Network Working Group B. Liu
Internet Draft S. Jiang
Intended status: Informational Huawei Technologies
Expires: August 18, 2014 February 14, 2014
Recommendations of Using Unique Local Addresses
draft-ietf-v6ops-ula-usage-recommendations-02.txt
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Abstract
This document provides guidance of how to use ULAs. It analyzes ULA
usage scenarios and recommends use cases where ULA addresses might be
beneficially used.
Table of Contents
1. Introduction ................................................. 3
2. The analysis of ULA features ................................. 3
2.1. Automatically Generated ................................. 3
2.2. Globally unique.......................................... 3
2.3. Independent address space ............................... 4
2.4. Well known prefix ....................................... 4
2.5. Stable or Temporary Prefix .............................. 4
3. Enumeration of Scenarios Using ULAs .......................... 4
3.1. Isolated network ........................................ 4
3.2. Connected network ....................................... 5
3.2.1. ULA-only Deployment ................................ 5
3.2.2. ULA along with GUA ................................. 7
3.3. IPv4 Co-existence consideration ......................... 9
4. General Guidelines of using ULA .............................. 9
4.1. Do not treat ULA equal to RFC1918 ....................... 9
4.2. Using ULAs in a limited scope .......................... 10
5. ULA usage recommendations ................................... 10
5.1. Used in Isolated Networks .............................. 10
5.2. ULA along with GUA ..................................... 10
5.3. Recommended Specific Use Cases ......................... 10
5.3.1. Special routing ................................... 11
5.3.2. Used as NAT64 prefix .............................. 11
5.3.3. Used as identifier ................................ 12
6. Security Considerations ..................................... 12
7. IANA Considerations ......................................... 13
8. Conclusions ................................................. 13
9. References .................................................. 13
9.1. Normative References ................................... 13
9.2. Informative References ................................. 13
10. Acknowledgments ............................................ 14
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1. Introduction
Unique Local Addresses (ULAs) are defined in [RFC4193] as provider-
independent prefixes that can be used on isolated networks, internal
networks, and VPNs. Although ULAs may be treated like global scope by
applications, normally they should not be used on the publicly
routable internet.
However, the ULAs haven't been widely used since IPv6 hasn't been
widely deployed yet. The use of ULA addresses in various types of
networks has been confusing to network operators. This document aims
to clarify the advantages and disadvantages of ULAs and how they can
be most appropriately used.
2. The analysis of ULA features
2.1. Automatically Generated
ULA prefixes could be automatically generated according to the
algorithms described in [RFC4193]. This feature allows automatic
address allocation, which is beneficial for some lightweight systems
and can leverage minimal human management.
2.2. Globally unique
ULA is intended to have an extremely low probability of collision.
Since the hosts assigned with ULA may occasionally be merged into one
network, this uniqueness is necessary. The prefix uniqueness is based
on randomization of 40 bits and is considered random enough to ensure
a high degree of uniqueness (refer to [RFC4193] section 3.2.3 for
details)and make merging of networks simple and without the need to
renumbering overlapping IP address space. Overlapping is cited as a
deficiency with how [RFC1918] addresses were deployed, and ULA was
designed to overcome this deficiency.
Notice that, as described in [RFC4864], in practice, applications may
treat ULAs like global-scope addresses, but address selection
algorithms may need to distinguish between ULAs and ordinary GUA
(Global-scope Unicast Address) to ensure bidirectional communications.
(Note: the new address selection standard has supported this in the
default policy table. [RFC6724])
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2.3. Independent address space
ULA provides an internal address independence capability in IPv6ULA
can be used for internal communications without having any permanent
or only intermittent Internet connectivity. And it needs no
registration so that it can support on-demand usage and does not
carry any RIR documentation burden or disclosures.
2.4. Well known prefix
The prefixes of ULAs are well known and they are easy to be
identified and easy to be filtered.
