Internet Engineering Task Force (IETF) S. Kiesel, Ed.

Internet Engineering Task Force (IETF) S. Kiesel, Ed.
Request for Comments: 6708 University of Stuttgart
Category: Informational S. Previdi
ISSN: 2070-1721 Cisco Systems, Inc.
M. Stiemerling
NEC Europe Ltd.
R. Woundy
Comcast Corporation
Y. Yang
Yale University
September 2012

Application-Layer Traffic Optimization (ALTO) Requirements

Abstract

Many Internet applications are used to access resources, such as
pieces of information or server processes that 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 that have to select one or several hosts
from a set of candidates capable of providing 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 the
utilization of the underlying network infrastructure.

This document enumerates requirements for specifying, assessing, or
comparing protocols and implementations.

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 a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6708.

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Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Architectural Framework . . . . . . . . . . . 3
2.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3
2.2. ALTO Terminology . . . . . . . . . . . . . . . . . . . . . 3
2.3. Architectural Framework for ALTO . . . . . . . . . . . . . 5
3. ALTO Requirements . . . . . . . . . . . . . . . . . . . . . . 5
3.1. ALTO Client Protocol . . . . . . . . . . . . . . . . . . . 5
3.1.1. General Requirements . . . . . . . . . . . . . . . . . 5
3.1.2. Host-Group Descriptor Support . . . . . . . . . . . . 6
3.1.3. Rating Criteria Support . . . . . . . . . . . . . . . 7
3.1.4. Placement of Entities and Timing of Transactions . . . 9
3.1.5. Protocol Extensibility . . . . . . . . . . . . . . . . 11
3.1.6. Error Handling and Overload Protection . . . . . . . . 11
3.2. ALTO Server Discovery . . . . . . . . . . . . . . . . . . 12
3.3. Security and Privacy . . . . . . . . . . . . . . . . . . . 13
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
5. Security Considerations . . . . . . . . . . . . . . . . . . . 14
5.1. High-Level Security Considerations . . . . . . . . . . . . 14
5.2. Information Disclosure Scenarios . . . . . . . . . . . . . 14
5.2.1. Classification of Information Disclosure Scenarios . . 14
5.2.2. Discussion of Information Disclosure Scenarios . . . . 16
5.3. ALTO Server Discovery . . . . . . . . . . . . . . . . . . 18
5.4. Security Requirements . . . . . . . . . . . . . . . . . . 18
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.1. Normative References . . . . . . . . . . . . . . . . . . . 18
6.2. Informative References . . . . . . . . . . . . . . . . . . 18
Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 19
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . . 19

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1. Introduction

The motivation for Application-Layer Traffic Optimization (ALTO) is
described in the ALTO problem statement [RFC5693].

The goal of ALTO is to provide information that can help peer-to-peer
(P2P) applications 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 the utilization of 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 functional 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. These functional entities 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, etc.

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 [RFC5693]:

Application, Peer, P2P, Resource, Resource Identifier, Resource
Provider, Resource Consumer, Transport Address, Overlay Network,
Resource Directory, ALTO Service, ALTO Server, ALTO Client, ALTO

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Query, ALTO Response, ALTO Transaction, Local Traffic, Peering
Traffic, Transit Traffic, Application Protocol, ALTO Client Protocol,
and Provisioning Protocol.

Furthermore, the following additional terms will be used:

o Host-Group Descriptor: Information used to describe one or more
Internet hosts (such as the resource consumer that seeks ALTO
guidance, or one or more candidate resource providers) and their
location within the network topology. There can be several
different types of host-group descriptors, for example, a single
IP address, an address prefix or address range that contains the
host(s), or an Autonomous System (AS) number. Different host-
group descriptor types may provide different levels of detail.
Depending on the system architecture, this may have implications
on the quality of the guidance ALTO is able to provide, on whether
recommendations can be aggregated, and on how much privacy-
sensitive information about users might be disclosed to additional
parties.

