arise in scenarios where
flash-renumbering events occur and proposes workarounds to mitigate
the aforementioned problem. This document does not introduce any new
security issues; therefore, the same security considerations as for
[RFC4861] and [RFC4862] apply.
7. References
7.1. Normative References
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<https://www.rfc-editor.org/info/rfc4862>.
[RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
<https://www.rfc-editor.org/info/rfc6724>.
[RFC8028] Baker, F. and B. Carpenter, "First-Hop Router Selection by
Hosts in a Multi-Prefix Network", RFC 8028,
DOI 10.17487/RFC8028, November 2016,
<https://www.rfc-editor.org/info/rfc8028>.
[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>.
7.2. Informative References
[DEFAULT-ADDR]
Linkova, J., "Default Address Selection and Subnet
Renumbering", Work in Progress, Internet-Draft, draft-
linkova-6man-default-addr-selection-update-00, 30 March
2017, <https://tools.ietf.org/html/draft-linkova-6man-
default-addr-selection-update-00>.
[FRITZ] Gont, F., "Quiz: Weird IPv6 Traffic on the Local Network
(updated with solution)", SI6 Networks, February 2016,
<https://www.si6networks.com/2016/02/16/quiz-weird-ipv6-
traffic-on-the-local-network-updated-with-solution/>.
[GERMAN-DP]
BFDI, "Einführung von IPv6 Hinweise für Provider im
Privatkundengeschäft und Hersteller" [Introduction of
IPv6: Notes for providers in the private sector and
manufacturers], Entschliessung der 84. Konferenz der
Datenschutzbeauftragten des Bundes und der Lander
[Resolution of the 84th Conference of the Federal and
State Data Protection Commissioners], November 2012,
<http://www.bfdi.bund.de/SharedDocs/Publikationen/
Entschliessungssammlung/DSBundLaender/84DSK_EinfuehrungIPv
6.pdf?__blob=publicationFile>.
[Linux-update]
Gont, F., "Subject: [net-next] ipv6: Honor all IPv6 PIO
Valid Lifetime values", message to the netdev mailing
list, 19 April 2020,
<https://patchwork.ozlabs.org/project/netdev/
patch/20200419122457.GA971@archlinux-
current.localdomain/>.
[RENUM-CPE]
Gont, F., Zorz, J., Patterson, R., and B. Volz, "Improving
the Reaction of Customer Edge Routers to IPv6 Renumbering
Events", Work in Progress, Internet-Draft, draft-ietf-
v6ops-cpe-slaac-renum-06, 11 December 2020,
v6ops-cpe-slaac-renum-07, 16 February 2021,
<https://tools.ietf.org/html/draft-ietf-v6ops-cpe-slaac-
renum-06>.
renum-07>.
[RENUM-RXN]
Gont, F., Zorz, J., and R. Patterson, "Improving the
Robustness of Stateless Address Autoconfiguration (SLAAC)
to Flash Renumbering Events", Work in Progress, Internet-
Draft, draft-ietf-6man-slaac-renum-02, 19 January 2021,
<https://tools.ietf.org/html/draft-ietf-6man-slaac-renum-
02>.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
<https://www.rfc-editor.org/info/rfc4941>.
[RFC7084] Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
Requirements for IPv6 Customer Edge Routers", RFC 7084,
DOI 10.17487/RFC7084, November 2013,
<https://www.rfc-editor.org/info/rfc7084>.
[RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC7721, March 2016,
<https://www.rfc-editor.org/info/rfc7721>.
[RFC7772] Yourtchenko, A. and L. Colitti, "Reducing Energy
Consumption of Router Advertisements", BCP 202, RFC 7772,
DOI 10.17487/RFC7772, February 2016,
<https://www.rfc-editor.org/info/rfc7772>.
[RIPE-690] Žorž, J., Steffann, S., Dražumerič, P., Townsley, M.,
Alston, A., Doering, G., Palet Martinez, J., Linkova, J.,
Balbinot, L., Meynell, K., and L. Howard, "Best Current
Operational Practice for Operators: IPv6 prefix assignment
for end-users - persistent vs non-persistent, and what
size to choose", RIPE 690, October 2017,
<https://www.ripe.net/publications/docs/ripe-690>.
[UK-NOF] Palet Martinez, J., "IPv6 Deployment Survey and BCOP", UK
NOF 39, January 2018,
<https://indico.uknof.org.uk/event/41/contributions/542/>.
Acknowledgments
The authors would like to thank (in alphabetical order) Brian
Carpenter, Alissa Cooper, Roman Danyliw, Owen DeLong, Martin Duke,
Guillermo Gont, Philip Homburg, Sheng Jiang, Benjamin Kaduk, Erik
Kline, Murray Kucherawy, Warren Kumari, Ted Lemon, Juergen
Schoenwaelder, Éric Vyncke, Klaas Wierenga, Robert Wilton, and Dale
Worley for providing valuable comments on earlier draft versions of
this document.
The authors would like to thank (in alphabetical order) Mikael
Abrahamsson, Luis Balbinot, Brian Carpenter, Tassos Chatzithomaoglou,
Uesley Correa, Owen DeLong, Gert Doering, Martin Duke, Fernando
Frediani, Steinar Haug, Nick Hilliard, Philip Homburg, Lee Howard,
Christian Huitema, Ted Lemon, Albert Manfredi, Jordi Palet Martinez,
Michael Richardson, Mark Smith, Tarko Tikan, and Ole Troan for
providing valuable comments on a previous document on which this
document is based.
Fernando would like to thank Alejandro D'Egidio and Sander Steffann
for a discussion of these issues. Fernando would also like to thank
Brian Carpenter who, over the years, has answered many questions and
provided valuable comments that have benefited his protocol-related
work.
The problem discussed in this document has been previously documented
by Jen Linkova in [DEFAULT-ADDR] and also in [RIPE-690]. Section 1
borrows text from [DEFAULT-ADDR], authored by Jen Linkova.
Authors' Addresses
Fernando Gont
SI6 Networks
Segurola y Habana 4310, 7mo Piso
Villa Devoto
Ciudad Autónoma de Buenos Aires
Argentina
Email: fgont@si6networks.com
URI: https://www.si6networks.com
Jan Žorž
6connect, Inc.
Email: jan@6connect.com
Richard Patterson
Sky UK
Email: richard.patterson@sky.uk