This feature may be convenient to management of security policies and
troubleshooting. For example, the administrators can decide what
parameters have to be assembled or transmitted globally, by a
separate function, through an appropriate gateway/firewall, to the
Internet or to the telecom network.
2.5. Stable or Temporary Prefix
A ULA prefix can be generated once, at installation time or "factory
reset", and then never change unless the network manager wants to
change. Alternatively, it could be regenerated regularly, if desired
for some reason.
3. Enumeration of Scenarios Using ULAs
In this section, we cover possible ULA use cases. Some of them might
have been discussed in other documents and are briefly reviewed in
this document as well as other potential valid usage is discussed.
3.1. Isolated network
IP is used ubiquitously. Some networks like industrial control bus
(e.g. [RS-485], [SCADA], or even non-networked digital interface like
[MIL-STD-1397] began to use IP protocol. In such kind of networks,
the system might lack the ability/requirement of connecting to the
Internet. or explicitly designed not to connect.
Besides, there might be some networks which could connect to the
Internet, but prohibited by administration or just temporally not
connected. These networks may include machine-to-machine (e.g.
vehichle networks), sensor networks, or even normal LANs, which may
include very large numbers of addresses.
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Since the serious disadvantages and impact on applications by
ambiguous address space have been well documented in [RFC1918], ULA
is a straightforward way to assign the IP addresses in these kinds of
networks with minimal administrative cost or burden. Also, ULAs fit
in multiple subnet scenarios, in which each subnet has its own ULA
prefix. For example, when we assign vehicles with ULA addresses, it
is then possible to separate in-vehicle embedded networks into
different subnets depending on real-time requirements, devices types,
services and more.
o Benefits of Using ULAs in Isolated Networks:
- No cost of RIR/LIR fees or operational burden
- Could be used instantly on-demand
- Scalable to support multiple subnets
- Extremely low probability of collision when isolated networks
merge
o Drawbacks:
- The uniqueness of the global ULA prefix is not guaranteed,
however, the probability of collision is extremely low
o Operational considerations
- Prefix generation: Randomly generated according to the
algorithms defined in [RFC4193] or literally assigned by human.
Normally, it is recommended to following the standard way to
automatically generate the prefixes; if there are some specific
reasons that need to be assigned by human, the administrators must
carefully plan the prefixes to avoid collision.
- Prefix announcement: In some cases, the networks might need to
announce prefixes to each other, for example in vehicle networks
with infrastructure-less settings such as Vehicle-to-Vehicle (V2V)
settings, prior knowledge of respective prefixes is unlikely.
Hence, a prefix announcement mechanism is needed to enable inter-
vehicles communications based on IP. For instance, such
announcement could rely on an extension of the Router
Advertisement message of Neighbor Discovery Protocol (e.g.
[I-D.petrescu-autoconf-ra-based-routing] and
[I-D.jhlee-mext-mnpp]).
3.2. Connected network
3.2.1. ULA-only Deployment
In some situations, hosts/interfaces are assigned with ULA-only, but
the networks need to communicate with the outside. It mainly includes
the following two models.
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o Using Network Prefix Translation
Network Prefix Translation (NPTv6) [RFC6296] is an experimental
specification that provides a stateless one to one mapping between
internal addresses and external addresses.
In some very constrained situations(for example, in the sensors), the
network needs ULA as the on-demand and stable addressing which
doesn't need much code to support address assignment mechanisms like
DHCP or full ND (Note: surely it needs SLAAC). If the network also
needs to connect to the outside, then there can be an NPTv6 gateway
which is not subject to extreme resource constraints. Especially when
a lightweight isolated network needs to add Internet connectivity,
this is quite a straightforward and efficient way.
This document does not intend to encourage the ULA binding with NPTv6
model, since in [RFC5902] the IAB had already gave opinions on IPv6
NAT; but this document considers it as an effective approach in some
specific situations as described above.
o Using application-layer proxies
The proxies terminate the network-layer connectivity of the hosts and
delegate the outgoing/incoming connections.