o Rating Criterion: The condition or relation that defines the
"better" in "better-than-random peer selection", which is the
ultimate goal of ALTO. Examples may include "host's Internet
access is not subject to volume-based charging (flat rate)" or
"low topological distance". Some rating criteria, such as "low
topological distance", need to include a reference point, e.g.,
"low topological distance from a given resource consumer". This
reference point can be described by means of a host-group
descriptor.

o Host-Characteristics Attribute: Properties of a host, other than
the host-group descriptor. It may be evaluated according to one
or more rating criteria. This information may be stored in an
ALTO server and transmitted via an ALTO protocol. One example for
a host-characteristics attribute would be a data field indicating
whether a host's Internet access is subject to volume-based
charging or not (flat rate).

o Target-Aware Query Mode: In this mode of operation, an ALTO client
performs the ALTO query when the desired resource and a set of
candidate resource providers are already known, i.e., after
Distributed Hash Table (DHT) lookups, queries to the resource
directory, etc. To this end, the ALTO client transmits a list of
host-group descriptors and optionally one or more rating criteria
to the ALTO server. The ALTO server evaluates the host-group
descriptors according to the indicated criteria or a default

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criterion. It returns a list of these host-group descriptors to
the ALTO client, which is sorted according to the rating criteria
and/or enriched with host-characteristics attributes.

o Target-Independent Query Mode: In this mode of operation, ALTO
queries are performed in advance or periodically, in order to
receive comprehensive guidance. The ALTO client indicates the
desired host-characteristics attributes in the ALTO query. The
ALTO server answers with a list that indicates for all known host-
group descriptors (possibly subject to the server's policies) the
desired host-characteristics attributes. These lists will be
cached locally and evaluated later, when a resource is to be
accessed.

2.3. Architectural Framework for ALTO

There are various architectural options for ALTO implementation.
Specifying or mandating one specific architecture is out of the scope
of this document.

In addition to the terminology (see Section 2 of [RFC5693] and
Section 2.2 of this document), [RFC5693] presents a figure that gives
a high-level overview of protocol interaction between these
components.

This document itemizes requirements for the following components:
ALTO client protocols, ALTO server discovery mechanisms, host-group
descriptors, rating criteria, and host-characteristics attributes.
Furthermore, requirements regarding the overall architecture,
especially with respect to security and privacy issues, are
presented.

Note that the detailed specification of such protocols and mechanisms
is out of the scope of this document. In fact, this document does
not even assume that there will be only one single specification for
each of these components, respectively. However, this document
enumerates requirements for ALTO to be considered when specifying,
assessing, or comparing protocols and implementations.

3. ALTO Requirements

3.1. ALTO Client Protocol

3.1.1. General Requirements

Req. AR-1: The ALTO service is provided by one or more ALTO servers.
It may be queried by ALTO clients seeking guidance for selecting
appropriate resource providers. ALTO clients and ALTO servers MUST

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implement an ALTO client protocol. An ALTO client protocol MUST be
able to transmit ALTO queries from an ALTO client to an ALTO server,
and it MUST be able to transmit the corresponding ALTO replies from
the ALTO server to the ALTO client.

The detailed specification of an ALTO client protocol is out of the
scope of this document. In fact, this document does not even assume
that there will be only one single protocol specification. However,
this document enumerates requirements for ALTO, to be considered when
specifying, assessing, or comparing protocols and implementations.

Req. AR-2: An ALTO client protocol MUST provide adequate mechanisms
for operations and management support, as outlined in RFC 5706
[RFC5706].

3.1.2. Host-Group Descriptor Support

The ALTO guidance is based on the evaluation of several resource
providers or groups of resource providers, considering one or more
rating criteria. The resource providers or groups of resource
providers are characterized by means of host-group descriptors.

Req. AR-3: An ALTO client protocol MUST support the usage of multiple
host-group descriptor types.

