Network Working Group S. Kiesel, Ed.
Internet-Draft NEC
Intended status: Informational L. Popkin
Expires: September 10, 2009 Pando Networks, Inc.
S. Previdi
Cisco Systems, Inc.
R. Woundy
Comcast Corporation
Y R. Yang
Yale University
March 9, 2009
Application-Layer Traffic Optimization (ALTO) Requirements
draft-kiesel-alto-reqs-02.txt
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Abstract
Many Internet applications are used to access resources, such as
pieces of information or server processes, which are available in
several equivalent replicas on different hosts. This includes, but
is not limited to, peer-to-peer file sharing applications. The goal
of Application-Layer Traffic Optimization (ALTO) is to provide
guidance to applications, which have to select one or several hosts
from a set of candidates, that are able to provide a desired
resource. This guidance shall be based on parameters that affect
performance and efficiency of the data transmission between the
hosts, e.g., the topological distance. The ultimate goal is to
improve performance (or Quality of Experience) in the application
while reducing resource consumption in the underlying network
infrastructure.
This document enumerates requirements for ALTO, which should be
considered when specifying, assessing, or comparing protocols and
implementations, and it solicits feedback and discussion.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Terminology and architectural framework . . . . . . . . . . . 6
2.1. Requirements notation . . . . . . . . . . . . . . . . . . 6
2.2. ALTO terminology . . . . . . . . . . . . . . . . . . . . . 6
2.3. Architectural framework for ALTO . . . . . . . . . . . . . 6
2.4. Sample use cases . . . . . . . . . . . . . . . . . . . . . 7
3. ALTO requirements . . . . . . . . . . . . . . . . . . . . . . 9
3.1. ALTO client protocol . . . . . . . . . . . . . . . . . . . 9
3.1.1. General requirements . . . . . . . . . . . . . . . . . 9
3.1.2. Protocol semantics . . . . . . . . . . . . . . . . . . 9
3.1.3. Error handling and overload protection . . . . . . . . 11
3.2. ALTO server discovery . . . . . . . . . . . . . . . . . . 11
3.3. Security and privacy . . . . . . . . . . . . . . . . . . . 12
4. Host location attributes . . . . . . . . . . . . . . . . . . . 13
5. Rating criteria . . . . . . . . . . . . . . . . . . . . . . . 14
5.1. Distance-related rating criteria . . . . . . . . . . . . . 14
5.2. Charging-related rating criteria . . . . . . . . . . . . . 15
5.3. Performance-related rating criteria . . . . . . . . . . . 15
5.4. Inappropriate rating criteria . . . . . . . . . . . . . . 16
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1. Normative References . . . . . . . . . . . . . . . . . . . 19
8.2. Informative References . . . . . . . . . . . . . . . . . . 19
Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 20
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
The motivation for Application-Layer Traffic Optimization (ALTO) is
described in the ALTO problem statement:
"Peer-to-peer applications, such as file sharing, real-time
communication, and live media streaming, use a significant amount of
Internet resources. Such applications often transfer large amounts
of data in direct peer-to-peer connections. However, they usually
have little knowledge of the underlying network topology. As a
result, they may choose their peers based on measurements and
statistics that, in many situations, may lead to suboptimal choices."
[I-D.marocco-alto-problem-statement].
The goal of ALTO is to provide information which can help peer-to-
peer (P2P) applications to make better decisions with respect to peer
selection. However, ALTO may be useful for non-P2P applications as
well. For example, clients of client-server applications may use
information provided by ALTO to select one of several servers or
information replicas. As another example, ALTO information could be
used to select a media relay needed for NAT traversal. The goal of
these informed decisions is to improve performance (or Quality of
Experience) in the application while reducing resource consumption in
the underlying network infrastructure.
Usually, it would be difficult or even impossible for application
entities to acquire this information by other mechanisms (e.g., using
measurements between the peers of a P2P overlay), because of
complexity or because it is based on network topology information,
network operational costs, or network policies, which the respective
network provider does not want to disclose in detail.