In some environments (e.g. information security sensitive enterprise
or government), the endpoints are default disconnected to the
Internet, and need the proxies to connect for central control. In
IPv4, using private address space with proxies is an effective and
common practice for this purpose, and it is natural to pick ULA in
IPv6.
o Benefits of Using ULAs in This Scenario:
- Allowing minimal management burden on address assignment for
some specific environments.
o Drawbacks:
- The serious disadvantages and impact on applications by NAT have
been well documented in [RFC2993] and [RFC3027]. However, it
should be noted that, For NPTv6, as described in [RFC6296], it is
"a mechanism that has fewer architectural problems than merely
implementing a traditional stateful Network Address Translator in
an IPv6 environment."
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o Operational considerations
- Firewall Deployment: As described in [RFC6296], firewall is
recommended to be implemented with NPTv6 translator. Then the
administrators need to know about where the firewall is located.
If the firewall is located outside the NPTv6 translator, then the
filtering is based on the translated GUA prefixes, and when the
internal ULA prefixes are renumbered, the filtering rules don't
need to be changed (however, when GUA prefixes of the NPTv6 are
renumbered, the filtering rules need to be updated accordingly.).
If the firewall is located inside the NPTv6 translator, the
filtering is then based on the ULA prefixes, and the rules need to
be updated according to the ULA prefixes renumbering (but don't
need to update when NPTv6 GUA prefixes are renumbered.).
3.2.2. ULA along with GUA
There are two classes of network probably to use ULA with GUA
addresses:
- Home network. Home networks are normally assigned with one or
more globally routed PA prefixes to connect to the uplink of some
an ISP. And besides, they may need internal routed networking even
when the ISP link is down. Then ULA is a proper tool to fit the
requirement. And in [RFC6204], it requires the CPE to support ULA.
Note, ULAs provide more benefit for multiple-segment home networks;
for home networks only containing one segment, link-local
addresses are better alternative.
- Enterprise network. An enterprise network is usually a managed
network with one or more PA prefixes or with a PI prefix, all of
which are globally routed. The ULA could be used for internal
connectivity redundancy and better internal connectivity or
isolation of certain functions like OAM of servers.
o Benefits of Using ULAs in This Scenario:
- Separated Local Communication Plane: For either home networks or
enterprise networks, the main purpose of using ULA along with GUA
is to provide a logically local routing plane separated from the
globally routing plane. The benefit is to ensure stable and
specific local communication regardless of the ISP uplink failure.
This benefit is especially meaningful for the home network or
private OAM function in an enterprise.
- Renumbering: In some special cases such as renumbering,
enterprise administrators may want to avoid the need to renumber
their internal-only, private nodes when they have to renumber the
PA addresses of the whole network because of changing ISPs, ISPs
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restructure their address allocations, or whatever reasons. In
these situations, ULA is an effective tool for the internal-only
nodes.
Besides the internal-only nodes, the public nodes can also benefit
from ULA for renumbering. When renumbering, as RFC4192 suggested,
it has a period to keep using the old prefix(es) before the new
prefix(es) is(are) stable. In the process of adding new prefix(es)
and deprecating old prefix(es), it is not easy to keep the local
communication immune of global routing plane change. If we use ULA
for the local communication, the separated local routing plane can
isolate the affecting by global routing change.
o Drawbacks:
- Operational Complexity: There are some arguments that in
practice the ULA+PA makes additional operational complexity. It is
not a ULA-specific problem; the multiple-addresses-per-interface
is an important feature of IPv6 protocol. Never the less, running
multiple prefixes needs more operational considerations than
running a single one.
o Operational considerations
- Default Routing: connectivity might be broken if ULAs are used
as default route.
- SLAAC/DHCPv6 co-existing: Since SLAAC and DHCPv6 might be
enabled in one network simultaneously; the administrators need to
carefully plan how to assign ULA and GUA prefixes in accordance
with the two mechanisms. The administrators need to know the
current issue of the SLAAC/DHCPv6 interaction (please refer to
[I-D.liu-bonica-dhcpv6-slaac-problems] for details).
