Routing Area Working Group A. Retana
Internet-Draft Cisco Systems, Inc.
Intended status: Informational R. White
Expires: April 27, 2015 Ericsson
M. Paul
Deutsche Telekom AG
October 24, 2014
A Framework and Requirements for Energy Aware Control Planes
draft-retana-rtgwg-eacp-03
Abstract
There has been, for several years, a rising concern over the energy
usage of large scale networks. This concern is strongly focused on
campus, data center, and other highly concentrated deployments of
network infrastructure. Given the steadily increasing demand for
higher network speeds, always-on service models, and ubiquitous
network coverage, it is also of growing importance for
telecommunication networks both local and wide area in scope. One of
the issues in moving forward to reduce energy usage is to ensure that
the network can still meet the performance specifications required to
support the applications running over it.
This document provides an overview of the various areas of concern in
the interaction between network performance and efforts at energy
aware control planes, as a guide for those working on modifying
current control planes or designing new control planes to improve the
energy efficiency of high density, highly complex, network
deployments.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on April 27, 2015.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Business Drivers . . . . . . . . . . . . . . . . . . . . . 6
3.3. Application Drivers . . . . . . . . . . . . . . . . . . . 6
4. Framework . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Modes of Reducing Energy Usage . . . . . . . . . . . . . . 7
4.1.1. Example Network . . . . . . . . . . . . . . . . . . . 8
4.1.2. Examples of Energy Reduction . . . . . . . . . . . . . 8
4.2. Global Verses Local Decisions . . . . . . . . . . . . . . 9
5. Considerations and Requirements . . . . . . . . . . . . . . . 9
5.1. Energy Efficiency and Bandwidth Reduction . . . . . . . . 9
5.1.1. An Example of Lowered Bandwidth . . . . . . . . . . . 10
5.1.2. Requirements . . . . . . . . . . . . . . . . . . . . . 10
5.2. Energy Efficiency and Stretch . . . . . . . . . . . . . . 10
5.2.1. An Example of Stretch . . . . . . . . . . . . . . . . 11
5.2.2. Requirements . . . . . . . . . . . . . . . . . . . . . 11
5.3. Energy Efficiency and Fast Recovery . . . . . . . . . . . 12
5.3.1. An Example of Impact on Fast Recovery . . . . . . . . 12
5.3.2. Requirements . . . . . . . . . . . . . . . . . . . . . 12
5.4. Introducing Jitter Through Microsleeps . . . . . . . . . . 13
5.4.1. An Example of Microsleeps to Reduce Energy Usage . . . 13
5.4.2. Requirements . . . . . . . . . . . . . . . . . . . . . 14
5.5. Other Operational Aspects . . . . . . . . . . . . . . . . 14
5.5.1. An Example of Operational Impact . . . . . . . . . . . 14
5.5.2. Requirements . . . . . . . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. Normative References . . . . . . . . . . . . . . . . . . . 15
8.2. Informative References . . . . . . . . . . . . . . . . . . 15
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 15
A.1. Changes between the -00 and -01 versions. . . . . . . . . 15
A.2. Changes between the -01 and -02 versions. . . . . . . . . 16
A.3. Changes between the -02 and -03 versions. . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
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1. Introduction
As energy prices continue to increase, and energy awareness becomes a
watchword for most large companies, places where the network
infrastructure used a good deal of power have come under increased
scrutiny for savings. There is a concern, however, in saving energy
at the cost of network operations --to reduce performance along with
energy consumption, negatively impacting the operation of a network
and the applications reliant on that network. This concern is
primarily focused on the network control plane, but will necessarily
apply to network performance and energy usage overall.
This document provides a background, a framework for understanding
and managing the tradeoffs between modifications made to network
protocols to conserve energy and network performance metrics and
requirements, and a set of requirements for protocol designers to
consider in proposals for new control plane protocols or
modifications to existing control plane protocols. It is intended to
encourage work on mechanisms that will reduce network energy usage
while providing perspective on balancing energy usage against
performance. The ultimate goal is to provide the tools and knowledge
necessary for protocol designers to modify network protocols to best
balance efficiency against performance, and to provide the background
information network operators will need to intelligently deploy and
use protocol modifications to network protocols.