Req. AR-4: ALTO clients and ALTO servers MUST clearly identify the
type of each host-group descriptor sent in ALTO queries or responses.
An ALTO protocol specification MUST provide appropriate protocol
elements.

Req. AR-5: An ALTO client protocol MUST support the host group
descriptor types "IPv4 address prefix" and "IPv6 address prefix".
They can be used to specify the IP address of one host, or an IP
address range (in Classless Inter-Domain Routing (CIDR) notation)
containing all hosts in question.

Req. AR-6: An ALTO client protocol MUST be extensible to enable
future support of other host-group descriptor types. An ALTO client
protocol specification MUST define an appropriate procedure for
adding new host-group descriptor types, e.g., by establishing an IANA
registry.

Req. AR-7: For host-group descriptor types other than "IPv4 address
prefix" and "IPv6 address prefix", the host-group descriptor type
identification MUST be supplemented by a reference to a facility that
can be used to translate host-group descriptors of this type to IPv4/
IPv6 address prefixes, e.g., by means of a mapping table or an
algorithm.

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Req. AR-8: Protocol functions for mapping other host-group descriptor
types to IPv4/IPv6 address prefixes SHOULD be designed and specified
as part of an ALTO client protocol, and the corresponding address
mapping information SHOULD be made available by the same entity that
wants to use these host-group descriptors within an ALTO client
protocol. However, an ALTO server or an ALTO client MAY also send a
reference to an external mapping facility, e.g., a translation table
to be obtained via an alternative mechanism.

Rationale for the previous two requirements: The preferred type of
host-group descriptors are IPv4 and IPv6 prefixes. However, in
some situations, one party may prefer to use another type, e.g.,
AS numbers. Usually, applications seeking ALTO guidance work with
IP addresses, e.g., when establishing connections. Understanding
guiding information that is based on other host-group descriptor
types, i.e., mapping from these other types to IP prefixes and
back, may be a non-trivial task. Therefore, before a party may
use other host-group descriptor types, they must provide a mapping
mechanism to IP prefixes.

Req. AR-9: An ALTO client protocol specification MUST define
mechanisms that can be used by the ALTO server to indicate that a
host-group descriptor used by the ALTO client is of an unsupported
type, or that the indicated mapping mechanism could not be used.

Req. AR-10: An ALTO client protocol specification MUST define
mechanisms that can be used by the ALTO client to indicate that a
host-group descriptor used by the ALTO server is of an unsupported
type, or that the indicated mapping mechanism could not be used.

3.1.3. Rating Criteria Support

Req. AR-11: An ALTO client protocol specification MUST define a
rating criterion that can be used to express and evaluate the
"relative operator's preference". This is a relative measure, i.e.,
it is not associated with any unit of measurement. A preferred
rating, according to this criterion, indicates that the application
should prefer the respective candidate resource provider over others
with less preferred ratings (unless information from non-ALTO sources
suggests a different choice, such as transmission attempts suggesting
that the path is currently congested). The operator of the ALTO
server does not have to disclose how and based on which data the
ratings are actually computed. Examples could be: cost for peering
or transit traffic, traffic engineering inside the network, and other
policies.

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Req. AR-12: An ALTO client protocol MUST be extensible to enable
future support of other rating criteria types. An ALTO client
protocol specification MUST define an appropriate procedure for
adding new rating criteria types, e.g., by establishing an IANA
registry.

Req. AR-13: ALTO client protocol specifications MUST NOT define
rating criteria closely related to the instantaneous network
congestion state, i.e., rating criteria that have the primary aim to
serve as an alternative to established congestion control strategies,
such as using TCP-based transport.

Req. AR-14: Applications using ALTO guidance MUST NOT rely solely on
the ALTO guidance to avoid causing network congestion. Instead, they
MUST use other appropriate means, such as TCP-based transport, to
avoid causing excessive congestion.