- Address Selection: As mentioned in [RFC5220], there is a
possibility that the longest matching rule will not be able to
choose the correct address between ULAs and global unicast
addresses for correct intra-site and extra-site communication. In
[RFC6724], it claimed that a site-specific policy entry can be
used to cause ULAs within a site to be preferred over global
addresses.
- DNS Relevant: if administrators chose not to do reverse DNS
delegation inside of their local control of ULA prefixes, a
significant amount of information about the ULA population might
leak to the outside world. Because reverse queries will be made
and naturally routed to the global reverse tree, so people will be
exposed to the existence of population the ULA uses, not in
traffic terms, but in knowledge terms. [ULA-IN-WILD] provides more
detailed situations on this issue.
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Administrators may need a split DNS to separate the queries from
internal and external for ULA entries and GUA entries.
3.3. IPv4 Co-existence consideration
Generally, this document does not consider IPv4 relevant as in the
scope But regarding ULA, there is a special case needs to be noticed,
which is described in section 2.2.2 of [RFC5220]. When an enterprise
has IPv4 Internet connectivity but does not yet have IPv6 Internet
connectivity, and the enterprise wants to provide site-local IPv6
connectivity, a ULA is the best choice for site-local IPv6
connectivity. Each employee host will have both an IPv4 global or
private address and a ULA. Here, when this host tries to connect to
an outside node that has registered both A and AAAA records in the
DNS, the host will choose AAAA as the destination address and the ULA
for the source address according to the IPv6 preference of the
default address selection policy. This will clearly result in a
connection failure.
Although with Happy Eyeballs [RFC6555] this connection failure
problem could be solved, but unwanted timeouts would obviously lower
the user experience. One possible approach of eliminating the
timeouts is deprecating IPv6 default route and simply configuring a
scoped route on hosts (in the context of this document, only
configure the ULA prefix routes).
Another alternative is configuring IPv4 preference on the hosts, and
not including DNS A records but only AAAA records for the internal
nodes in the internal DNS server, then outside nodes have both A and
AAAA records could be connected through IPv4 as default and internal
nodes could be always connected through IPv6. But since IPv6
preference is default, changing the default in all nodes is not
suitable at scale.
4. General Guidelines of using ULA
4.1. Do not treat ULA equal to RFC1918
ULA and [RFC1918] are similar in some aspects. The most obvious one
is as described in section 2.1.3 that ULA provides an internal
address independence capability in IPv6 that is similar to how
[RFC1918] is commonly used. ULA allows administrators to configure
the internal network of each platform the same way it is configured
in IPv4. Many organizations have security policies and architectures
based around the local-only routing of [RFC1918] addresses and those
policies may directly map to ULA [RFC4864].
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But it doesn't mean ULA is equal to an IPv6 version of [RFC1918]
deployment. [RFC1918] usually combines with NAT/NAPT for global
connectivity. But it is not necessarily to combine ULAs with any kind
of NAT. People could use ULA for local communications along with
global addresses for global communications (see section 5.2). This is
a big advantage brought by default support of multiple-addresses-per-
interface feature in IPv6. (People might still have requirement of
ULA with NAT, this is discussed in section 3.2.1. But people also
should keep in mind that ULA is not intentionally designed for this
kind of use case.)
Another important difference is the ability to merge two ULA networks
without renumbering (because of the uniqueness), which is a big
advantage over [RFC1918].
4.2. Using ULAs in a limited scope
A ULA is by definition a prefix that is never advertised outside a
given domain, and is used within that domain by agreement of those
networked by the domain.
So when using ULAs in a network, the administrators should clearly
set the scope of the ULAs and configure ACLs on relevant border
routers to block them out of the scope. And if internal DNS are
enabled, the administrators might also need to use internal-only DNS
names for ULAs and should split the DNS so that the internal DNS
server includes records that are not presented in the external DNS
server.