The document is organized as follows. Section 3 provides material
the reader needs to understand to appreciate the challenges inherent
in balancing energy reduction with effective network performance.
This section includes subsections considering the application and
business requirements that are the basis of the reset of the
document. Section 4 provides a framework for understanding
mechanisms common to all energy management schemes proposed to date
in general terms. Section 5 provides an analysis of the areas
highlighted, including an explanation of how the specific area
interacts with energy management, and example of the interaction,
and, finally, a set of requirements protocol designers should
consider when proposing either new protocols or modifications to
existing protocols to reduce energy usage.
2. 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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3. Background
The background covered here describes the underlying business and
application drivers for the consideration and requirements sections
below. This section also contains a small example network used
throughout the remainder of this document for explaining various
mechanisms and technical points.
3.1. Scope
The reader should differentiate between radio based and wireline (or
rather, "plugged in"), networks. Radio based networks designed for
rapid deployment for highly mobile users (often called Mobile Ad Hoc
Networks, or MANETs [MANET]), and sensor networks designed for low
power, processing, and memory (such as those described in [ROLL]),
are not the target of this document. Readers should refer to the
groups working within those areas for energy management requirements
based on those specialized environment. While protocol developers
for those environments may draw useful information from this
document, this work is not intended to address those specialized
networks specifically. Mobile cellular networks however are
similarly affected by excess energy consumption as wireline networks
and seek to save energy by methods as described in the following (see
e.g. [3GPP]).
The reader should also differentiate between intradomain and
interdomain applications. Interdomain applications require more work
in policy than in technical and business considerations, and
therefore fall outside the scope of this document. Intradomain
control planes are (intuitively) where most energy savings will be
attained, at any rate. Most high concentrations of routers, such as
data centers and campus networks, are under a single administrative
domain. Therefore, placing interdomain control planes outside the
scope of this document does not limit its usefulness in any
meaningful way.
The reader should further differentiate between the components of an
energy management system, namely energy monitoring and energy
control. Energy monitoring deals with the collection of information
related to energy utilization and characteristics, as described in
[EMAN]. Energy control relates to directly influencing the
optimization and/or efficiency of devices in the network. The focus
of this document is on understanding the tradeoffs between
modifications made to network protocols to conserve energy and
network performance metrics and requirements, not on the functions,
steps or procedures required for energy monitoring.
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3.2. Business Drivers
Networks are primarily built to support both broad and narrow
business requirements. Broad business requirements might include
general communication requirements, such as providing email service
between internal and external personnel, or providing general access
to the World Wide Web for research and business support. Narrow
requirements would relate to specific applications, such as
supporting a particular financial application in the case of a bank
or other financial enterprise, or supporting customer traffic in the
case of a service provider. Application requirements will be
considered in greater detail in the next section.
Another class of requirements business place on networks can be
called operational requirements. These include (but are not limited
to), capital expense, operational expense, and the restrictions the
network architecture places on the growth and operation of the
business itself. These, in turn, drive requirements such as change
management, total uptime (availability), and the ability of the
network to be easily and quickly modified to meet new business
demands, or to shed old business demands. Operational expense is the
primary area this document covers in relation to business
requirements, because this is where energy management most obviously
overlaps with network performance.
3.3. Application Drivers
Applications drivers provide the background for each of the technical
sections below. When approaching a specific application, there are
only a small number of questions network and protocol designers need
to fully understand to shape networks and protocols so a specific
application can be supported. The first two questions revolve around
bandwidth; how much bandwidth will the application consume, and is
this bandwidth consumption fairly steady, or highly variable? For
instance, applications such as streaming video tend to have long
lasting flows with high bandwidth requirements, file transfers tend
to produce shorter flows requiring high bandwidth, and HTML traffic
tends to be bursty, with much lower bandwidth requirements.