Rationale for the previous requirement: One design assumption for
ALTO is that it is acceptable for the host-characteristics
attributes, which are stored and processed in the ALTO servers for
giving guidance, to be updated rather infrequently. Typical
update intervals may be several orders of magnitude longer than
the typical network-layer packet round-trip time (RTT).
Therefore, ALTO cannot be a replacement for TCP-like congestion
control mechanisms.

Req. AR-15: In the target-independent query mode, the ALTO query
message SHOULD allow the ALTO client to express which host-
characteristics attributes should be returned.

Req. AR-16: In the target-aware query mode, the ALTO query message
SHOULD allow the ALTO client to express which rating criteria should
be considered by the server, as well as their relative relevance for
the specific application that will eventually make use of the
guidance. The corresponding ALTO response message SHOULD allow the
ALTO server to express which rating criteria have been considered
when generating the response.

Req. AR-17: An ALTO client protocol specification MUST define
mechanisms that can be used by the ALTO client and the ALTO server to
indicate that a rating criteria used by the other party is of an
unsupported type.

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3.1.4. Placement of Entities and Timing of Transactions

With respect to the placement of ALTO clients, several modes of
operation exist:

o One mode of ALTO operation is that an ALTO client may be embedded
directly in the resource consumer, i.e., the application protocol
entity that will eventually initiate data transmission to/from the
selected resource provider(s) in order to access the desired
resource. For example, an ALTO client could be integrated into
the peer of a P2P application that uses a distributed algorithm
such as "query flooding" for resource discovery.

o Another mode of operation is to integrate the ALTO client into a
third party, such as a resource directory. This third party may
issue ALTO queries to solicit preference on potential resource
providers, considering the respective resource consumer. For
example, an ALTO client could be integrated into the tracker of a
tracker-based P2P application, in order to request ALTO guidance
on behalf of the peers contacting the tracker.

Req. AR-18: An ALTO client protocol MUST support the mode of
operation in which the ALTO client is directly embedded in the
resource consumer.

Req. AR-19: An ALTO client protocol MUST support the mode of
operation in which the ALTO client is embedded in a third party.
This third party performs queries on behalf of resource consumers.

Req. AR-20: An ALTO client protocol MUST be designed in a way that
the ALTO service can be provided by an entity that is not the
operator of the underlying IP network.

Req. AR-21: An ALTO client protocol MUST be designed in a way that
different instances of the ALTO service operated by different
providers can coexist.

Req. AR-22: An ALTO client protocol specification MUST specify at
least one query mode, either the target-aware or the target-
independent query mode.

Note that this requirements document does not assume that there will
be only one single protocol specification.

Req. AR-23: An ALTO client protocol specification SHOULD specify both
the target-aware and the target-independent query mode. If an ALTO
client protocol specification specifies more than one query mode, it
MUST define at least one of these modes as REQUIRED to implement by

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ALTO clients and ALTO servers. Furthermore, it MUST specify an
appropriate protocol mechanism for negotiating between the ALTO
client and ALTO server, which query mode to use.

Req. AR-24: An ALTO client protocol SHOULD support version numbering,
TTL (time-to-live) attributes, and/or similar mechanisms in ALTO
transactions, in order to enable time validity checking for caching,
and to enable comparisons of multiple recommendations obtained
through redistribution.

Req. AR-25: An ALTO client protocol SHOULD allow the ALTO server to
add information about appropriate modes of reuse to its ALTO
responses. Reuse may include redistributing an ALTO response to
other parties, as well as using the same ALTO information in a
resource directory to improve the responses to different resource
consumers within the specified lifetime of the ALTO response. The
ALTO server SHOULD be able to express that

o no reuse should occur.

o reuse is appropriate for a specific "target audience", i.e., a set
of resource consumers explicitly defined by a list of host-group
descriptors. The ALTO server MAY specify a "target audience" in
the ALTO response that is only a subset of the known actual
"target audience", e.g., if required by operator policies.