5. ULA usage recommendations
5.1. Used in Isolated Networks
As analyzed in section 3.1, ULA is very suitable for isolated
networks. Especially when you have subnets in the isolated networks,
ULA is the best choice.
5.2. ULA along with GUA
As the benefits described in Section 3.2.2, using ULA along with GUA
to provide a logically separated local plane could benefit to OAM
functions and renumbering.
5.3. Recommended Specific Use Cases
Along with the general scenarios, this section provides some specific
use cases that could benefit from using ULA.
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5.3.1. Special routing
For various reasons the administrators might want to have private
routing be controlled and separated from other routing. For example,
in the b2b case described in
[I-D.baker-v6ops-b2b-private-routing], two companies might want to
use direct connectivity that only connects stated machines, such as a
silicon foundry with client engineers that use it. A ULA provides a
simple way to assign such prefixes that would be used in accordance
with an agreement between the parties.
5.3.2. Used as NAT64 prefix
Since the NAT64 pref64 is just a group of local fake addresses for
the DNS64 to point traffic to a NAT64, When using a ULA prefix as the
pref64, it could easily ensures that only local systems can use the
translation resources of the NAT64 system since the ULA is not
intended to be globally routable and helps clearly identify traffic
that is locally contained and destine to a NAT64. Using ULA for
Pref64 is deployed and it is an operational model.
But there's an issue should be noticed. The NAT64 standard [RFC6146)
mentioned the pref64 should align with [RFC6052], in which the IPv4-
Embedded IPv6 Address format was specified. If we pick a /48 for
NAT64, it happened to be a standard 48/ part of ULA (7bit ULA famous
prefix+ 1 "L" bit + 40bit Global ID). Then the 40bit of ULA is not
violated to be filled with part of the 32bit IPv4 address. This is
important, because the 40bit assures the uniqueness of ULA, if the
prefix is shorter than /48, the 40bit would be violated, and this may
cause conformance issue. But it is considered that the most common
use case will be a /96 PREF64, or even /64 will be used. So it seems
this issue is not common in current practice.
It is most common that ULA Pref64 will be deployed on a single
internal network, where the clients and the NAT64 share a common
internal network. ULA will not be effective as Pref64 when the access
network must use an Internet transit to receive the translation
service of a NAT64 since the ULA will not route across the internet.
According to the default address selection table specified in
[RFC6724], the host would always prefer IPv4 over ULA. This could be
a problem in NAT64-CGN scenario as analyzed in Section 8 of
[I-D.ietf-v6ops-nat64-experience]. So administrators need to add
additional site-specific address selection rules to the default table
to steer traffic flows going through NAT64-CGN. However, updating the
default policy tables in all hosts involves significant management
cost. This may be possible in an enterprise (using a group policy
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object, or other configuration mechanisms), but it is not suitable at
scale for home networks.
5.3.3. Used as identifier
ULAs could be self-generated and easily grabbed from the standard
IPv6 stack. And ULAs don't need to be changed as the GUA prefixes do.
So they are very suitable to be used as identifiers by the up layer
applications. And since ULA is not intended to be globally routed, it
is not harmful to the routing system.
Such kind of benefit has been utilized in real implementations. For
example, in [RFC6281], the protocol BTMM (Back To My Mac) needs to
assign a topology-independent identifier to each client host
according to the following considerations:
o TCP connections between two end hosts wish to survive in network
changes.
o Sometimes one needs a constant identifier to be associated with a
key so that the Security Association can survive the location
changes.
It should be noticed again that in theory ULA has the possibility of
collision. However, the probability is desirable small enough and
could be ignored by most of the cases when used as identifiers.
6. Security Considerations
Security considerations regarding ULAs, in general, please refer to
the ULA specification [RFC4193].
Also refer to [RFC4864], which shows how ULAs help with local network
protection.
As mentioned in Section 3.2.1, when use NPTv6, the administrators
need to know about where the firewall is located to set proper
filtering rules.