The next question a protocol or network designer might ask about a
specific application is it's tolerance to jitter. Real time
applications, such as voice and video conferencing, have a very low
toleration for jitter. File transfers and streaming video, on the
other hand, can often handle large variations in packet arrival
times. If packets are delayed long enough, the application may
actually time out, shutting down sessions. Users will often "hang
up" after a short period of time, as well, causing loss of revenue
and productivity.
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Delay is another crucial factor in the performance of many
applications. Many server virtualization protocols, for instance,
have very low tolerance for delay, having been written with a short
wire local broadcast segment in mind. Applications such as stock and
commodity trading, remote medical, and collaborative video editing
also exhibit very little tolerance for delay.
These last two application drivers, jitter and delay, are normally
the result of two underlying causes within a network's control plane:
stretch and convergence. Stretch (defined more fully in the section
considering stretch below) causes longer paths to be taken through
the network. Each hop in the network path adds serialization into
and out of a set of queues in device memory, along with the delays of
various queuing mechanisms implemented on that device. Each hop in
the network increases delay directly, and has the potential to
increase jitter as packets pass into and out of the additional
devices.
Network convergence will also show up as jitter in an application's
stream; if packets are held up or looped for hundreds of milliseconds
during a network convergence event, applications running over the
converging topology will see this convergence time as a massive
jitter event, or a short term delay in the delivery of packets.
Jitter and delay can also be introduced directly into the packet
stream by reducing the throughput of individual links, or putting
devices and/or links into energy reduced modes for very short periods
of time (microsleeps). If a link is asleep when the first and third
packets from a flow arrive at the head end of the link, and not when
the second packet from that same flow arrives, each packet is going
to be processed differently, and hence will have a different delay
across the path.
The specific technical problems addressed in the following sections,
then, are bandwidth reduction, increasing stretch, network
convergence, and introducing jitter through microsleeps.
4. Framework
4.1. Modes of Reducing Energy Usage
Regardless of whether the control plane is centralized (such as some
form of centrally computed traffic engineering or software defined
network), or distributed (traditional routing protocols), there are
four primary ways in which energy usage can be reduced:
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o Removing redundant links from the network topology
o Removing redundant network equipment from the network topology
o Reducing the amount of time equipment or links are operational
o Reducing the link speed or processing rate of equipment
4.1.1. Example Network
To illustrate the impacts of link and device removal throughout the
rest of this document, the following network is used.
/---R2---\ /---\
R1 R4 R5
\---R3---/ \---/
This network is overly simplistic so the impact of removing various
links and devices from the topology can be more clearly illustrated.
More complex topologies will often exhibit these same impacts without
being so obvious.
4.1.2. Examples of Energy Reduction
In the example network above, several different modes of energy
reduction might be:
o Shutting down one of the two links between R4 and R5
o Shutting down one of the two links between R4 and R5, and shutting
down any line cards (or part of the nodes themselves) associated
with the removal of these links
o Shutting down R2 or R3, since these represent alternate paths to
reach the same set of destinations
o Shutting down the link between R2 and R4, since similar
connectivity is provided through R1->R3->R4
o Shutting down all links and devices for fractions of time in a
coordinated fashion
o Shutting down individual links as traffic or the control plane
permits for fractions of time (here the momentary shutdown of
various links is not coordinated, but undertaken hop by hop)
o Reducing the speed of all links and devices for fractions of time
in a coordinated fashion
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o Reducing the speed of individual links as traffic or the control
plane permits for fractions of time (here the momentary slowdown
of various links is not coordinated, but undertaken hop by hop)
4.2. Global Verses Local Decisions
Independent of whether the control plane is centralized or
distributed, the scope considered when making a decision about energy
efficiency may affect the result and effectiveness of the system.
There are clearly two extreme options when looking at the scope of
the information used to make decisions. The first extreme is that of
every device in the network considering only local conditions, and
determining the proper local state from that information. An example
of this mode of operation might be a local link where the devices on
either side of that link measure the link utilization, and
independently decide to automatically shut the link down when
utilization reaches a specific threshold. An example of the other
end of the spectrum might be a network control plane in which all the
nodes involved agree before taking a specific action; in the case of
two parallel links, the devices on each end not only would have
similar configured policies, but would coordinate if one of the links
was to be turned off. It is outside the scope of this document to
determine which of these two options may be optimal or "best."