o reuse is appropriate for any resource consumer that would send (or
cause a third party to send on behalf of it) the same ALTO query
(i.e., with the same query parameters, except for the resource
consumer ID, if applicable) to this ALTO server.

o reuse is appropriate for any resource consumer that would send (or
cause a third party to send on behalf of it) the same ALTO query
(i.e., with the same query parameters, except for the resource
consumer ID, if applicable) to any other ALTO server that was
discovered (using an ALTO discovery mechanism) together with this
ALTO server.

o reuse is appropriate for any resource consumer that would send (or
cause a third party to send on behalf of it) the same ALTO query
(i.e., with the same query parameters, except for the resource
consumer ID, if applicable) to any ALTO server in the whole
network.

Req. AR-26: An ALTO client protocol MUST support the transport of
ALTO transactions, even if the ALTO client is located in the private
address realm behind a network address translator (NAT). There are
different types of NAT, see [RFC4787] and [RFC5382].

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3.1.5. Protocol Extensibility

Req. AR-27: An ALTO client protocol MUST include support for adding
protocol extensions in a non-disruptive, backward-compatible way.

Req. AR-28: An ALTO client protocol MUST include protocol versioning
support, in order to clearly distinguish between incompatible
versions of the protocol.

3.1.6. Error Handling and Overload Protection

Req. AR-29: An ALTO client protocol MUST use congestion-aware
transport, e.g., by using TCP.

Req. AR-30: An ALTO client protocol specification MUST specify
mechanisms for an ALTO server to inform clients about an impending or
occurring overload situation, or how to leverage appropriate
mechanisms provided by underlying protocol layers. The mechanisms
MUST provide all of the following options to the server:

o terminate the conversation with the client,

o redirect the client to another ALTO server, and

o request that the client throttle its query rate.

In particular, a simple form of throttling is to let an ALTO
server answer a query with an error message advising the client to
retry the query later (e.g., using a protocol function such as
HTTP's Retry-After header ([RFC2616], Section 14.37)). Another
simple option is to actually answer the query with the desired
information, but adding an indication that the ALTO client should
not send further queries to this ALTO server before an indicated
period of time has elapsed.

Req. AR-31: An ALTO client protocol specification MUST specify
mechanisms for an ALTO server to inform clients about its inability
to answer queries due to technical problems or system maintenance, or
how to leverage appropriate mechanisms provided by underlying
protocol layers. The mechanisms MUST provide all of the following
options to the server:

o terminate the conversation with the client,

o redirect the client to another ALTO server, and

o request that the client retry the query later.

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Note: The existence of the above-mentioned protocol mechanisms does
not imply that an ALTO server must use them when facing an overload,
technical problem, or maintenance situation, respectively. Some
servers may be unable to use them in that situation, or they may
prefer to simply refuse the connection or not to send any answer at
all.

3.2. ALTO Server Discovery

An ALTO client protocol is supported by one or more ALTO server
discovery mechanisms, which may be used by ALTO clients to determine
one or more ALTO servers, to which ALTO requests can be sent. This
section enumerates requirements for an ALTO client, as well as
general requirements to be fulfilled by the ALTO server discovery
mechanisms.

Req. AR-32: An ALTO server discovery mechanism MUST support features
allowing ALTO clients that are embedded in the resource consumer to
find one or several ALTO servers that can provide ALTO guidance
suitable for the resource consumer, using an ALTO protocol version
compatible with the ALTO client. This mode of operation is called
"resource consumer initiated ALTO server discovery".

Req. AR-33: An ALTO server discovery mechanism MUST support features
allowing ALTO clients that are embedded in a resource directory and
perform third-party ALTO queries on behalf of a remote resource
consumer to find one or several ALTO servers that can provide ALTO
guidance suitable for the respective resource consumer, using an ALTO
protocol version compatible with the ALTO client. This mode of
operation is called "third-party ALTO server discovery".

Req. AR-34: ALTO clients MUST be able to perform resource consumer
initiated ALTO server discovery, even if they are located behind a
NAT.