As mentioned in Section 3.2.2, if administrators chose not to do
reverse DNS delegation inside their local control of ULA prefixes, a
significant amount of information about the ULA population might leak
to the outside world.
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7. IANA Considerations
IANA considerations should be updated to point to RFC4193 in a
similar manner to section 4.
8. Conclusions
ULA is a useful tool; it could be successfully deployed in a diverse
set of circumstances including large private machine-to-machine type
networks, enterprise networks with private systems, and within
service providers to limit Internet communication with non-public
services such as caching DNS servers and NAT64 translation resources.
Some of the deployment is already in real production networks.
We should eliminate the misunderstanding that ULA is just an IPv6
version of [RFC1918]. The features of ULA could be beneficial for
various use cases.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP14, March 1997.
[RFC4193] Hinden, R., B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
9.2. Informative References
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",BCP 5,
RFC 1918, February 1996.
[RFC4864] Van de Velde, G., Hain, T., Droms, R., Carpenter, B., and
E. Klein, "Local Network Protection for IPv6", RFC 4864,
May 2007.
[RFC5220] Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama,
"Problem Statement for Default Address Selection in Multi-
Prefix Environments: Operational Issues of RFC 3484 Default
Rules", RFC 5220, July 2008.
[RFC6281] Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang,
"Understanding Apple's Back to My Mac (BTMM) Service", RFC
6281, June 2011.
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[RFC6296] Wasserman, M., and F. Baker, "IPv6-to-IPv6 Network Prefix
Translation", RFC 6296, June 2011.
[RFC6555] Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
Dual-Stack Hosts", RFC 6555, April 2012.
[RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724,
[I-D.ietf-v6ops-nat64-experience]
Chen G., Cao Z., Xie C., and D. Binet, "NAT64 Operational
Experiences", Working in Progress, October, 2013
[I-D.baker-v6ops-b2b-private-routing]
F. Baker, "Business to Business Private Routing", Expired
[I-D.petrescu-autoconf-ra-based-routing]
Petrescu, A., Janneteau, C., Demailly, N. and S. Imadali,
"Router Advertisements for Routing between Moving Networks",
Working in Progress, October, 2013
[I-D.jhlee-mext-mnpp]
Lee, J.-H., and T. Ernst, "Mobile Network Prefix
Provisioning", Expired
[RS-485] http://en.wikipedia.org/wiki/RS-485
[MIL-STD-1397]
http://en.wikipedia.org/wiki/MIL-STD-1397
[SCADA] http://en.wikipedia.org/wiki/SCADA
[ULA-IN-WILD]
G. Michaelson, "www.ietf.org/proceedings/87/slides/slides-
87-v6ops-0.pdf"
10. Acknowledgments
Many valuable comments were received in the IETF v6ops WG mail list,
especially from Fred Baker, Brian Carpenter, Lee Howard, Victor
Kuarsingh, Alexandru Petrescu, Mikael Abrahamsson, Jong-Hyouk Lee,
Doug Barton, Owen Delong, Anders Brandt, Tim Chown, Jen Linkova,
Christopher Palmer and Wesley George.
Some test of using ULA in the lab was done by our research partner
BNRC-BUPT (Broad Network Research Centre in Beijing University of
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Posts and Telecommunications). Thanks for the work of Prof. Xiangyang
Gong and student Dengjia Xu.
This document was prepared using 2-Word-v2.0.template.dot.
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Authors' Addresses
Bing Liu
Huawei Technologies Co., Ltd
Huawei Q14 Building, No.156 Beiqing Rd.,
Zhong-Guan-Cun Environmental Protection Park, Beijing
P.R. China
EMail: leo.liubing@huawei.com
Sheng Jiang
Huawei Technologies Co., Ltd
Huawei Q14 Building, No.156 Beiqing Rd.,
Zhong-Guan-Cun Environmental Protection Park, Beijing
P.R. China
EMail: jiangsheng@huawei.com