There are some considerations and tradeoffs which need to be outlined
in considering the global versus local decisions in relation to
energy efficiency. System designers should take note of the
difficulties with preventing pathological conditions when purely
localized decisions are made. For instance, in the example network,
assume R1 determines to put the R1->R2 link into an energy saving
mode, while R4 determines to put the R4->R3 link into an energy
saving mode. In this case, no path will remain available through the
network. It is also possible for the opposite to occur, that is for
no links or devices to be placed into a reduced energy state because
R1 and R4 don't agree through the control plane which links and
devices should be removed from the topology.
Protocol designers should consider these tradeoffs in proposals for
energy aware control planes.
5. Considerations and Requirements
5.1. Energy Efficiency and Bandwidth Reduction
Bandwidth is an important consideration in high density networks;
most data centers are designed to provide a specific amount of
bandwidth into and out of each server and to facilitate virtual
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server movement among physical devices. In campus and core networks
bandwidth is finely coupled with quality of service guarantees for
applications and services. It should be obvious that removing links
or devices from a network topology will adversely affect the amount
of available bandwidth, which could, in turn, cause well thought out
quality of service mechanisms to degrade or fail.
What might not be so obvious is the relationship between available
bandwidth and jitter, or other network quality of service measures.
If higher speed links are removed from the topology in order to
continue using lower speed (and therefore presumably lower power)
links, then serialization delays will have a larger impact on traffic
flow. Longer serialization delays can cause input queues to back up,
which impacts not only delay but jitter, and possibly even traffic
delivery.
5.1.1. An Example of Lowered Bandwidth
In the network illustrated above, one of the two links between R4 and
R5 could be an obvious candidate for removal from the network.
Especially if the network load can easily be transferred to the
remaining link without failure, and without serious consequences for
delay or jitter in the network, there is a strong case to be made for
doing so --particularly if the accompanying line cards could also be
shut down to add to the energy savings.
5.1.2. Requirements
Modifications to control plane protocols to achieve network energy
efficiency SHOULD provide the ability to set the minimal bandwidth,
jitter, and delay through the network, and not shut down links or
devices that would violate those minimal requirements.
5.2. Energy Efficiency and Stretch
In any given network, there is a shortest path between any source and
any destination. Network protocols discover these paths from the
destination's perspective --routing draws traffic along a path,
rather than driving along a path. Along with the shortest path,
there are a number of paths that can also carry traffic from a given
source to a given destination without the packets passing along the
same logical link, or through the same logical device, more than
once. These are considered loop-free alternate [RFC5714] paths.
The primary difference between the shortest path and the loop-free
alternate paths is the total cost of using the path. In simple
terms, this difference can be calculated as the number of links and
devices a packet must pass through when being carried from the source
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to the destination --the hop count. While most networks use much
more sophisticated metrics based on bandwidth, congestion, and other
factors, the hop count will stand in as the only metric used
throughout this document.
When the control plane causes traffic to pass from the source to the
destination along a path which is longer than the shortest path, the
network is said to have stretch (see [Krioukov] for a more in depth
explanation of network stretch). To measure stretch, simply subtract
the metric of the shortest path from the metric of the longer path.
For example, in hop count terms, if the best path is three hops, and
the current path is four hops, the network exhibits a stretch of 1.
5.2.1. An Example of Stretch
In the network illustrated above, if a modification is made to the
control plane in order to remove the link between R1 and R4 in order
to save energy, all the destinations shown in the diagram remain
reachable. However, from the perspective of R1, the best path
available to reach R2 has increased in length by two hops. The
original path is R1->R2, the new path is R1->R3->R4->R2. This
represents a stretch of 2.