Req. AR-35: ALTO clients MUST be able to perform third-party ALTO
server discovery, even if they are located behind a NAT.

Req. AR-36: ALTO clients MUST be able to perform third-party ALTO
server discovery, even if the resource consumer, on behalf of which
the ALTO query will be sent, is located behind a NAT.

Req. AR-37: ALTO server discovery mechanisms SHOULD leverage an
existing protocol or mechanism, such as DNS-, DHCP-, or PPP-based
automatic configuration, etc. A single mechanism with a broad
spectrum of applicability SHOULD be preferred over several different
mechanisms with narrower scopes.

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Req. AR-38: Every ALTO server discovery mechanism SHOULD be able to
return the respective contact information for multiple ALTO servers.

Req. AR-39: Every ALTO server discovery mechanism SHOULD be able to
indicate preferences for each returned ALTO server contact
information.

3.3. Security and Privacy

Note: The following requirements mandate the inclusion of certain
security mechanisms at a protocol specification level. Whether it
makes sense to enable these mechanisms in a given deployment scenario
depends on a threat analysis for this specific scenario. For a
classification of potential information disclosure risks, refer to
Section 5.2.

Req. AR-40: An ALTO client protocol specification MUST specify
mechanisms for the authentication of ALTO servers or specify how to
leverage appropriate mechanisms provided by underlying protocol
layers.

Req. AR-41: An ALTO client protocol specification MUST specify
mechanisms for the authentication of ALTO clients or specify how to
leverage appropriate mechanisms provided by underlying protocol
layers.

Req. AR-42: An ALTO client protocol specification MUST specify
mechanisms for the encryption of messages or specify how to leverage
appropriate mechanisms provided by underlying protocol layers.

Req. AR-43: An ALTO client is not required to implement mechanisms or
to comply with rules that limit its ability to redistribute
information retrieved from the ALTO server to third parties.

Req. AR-44: An 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. AR-45: An ALTO client protocol MAY include mechanisms that can
be used by the ALTO client when requesting guidance to specify the
resource (e.g., content identifiers) it wants to access. An ALTO
server MUST provide adequate guidance, even if the ALTO client
prefers not to specify the desired resource (e.g., keeps the data
field empty). The mechanism MUST be designed in a way that the
operator of the ALTO server cannot easily deduce the resource
identifier (e.g., file name in P2P file sharing) if the ALTO client
prefers not to specify it.

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Req. AR-46: An ALTO client protocol specification MUST specify
appropriate mechanisms for protecting the ALTO service against
Denial-of-Service (DoS) attacks or specify how to leverage
appropriate mechanisms provided by underlying protocol layers.

4. IANA Considerations

This requirements document does not mandate any immediate IANA
actions. However, such IANA considerations may arise from future
ALTO specification documents that try to meet the requirements given
here.

5. Security Considerations

5.1. High-Level Security Considerations

High-level security considerations for the ALTO service can be found
in the "Security Considerations" section of the ALTO problem
statement document [RFC5693].

5.2. Information Disclosure Scenarios

The unwanted disclosure of information is one key concern related to
ALTO. Neither the ALTO server nor a third party using or misusing
the ALTO service should be able to infer the application behavior or
correlate data in such a way that would violate user privacy, e.g.,
who is exchanging which files with whom using a P2P file-sharing
application. Furthermore, many network operators are concerned about
the amount of information related to their network infrastructure
(e.g., topology information, number of "premium customers", or
utilization statistics) that might be released through ALTO. This
section presents a classification and discussion of information
disclosure scenarios and potential countermeasures.

5.2.1. Classification of Information Disclosure Scenarios

The following issues may be considered a risk for the operator of an
ALTO server, depending on the specific deployment scenario:

(1) Excess disclosure of the ALTO server operator's data to an
authorized ALTO client. The operator of an ALTO server has to
feed information, such as tables mapping host-group descriptors
to host-characteristics attributes, into the server, thereby
enabling it to give guidance to ALTO clients. Some operators
might consider the full set of this information confidential
(e.g., a detailed map of the operator's network topology) and
might want to disclose only a subset of it or disclose somehow
obfuscated information to an ALTO client.