The logical entities that provide the ALTO service do not take part
in the actual user data transport, i.e., they do not implement
functions for relaying user data. They may be placed on various
kinds of physical nodes, e.g., on dedicated servers, as auxiliary
processes in routers, on "trackers" or "super peers" of a P2P
application operated by the network provider, etc.
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2. Terminology and architectural framework
2.1. Requirements notation
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].
2.2. ALTO terminology
This document uses the following ALTO-related terms, which are
defined in [I-D.marocco-alto-problem-statement]:
Application, Overlay Network, Application protocol, Peer, P2P,
Resource, Resource Identifier, Resource Provider, Resource Consumer,
Resource Directory, Transport Address, Host Location Attribute, ALTO
Service, ALTO Server, ALTO Client, ALTO Client Protocol, ALTO Query,
ALTO Reply, ALTO Transaction, Provisioning protocol, Inter ALTO-
Server Protocol, Local Traffic, Peering Traffic, Transit Traffic.
2.3. Architectural framework for ALTO
There are various architectural options how ALTO could be
implemented, and specifying or mandating one specific architecture is
out of the scope of this document.
The ALTO Working Group Charter [ALTO-charter] itemizes several key
components, which shall be elaborated and specified by the ALTO
Working Group. The ALTO problem statement
[I-D.marocco-alto-problem-statement] defines a terminology (see
Section 2.2) and presents a figure that gives a high-level overview
of protocol interaction between ALTO elements.
This document itemizes requirements for the following components of
the abovementioned architecture:
o The ALTO client protocol, which is used for sending ALTO queries
and ALTO replies between ALTO client and ALTO server.
o The discovery mechanism, which will be used by ALTO clients in
order to find out where to send ALTO requests.
o The overall architecture, especially with respect to security and
privacy issues.
Furthermore, this document describes the following data structures,
which might be used in the ALTO client protocol:
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o Host location attributes, which are used to describe the location
of a host in the network topology.
o Rating criteria, i. e., properties that shall be evaluated in
order to generate the ALTO guidance.
Requirements regarding other components are not considered in the
current version of this document, but may be added later.
2.4. Sample use cases
The ALTO problem statement [I-D.marocco-alto-problem-statement]
presents a figure that gives a high-level overview of protocol
interaction between ALTO elements. The following figures are
somewhat more elaborated and extended versions of it, in order to
give some non-normative examples of ALTO usage. It can also be seen
that, in some use cases, some of the requirements presented in later
sections are more relevant than in others.
Figure 1 shows an ALTO use case with a DHT-based P2P application.
Using this distributed lookup mechanism, a peer can figure out which
other peers are candidate resource providers for a desired resource.
Every peer software includes an ALTO client, in order to request and
receive guidance on peer selection from the ALTO servers.
From an ALTO perspective this means that the ALTO servers will
receive ALTO queries from a rather large number of different ALTO
clients. The performance of many clients and their Internet
connectivity may be rather limited and therefore, this puts certain
restrictions on the amount of guiding data that can be sent to them.
Furthermore, the privacy-sensitive IP addresses of the peers are
visible to the (operators of the) ALTO servers, as these are also the
source addresses of the ALTO query messages.
Figure 2 shows an ALTO use case with a P2P application that makes use
of a centralized resource directory (in some specific P2P
implementations called a "tracker"). In this scenario the ALTO
servers receive queries only from few entities, i.e., the resource
directories. As these resource directories must be powerful machines
anyway, it may be reasonable to send large amounts of ALTO guidance
data to them, which will be cached there. Furthermore, in this
scenario it may be possible to hide the exact addresses of the peers
from the ALTO server.