Along with this increased stretch will most likely also come
increased delay through the network; each hop in the network
represents a measurable amount of delay. This increased stretch
might also represent an increased amount of jitter, as there are more
queues and more serialization events in the path of each packet
carried. There will also be the modifications in jitter as the
network switches between the optimal performance configuration and an
energy efficient configuration.
5.2.2. Requirements
Designers who propose modifications to control plane protocols to
achieve network energy efficiency SHOULD analyze the impact of their
mechanisms on the stretch in typical network topologies, and SHOULD
include such analysis when explaining the applicability of their
proposals. This analysis may include an examination of the absolute,
or maximum, stretch caused by the modifications to the control plane
as well as analysis at the 95th percentile, the average stretch
increase in a given set of topologies, and/or the mean increase in
stretch.
Mechanisms that could impact the stretch of a network SHOULD provide
the ability for the network administrator to limit the amount of
stretch the network will encounter when moving into a more energy
efficient mode.
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5.3. Energy Efficiency and Fast Recovery
A final area where modifications to the control plane for energy
efficiency is fast convergence or fast recovery. Many networks are
now designed to recover from failures quickly enough to only cause a
handful of traffic to be lost; recovery on the order of half a second
is not an uncommon goal. It should be obvious that removing
redundant links and devices from the network to reduce energy
consumption could adversely affect these goals.
5.3.1. An Example of Impact on Fast Recovery
In the network shown, assume R2 and its associated links are removed
from the topology in order to save energy. Rather than this second
path being available for immediate recovery on the failure of the
R1->R3 link, some process must be followed to bring R2 and its
associated links back up, reinject them into the topology, and
finally begin routing traffic across this path.
In many situations, only links and devices which are a "third point
of failure" may be acceptable as removal candidates in order to
conserve energy.
5.3.2. Requirements
Modifications to the control plane in order to remove links or nodes
to conserve energy SHOULD entail the ability to choose the level of
redundancy available after the network topology has been trimmed.
For instance, it might be acceptable in some situations to move to
single points of failure throughout the network, or in specific
sections of the network, for certain periods of time. In other
situations, it may only be acceptable to reduce the network to a
double point of failure, and never to a single point of failure.
The complete removal of nodes or links from the network topology has
several impacts on the control plane which must be considered. In
these cases, the control plane must:
o Modify the network topology so removed links or devices are not
used to forward traffic
o Remember that such links exist, possibly including the neighbors
and destinations reachable through those links or devices
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5.4. Introducing Jitter Through Microsleeps
One proposed mechanism to reduce energy usage in a network is to
sleep links or devices for very short periods of time, called
microsleeps. For instance, if a particular link is only used at 50%
of the actual available bandwidth, it should be possible to place the
link in some lower power state for 50% of the time, thus reducing
energy usage by something percentage.
Such schemes introduce delay and jitter into the network path
directly; if a packet arrives while the link to the next hop, or the
next hop itself, is in a reduced energy state, the packet must wait
until the link or next hop device enter a normal operational mode
before it can be forwarded. Most of the time the proposed sleep
states are so small as to be presumably inconsequential on overall
packet delay, but multiple packets crossing a series of links, each
encountering different links in different states, could take very
different amounts of time to pass along the path.
One possible way to resolve this somewhat random accrual of delays on
a per packet basis is to coordinate these sleep states such that
packets accepted at the entry of the network are consistently passed
through the network when all links and devices are in a normal
operating mode, and simply delaying all packets at the entry point
into the network while the devices in the network are in some energy
reduced state. This solution still introduces some amount of jitter;
some packets will be delayed by the sleep state at the edge of the
network, while others will not. This solution also requires
coordinated timers at the speed of forwarding itself to effectively
control the sleep and wake cycles of the network.