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(2) Disclosure of the ALTO server operator's data (e.g., network
topology information) to an unauthorized third party. There are
three subcases here:

(2a) An ALTO server receives and answers queries originating
from an unauthorized ALTO client.

(2b) An unauthorized party snoops on the data transmission from
the ALTO server to an authorized ALTO client.

(2c) An authorized ALTO client knowingly forwards the
information it has received from the ALTO server to an
unauthorized party.

(3) Excess retrieval of the ALTO server operator's data by
collaborating ALTO clients. Several authorized ALTO clients
could ask one or more ALTO servers for guidance, possibly
several times during an extended period of time, and
redistribute the responses among each other (see also case 2c).
By aggregating and correlating the ALTO responses, they could
find out more information than intended to be disclosed by the
ALTO server operator(s).

The following issues may be considered a risk for the user of an ALTO
client, depending on the specific deployment scenario:

(4) Disclosure of the application behavior or other user private
data to the (authorized) ALTO server. The operator of an ALTO
server could infer the application behavior (e.g., content
identifiers in P2P file sharing applications, or lists of
resource providers that are considered for establishing a
connection) from the ALTO queries sent by an ALTO client.

(5) Disclosure of the application behavior or other user private
data to an unauthorized third party. There are three subcases
here:

(5a) An ALTO client willingly sends queries directly to an
untrusted or malicious ALTO server, possibly due to a
forged response of the ALTO server discovery mechanism.

(5b) An unauthorized party snoops on the data transmission from
the ALTO client to an authorized ALTO server.

(5c) An authorized ALTO server knowingly forwards the
information it has received from the ALTO client to an
unauthorized party.

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(6) One or several collaborating (see case 5c) ALTO servers could
try to infer the application behavior or other user private data
by aggregating and correlating queries from one or more ALTO
clients, possibly over an extended period of time.

5.2.2. Discussion of Information Disclosure Scenarios

An ALTO server operator should consider:

o Issue (1) may be addressed by the ALTO server operator choosing
the level of detail of the information to be populated into the
ALTO server and returned in the responses. For example, by
specifying a broader address range (i.e., a shorter prefix length)
than a group of hosts in question actually uses, an ALTO server
operator may control to some extent how much information about the
network topology is disclosed. Furthermore, access control
mechanisms for filtering ALTO responses according to the
authenticated ALTO client identity might be installed in the ALTO
server, although this might not be effective given the lack of
efficient mechanisms for addressing (2c) and (3), see below.

o (2a) and (2b) may be addressed by authentication, access control,
and encryption schemes for the ALTO client protocol. However,
deployment of encryption schemes might not be effective given the
lack of efficient mechanisms for addressing (2c) and (3), see
below.

o Straightforward authentication and encryption schemes will not
help solving (2c) and (3), and there is no other simple and
efficient mechanism known. The cost of complex approaches, e.g.,
based on Digital Rights Management (DRM), might easily outweigh
the benefits of the whole ALTO solution; therefore, they are not
considered as a viable solution. That is, ALTO server operators
must be aware that (2c) and (3) cannot be prevented from
happening; therefore, they should feed only such data into an ALTO
server that they do not consider sensitive with respect to (2c)
and (3).

A user of an ALTO client should consider:

o Issue (4) can and needs to be addressed in several ways: If the
ALTO client is embedded in the resource consumer, the resource
consumer's IP address (or the "public" IP address of the outermost
NAT in front of the resource consumer) is disclosed to the ALTO
server as a matter of principle, because it is in the source
address fields of the IP headers. By using a proxy, the
disclosure of source addresses to the ALTO server can be avoided
at the cost of disclosing them to said proxy. If, in contrast,

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the ALTO client is embedded in a third party (e.g., a resource
directory), which issues ALTO requests on behalf of resource
consumers, it is possible to hide the exact addresses of the
resource consumers from the ALTO server, e.g., by zeroing out or
randomizing the last few bits of IP addresses. However, there is
the potential side effect of yielding inaccurate results.