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+-----+
=====| |**
==== +-----+ *
==== * *
==== * *
+-----+ +------+===== +-----+ *
| |.....| |======================| | *
+-----+ +------+===== +-----+ *
Source of ALTO ==== * *
topological service ==== * *
information ==== +-----+ *
=====| |**
+-----+
Legend:
=== ALTO client protocol
*** Application protocol
... Provisioning protocol
Figure 1: Overview of protocol interaction between ALTO elements,
scenario without resource directory
+-----+
**| |**
** +-----+ *
** * *
** * *
+-----+ +------+ +-----+** +-----+ *
| |.....| |=====| |**********| | *
+-----+ +------+ +-----+** +-----+ *
Source of ALTO Resource ** * *
topological service directory ** * *
information ("tracker") ** +-----+ *
**| |**
+-----+
Peers
Legend:
=== ALTO client protocol
*** Application protocol
... Provisioning protocol
Figure 2: Overview of protocol interaction between ALTO elements,
scenario with resource directory
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3. ALTO requirements
3.1. ALTO client protocol
3.1.1. General requirements
REQ. RQv02-1: The ALTO service is provided by one or more ALTO
servers. ALTO servers MUST implement the ALTO client protocol, for
receiving ALTO queries from ALTO clients and for sending the
corresponding ALTO replies.
REQ. RQv02-2: ALTO clients MUST implement the ALTO client protocol,
for sending ALTO queries to ALTO servers and for receiving the
corresponding ALTO replies.
REQ. RQv02-3: The detailed specification of a protocol is out of the
scope of this document. However, any protocol specification that
claims to implement the ALTO client protocol MUST be compliant to the
requirements itemized in this document.
3.1.2. Protocol semantics
REQ. RQv02-4: The format of the ALTO query message MUST allow the
ALTO client to solicit guidance for selecting appropriate resource
providers.
REQ. RQv02-5: The ALTO guidance is be based on the evaluation of one
or several rating criteria (see Section 5). The ALTO query message
SHOULD allow the ALTO client to express which rating criteria should
be considered, as well as their relative relevance for the specific
application that will eventually make use of the guidance.
REQ. RQv02-6: The format of the ALTO reply message MUST allow the
ALTO server to express his guidance for selecting appropriate
resource providers.
With respect to the placement of ALTO clients, several modes of
operation exist:
o One mode of ALTO operation is that ALTO clients may be embedded
directly in the resource consumer (e.g., peer of a DHT-based P2P
application), which wants to access a resource.
o Another mode of operation is to perform ALTO queries indirectly,
via resource directories (e.g., tracker of a P2P application),
which may issue ALTO queries to solicit preference on potential
resource providers, considering the respective resource consumer.
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REQ. RQv02-7: The ALTO client protocol MUST support the mode of
operation, in which the ALTO client is directly embedded in the
resource consumer.
REQ. RQv02-8: The ALTO client protocol MUST support the mode of
operation, in which the ALTO client is embedded in the resource
directory.
With respect to the timing of ALTO queries, several modes of
operation exist:
o In target-aware query mode, an ALTO client performs the ALTO query
when the desired resource and a set of candidate resource
providers are already known, i. e., after DHT lookups, queries to
the resource directory, etc.
o In target-independent query mode, ALTO queries are performed in
advance or periodically, in order to receive "target-independent"
guidance, which will be cached locally and evaluated later, when a
resource is to be accessed.
REQ. RQv02-9: The ALTO client protocol MUST support at least one of
these two modes, either the target-aware or the target-independent
query mode.
REQ. RQv02-10: The ALTO client protocol SHOULD support both the
target-aware and the target-independent query mode.
REQ. RQv02-11: The ALTO client protocol SHOULD support lifetime
attributes, to enable caching of recommendations at ALTO clients.
REQ. RQv02-12: The ALTO client protocol SHOULD specify an aging
mechanism, which allows to give newer recommendations precedence over
older ones.
REQ. RQv02-13: The ALTO client protocol MUST be designed in a way
that the ALTO service can be provided by an operator which is not the
operator of the IP access network.
REQ. RQv02-14: The ALTO client protocol MUST be designed in a way
that different instances of the ALTO service operated by different
providers can coexist.