5.4.1. An Example of Microsleeps to Reduce Energy Usage
In the example network, assume the bandwidth utilization along the
path R1->R2->R4->R5 is 50% of the actual available bandwidth along
this path. It is possible to consider a scheme where R1->R2, R2,
R2->R4, and R4->R5 are all put into some energy reduced operational
mode 50% of the time, since packets are only available to send 50% of
the time. A packet entering at R1 may encounter a short delay at
R1->R2, at R2->R4, and at R4->R5, or it might not. Even if these
delays are very small, say 200ms at each hop, the accumulated delay
through the network due to sleep states may be 0ms (all links and
devices awake) or 600ms (all links and devices asleep) as the packet
passes through the network.
As network paths lengthen to more realistic path lengths in real
deployments, the jitter introduced varies more widely, which could
cause problems for the operation of a number of applications.
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5.4.2. Requirements
Protocol designers SHOULD analyze the impact of accumulated jitter
when proposing mechanisms that rely on microsleeps in either
equipment or links. This analysis SHOULD include both worst case and
best case scenarios, as well as an analysis of how coordinated clocks
are to be handled in the case of coordinated sleep states.
5.5. Other Operational Aspects
Modification of the network topology in order to save energy needs to
consider the operational needs of the network as well as application
requirements. Change management, operational downtime, and business
usage of the network need to be considered when determining which
links and nodes should be placed into a low energy state. Energy
provisions have to be assigned and changed for nodes and links,
optimally according to network usage profiles over the time of day.
Control plane protocol operation, in terms of operational efficiency
on the wire, also needs to be considered when modifying protocol
parameters. Any changes that negatively impact the operation of the
protocol, in terms of the amount of traffic, the size of routing
information transmitted over the network, and interaction with
network management operations need to be carefully analyzed for
scaling and operational implications.
5.5.1. An Example of Operational Impact
Time of day is an important consideration in business operations.
During normal operational hours, the network needs to be fully
available, including all available redundancy and bandwidth. During
holidays, night hours, and other times when a campus might not be
used, or when there are lower traffic and resiliency demands on the
network, network elements can be removed to reduce energy usage.
5.5.2. Requirements
Protocol designers SHOULD analyze operational requirements, such as
time of day and network traffic load considerations, and explain how
proposed protocols or modifications to protocols will interact with
these types of requirements. Protocols designers SHOULD analyze
increases in network traffic and the operational efficiency impact of
proposed changes or protocols.
6. Security Considerations
None.
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7. Acknowledgements
The authors of this document would like to acknowledge the
suggestions and ideas provided by Sujata Banerjee, Puneet Sharma and
Dirk Von Hugo.
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.
8.2. Informative References
[3GPP] 3GPP, "3GPP TR 25-927 Solutions for energy saving within
UTRA Node B", 2011,
<http://3gpp.org/ftp/Specs/html-info/25927.htm>.
[EMAN] IETF, "Energy Management Working Group Charter", 2012,
<http://datatracker.ietf.org/wg/eman/charter/>.
[Krioukov]
Krioukov, D., "On Compact Routing for the Internet", 2007,
<http://www.caida.org/publications/papers/2007/
compact_routing/>.
[MANET] IETF, "Mobile Ad Hoc Networks Charter", 2012,
<http://datatracker.ietf.org/wg/manet/charter/>.
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
RFC 5714, January 2010.
[ROLL] IETF, "Routing Over Lowe power and Lossy networks
Charter", 2012,
<http://datatracker.ietf.org/wg/roll/charter/>.
Appendix A. Change Log
A.1. Changes between the -00 and -01 versions.
o Updated authors' contact information.
o Modified some of the rfc2119 keywords.
Retana, et al. Expires April 27, 2015 [Page 15]
Internet-Draft Energy Aware Control Plane October 2014
A.2. Changes between the -01 and -02 versions.
o Updated authors' contact information.
A.3. Changes between the -02 and -03 versions.
o Updated authors' contact information.
Authors' Addresses
Alvaro Retana
Cisco Systems, Inc.
7025 Kit Creek Rd.
Raleigh, NC 27709
USA
Email: aretana@cisco.com
Russ White
Ericsson
Email: russw@riw.us
Manuel Paul
Deutsche Telekom AG
Winterfeldtstr. 21-27
Berlin 10781
Germany
Email: Manuel.Paul@telekom.de
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