The disclosure of candidate resource providers' addresses to the
ALTO server can be avoided by allowing ALTO clients to use the
target-independent query mode. In this mode of operation, guiding
information (e.g., "maps") is retrieved from the ALTO server and
used entirely locally by the ALTO client, i.e., without sending
host-location attributes of candidate resource providers to the
ALTO server. In the target-aware query mode, this issue can be
addressed by ALTO clients through obfuscating the identity of
candidate resource consumers, e.g., by specifying a broader
address range (i.e., a shorter prefix length) than a group of
hosts in question actually uses, or by zeroing out or randomizing
the last few bits of IP addresses. However, there is the
potential side effect of yielding inaccurate results.

o (5a) may be addressed by mandating that the ALTO server discovery
procedure, as a whole, must be secure against spoofing.

Note: Given that this document does not mandate a specific system
architecture, it is difficult to specify more details than that
the discovery procedure, as a whole, should be secure against
spoofing. There are many different architectural options, e.g.,
have an insecure discovery mechanism and use server certificates
to later verify its response (cf. the DNS + HTTPS security model
widely used in the World Wide Web). Therefore, at this
requirements stage, it is not mandatory for the discovery
mechanism itself to be secure against spoofing attacks.

o (5b) may be addressed by encryption schemes for the ALTO client
protocol. However, the effort vs. benefit should be evaluated for
any specific deployment scenario, while also considering the risks
and solution approaches for issues (4), (5c), and (6).

o Straightforward authentication and encryption schemes will not
help solving (5c) and (6). However, potential risks can be
mitigated using the same approaches as used for issue (4), see
above.

These insights are reflected in the requirements in this document.

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5.3. ALTO Server Discovery

See discussion of (5a) above.

5.4. Security Requirements

For a set of specific security requirements, please refer to
Section 3.3 of this document.

6. References

6.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC5693] Seedorf, J. and E. Burger, "Application-Layer Traffic
Optimization (ALTO) Problem Statement", RFC 5693,
October 2009.

6.2. Informative References

[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.

[RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, October 2008.

[RFC5706] Harrington, D., "Guidelines for Considering Operations and
Management of New Protocols and Protocol Extensions",
RFC 5706, November 2009.

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Appendix A. Contributors

Early draft versions of this document were co-authored by Laird
Popkin.

Appendix B. Acknowledgments

The authors would like to thank Vijay K. Gurbani and Enrico Marocco
for fostering discussions that lead to the creation of this document,
and for giving valuable comments on it.

The authors would like to thank the members of the P2PI and ALTO
mailing lists for contributions and feedback, in particular: Richard
Alimi, Jason Livingood, Michael Scharf, Nico Schwan, and Jan Seedorf.

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.

Martin Stiemerling is partially supported by the COAST project
(COntent Aware Searching, retrieval and sTreaming,
http://www.coast-fp7.eu), a research project supported by the
European Commission under its 7th Framework Program (contract no.
248036). 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 COAST project or the European Commission.

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Authors' Addresses

Sebastian Kiesel (editor)
University of Stuttgart Computing Center
Networks and Communication Systems Department
Allmandring 30
70550 Stuttgart
Germany

EMail: [email protected]
URI: http://www.rus.uni-stuttgart.de/nks/

Stefano Previdi
Cisco Systems, Inc.

EMail: [email protected]

Martin Stiemerling
NEC Laboratories Europe

EMail: [email protected]
URI: http://ietf.stiemerling.org

Richard Woundy
Comcast Corporation

EMail: [email protected]

Yang Richard Yang
Yale University

EMail: [email protected]

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