REQ. RQv02-15: The ALTO client protocol MUST include support for
adding protocol extensions in a non-disruptive, backward-compatible
way.
REQ. RQv02-16: The ALTO client protocol MUST include protocol
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versioning support, in order to clearly distinguish between
incompatible major versions of the protocol.
3.1.3. Error handling and overload protection
REQ. RQv02-17: Any application designed to use ALTO MUST also work
if no ALTO servers can be found or if no responses to ALTO queries
are received, e.g., due to connectivity problems or overload
situation.
REQ. RQv02-18: The ALTO client protocol MUST use TCP based
transport.
REQ. RQv02-19: An ALTO server, which is operating close to its
capacity limit, MUST be able to inform clients about its impending
overload situation, and require them to throttle their query rate.
REQ. RQv02-20: An ALTO server, which is operating close to its
capacity limit, MUST be able to inform clients about its impending
overload situation, and redirect them to another ALTO server.
REQ. RQv02-21: An ALTO server, which is operating close to its
capacity limit, MUST be able to inform clients about its impending
overload situation, and terminate the converstation with the ALTO
client.
REQ. RQv02-22: An ALTO server, which is operating close to its
capacity limit, MUST be able to inform clients about its impending
overload situation, and reject new conversation attempts.
3.2. ALTO server discovery
REQ. RQv02-23: ALTO clients MUST be able to use the ALTO server
discovery mechanism, in order to find out where to send ALTO queries.
REQ. RQv02-24: The ALTO server discovery mechanism SHOULD be able to
return the respective contact information for several ALTO servers.
REQ. RQv02-25: The ALTO server discovery mechanism SHOULD be able to
indicate preferences for each returned ALTO server contact
information.
REQ. RQv02-26: The ALTO server discovery mechanism SHOULD be
independent of specific link-layer protocols or access network
architectures. For example, many broadband access networks use DHCP
for configuration, while others use PPPoE. In contrast, DNS is
available in virtually all Internet access networks.
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3.3. Security and privacy
REQ. RQv02-27: The ALTO client protocol MUST support mechanisms for
mutual authentication and authorization of ALTO clients and servers.
REQ. RQv02-28: The ALTO client protocol MUST support different
levels of detail in queries and responses, in order for the operator
of an ALTO service to be able to control how much information (e.g.,
about the network topology) is disclosed.
REQ. RQv02-29: The ALTO client protocol MUST support different
levels of detail in queries and responses, in order to protect the
privacy of users, to ensure that the operators of ALTO servers and
other users of the same application cannot derive sensitive
information.
REQ. RQv02-30: The ALTO client protocol SHOULD be defined in a way,
that the operator of one ALTO server cannot easily deduce the
resource identifier (e.g., file name in P2P file sharing) which the
resource consumer seeking ALTO guidance wants to access.
REQ. RQv02-31: The ALTO protocol MUST include appropriate mechanisms
to protect the ALTO service against DoS attacks.
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4. Host location attributes
Host location attributes are used in the ALTO client protocol to
describe the location of a host in the network topology. The
following list gives an overview on such attributes that have been
proposed in the past, or which are in use by by ALTO-related
prototype implementations.
One possible way forward is to define the syntax and semantics of a
mandatory set of attributes, which have to be understood by all
entities that implement the ALTO client protocol. Furthermore,
defining a set of optional attributes, as well as a procedure for
allocating new attributes (e.g., an IANA registry) may be required.
However, there was no broad discussion of this issue so far and no
consensus has been reached. Therefore, the only purpose of the
following list is to document the attributes that have been proposed
so far, and to solicit further feedback and discussion:
o IP address or range of IP addresses (in CIDR notation)
o Autonomous System (AS) number
o Protocol-specific group identifiers, which expand to a set of IP
address ranges (CIDR) and/or AS numbers. In one specific solution
proposal, these are called Partition ID (PID).
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5. Rating criteria
Rating criteria are used in the ALTO client protocol to express
topology- or connectivity-related properties, which are evaluated in
order to generate the ALTO guidance. The following list gives an
overview on such rating criteria that have been proposed in the past,
or which are in use by by ALTO-related prototype implementations.
One possible way forward is to define the syntax and semantics of a
mandatory set of criteria, which have to be understood by all
entities that implement the ALTO client protocol. Furthermore,
defining a set of optional criteria, as well as a procedure for
allocating new criteria (e.g., an IANA registry) may be required.
However, there was no broad discussion of this issue so far and no
consensus has been reached. Therefore, the only purpose of the
following list is to document the attributes that have been proposed
so far, and to solicit further feedback and discussion.
5.1. Distance-related rating criteria
o Relative topological distance: relative means that a larger
numerical value means greater distance, but it is up to the ALTO
service how to compute the values, and the ALTO client will not be
informed about the nature of the information. One way of
generating this kind of information MAY be counting AS hops, but
when querying this parameter, the ALTO client MUST NOT assume that
the numbers actually are AS hops.
o Absolute topological distance, expressed in the number of
traversed autonomous systems (AS).
o Absolute topological distance, expressed in the number of router
hops (i.e., how much the TTL value of an IP packet will be
decreased during transit).
o Absolute physical distance, based on knowledge of the approximate
geolocation (continent, country) of an IP address.
o Relative operator's preference: higher numerical value indicates
that the application should prefer this candidate resource
provider over others with lower values (if no other reasons speak
against it, such as probed througput). Again, as this is a
relative measure, the ALTO service does not have to indicate how
the values have been computed. Examples could be: cost for
peering or transit traffic, traffic engineering inside the own
network, and other policies.
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5.2. Charging-related rating criteria
o Traffic volume caps, in case the Internet access of the resource
consumer is not charged by "flat rate". For each candidate
resource provider, the ALTO service could indicate the amount of
data that may be transferred from/to this resource provider until
a given point in time, and how much of this amount has already
been consumed. Furthermore, it would have to be indicated how
excess traffic would be handled (e.g., blocked, throtteled, or
charged separately at an indicated price). The interaction of
several applications running on a host, out of which some use this
attribute while others don't, as well as the evaluation of this
attribute in resource directories, which issue ALTO queries on
behalf of other peers, are for further study.
5.3. Performance-related rating criteria
The following rating criteria are subject to the remarks below.
o The minimum achievable throughput between the resource consumer
and the candidate resource provider, which is considered useful by
the application (only in ALTO queries), or
o An arbitrary upper bound for the througput from/to the candidate
resource provider (only in ALTO replies). This may be, but is not
necessarily the provisioned access bandwith of the candidate
resource provider.
o The maximum round-trip time (RTT) between resource consumer and
the candidate resource provider, which is acceptable for the
application for useful communication with the candidate resource
provider (only in ALTO queries), or
o An arbitrary lower bound for the RTT between resource consumer and
the candidate resource provider (only in ALTO replies). This may
be, for example, based on measurements of the propagation delay in
a completely unloaded network.
The ALTO client MUST be aware, that with high probability, the actual
performace values differ significantly from these upper and lower
bounds. In particular, an ALTO client MUST NOT consider the "upper
bound for througput" parameter as a permission to send data at the
indicated rate without using congestion control mechanisms.
The discrepancies are due to various reasons, including, but not
limited to the facts that
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o the ALTO service is not an admission control system
o the ALTO service may not know the instantaneous congestion status
of the network
o the ALTO service may not know all link bandwidths, i.e., where the
bottleneck really is, and there may be shared bottlenecks
o the ALTO service may not know whether the candidate peer itself is
overloaded
o the ALTO service may not know whether the candidate peer throttles
the bandwidth it devotes for the considered application
o the ALTO service may not know whether the candidate peer will
throttle the data it sends to us (e.g., because of some fairness
algorithm, such as tit-for-tat)
Because of these inacuracies and the lack of complete, instantaneous
state information, which are inherent to the ALTO service, the
application must use other mechanisms (such as passive measurements
on actual data transmissions) to assess the currently achievable
throughput, and it MUST use appropriate congestion control mechanisms
in order to avoid a congestion collapse. Nevertheless, these rating
criteria may provide a useful shortcut for quickly excluding
candidate resource providers from such probing, if it is known in
advance that connectivity is in any case worse than what is
considered the minimum useful value by the respective application.
5.4. Inappropriate rating criteria
Rating criteria that SHOULD NOT be defined for and used by the ALTO
service include:
o Performance metrics that are closely related to the instantaneous
congestion status. The definition of alternate approaches for
congestion control is explicitly out of the scope of ALTO.
Instead, other appropriate means, such as using TCP based
transport, have to be used to avoid congestion.
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6. IANA Considerations
This requirements document does not mandate any immediate IANA
actions. However, such IANA considerations may arise from future
ALTO specification documents which try to meet the requirements given
here.
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7. Security Considerations
High-level security considerations for the ALTO service can be found
in the "Security Considerations" section of the ALTO problem
statement [I-D.marocco-alto-problem-statement]. For a set of
specific security requirements please refer to Section 3.3 of this
document.
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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
[ALTO-charter]
Marocco, E. and V. Gurbani, "Application-Layer Traffic
Optimization (ALTO) Working Group Charter", February 2009.
[I-D.marocco-alto-problem-statement]
Seedorf, J. and E. Burger, "Application-Layer Traffic
Optimization (ALTO) Problem Statement",
draft-marocco-alto-problem-statement-05 (work in
progress), March 2009.
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Appendix A. Contributors
The authors were supported by the following people, who have
contributed to this document:
o Zoran Despotovic <despotovic@docomolab-euro.com>
o Jason Livingood <Jason_Livingood@cable.comcast.com>
o Saverio Niccolini <saverio.niccolini@nw.neclab.eu>
o Jan Seedorf <jan.seedorf@nw.neclab.eu>
o Martin Stiemerling <martin.stiemerling@nw.neclab.eu>
The authors would like to thank the members of the P2PI and ALTO
mailing lists for their feedback.
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Appendix B. Acknowledgments
The authors would like to thank
o Vijay K. Gurbani <vkg@alcatel-lucent.com>
o Enrico Marocco <enrico.marocco@telecomitalia.it>
for fostering discussions that lead to the creation of this document,
and for giving valuable comments on it.
Sebastian Kiesel, Saverio Niccolini, Jan Seedorf, and Martin
Stiemerling are partially supported by the NAPA-WINE project
(Network-Aware P2P-TV Application over Wise Networks,
http://www.napa-wine.org), a research project supported by the
European Commission under its 7th Framework Program (contract no.
214412). The views and conclusions contained herein are those of the
authors and should not be interpreted as necessarily representing the
official policies or endorsements, either expressed or implied, of
the NAPA-WINE project or the European Commission.
Laird Popkin and Y. Richard Yang are grateful to the many
contributions made by the members of the P4P working group and Yale
Laboratory of Networked Systems. The P4P working group is hosted by
DCIA.
Kiesel, et al. Expires September 10, 2009 [Page 21]
Internet-Draft ALTO Requirements March 2009
Authors' Addresses
Sebastian Kiesel (editor)
NEC Europe Ltd., Network Laboratories Europe
Kurfuersten-Anlage 36
Heidelberg 69115
Germany
Phone: +49 6221 4342 232
Email: sebastian.kiesel@nw.neclab.eu
Laird Popkin
Pando Networks, Inc.
Email: laird@pando.com
Stefano Previdi
Cisco Systems, Inc.
Email: sprevidi@cisco.com
Richard Woundy
Comcast Corporation
Email: Richard_Woundy@cable.comcast.com
Yang Richard Yang
Yale University
Email: yry@cs.yale.edu
Kiesel, et al. Expires September 10, 2009 [Page 22]