Internet DRAFT - draft-ietf-alto-reqs
draft-ietf-alto-reqs
Network Working Group S. Kiesel, Ed.
Internet-Draft University of Stuttgart
Intended status: Informational S. Previdi
Expires: December 17, 2012 Cisco Systems, Inc.
M. Stiemerling
NEC Europe Ltd.
R. Woundy
Comcast Corporation
Y R. Yang
Yale University
June 15, 2012
Application-Layer Traffic Optimization (ALTO) Requirements
draft-ietf-alto-reqs-16.txt
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 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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on December 17, 2012.
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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology and Architectural Framework . . . . . . . . . . . 5
2.1. Requirements Notation . . . . . . . . . . . . . . . . . . 5
2.2. ALTO Terminology . . . . . . . . . . . . . . . . . . . . . 5
2.3. Architectural Framework for ALTO . . . . . . . . . . . . . 6
3. ALTO Requirements . . . . . . . . . . . . . . . . . . . . . . 8
3.1. ALTO Client Protocol . . . . . . . . . . . . . . . . . . . 8
3.1.1. General Requirements . . . . . . . . . . . . . . . . . 8
3.1.2. Host Group Descriptor Support . . . . . . . . . . . . 8
3.1.3. Rating Criteria Support . . . . . . . . . . . . . . . 9
3.1.4. Placement of Entities and Timing of Transactions . . . 11
3.1.5. Protocol Extensibility . . . . . . . . . . . . . . . . 13
3.1.6. Error Handling and Overload Protection . . . . . . . . 13
3.2. ALTO Server Discovery . . . . . . . . . . . . . . . . . . 14
3.3. Security and Privacy . . . . . . . . . . . . . . . . . . . 15
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
5. Security Considerations . . . . . . . . . . . . . . . . . . . 18
5.1. High-level security considerations . . . . . . . . . . . . 18
5.2. Information Disclosure Scenarios . . . . . . . . . . . . . 18
5.2.1. Classification of Information Disclosure Scenarios . . 18
5.2.2. Discussion of Information Disclosure Scenarios . . . . 19
5.3. ALTO Server Discovery . . . . . . . . . . . . . . . . . . 21
5.4. Security Requirements . . . . . . . . . . . . . . . . . . 21
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
6.1. Normative References . . . . . . . . . . . . . . . . . . . 22
6.2. Informative References . . . . . . . . . . . . . . . . . . 22
Appendix A. Contributors List and Acknowledgments . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
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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 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 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.
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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 [RFC5693]:
Application, Peer, P2P, Resource, Resource Identifier, Resource
Provider, Resource Consumer, Transport Address, Overlay Network,
Resource Directory, ALTO Service, ALTO Server, ALTO Client, ALTO
Query, ALTO Response, ALTO Transaction, Local Traffic, Peering
Traffic, Transit Traffic, Application protocol, ALTO Client Protocol,
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
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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
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 characteristic 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 characteristic 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 characteristic 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 how ALTO could be
implemented, and 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
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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.
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3. ALTO Requirements
[*** Note to the RFC editor: before publication as an RFC, please
remove the draft version number from the requirements numbering,
i.e., change ARv16-1 to AR-1, and so on. Furthermore, remove this
note. ***]
3.1. ALTO Client Protocol
3.1.1. General Requirements
REQ. ARv16-1: 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 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. ARv16-2: 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. ARv16-3: An ALTO client protocol MUST support the usage of
multiple host group descriptor types.
REQ. ARv16-4: 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. ARv16-5: 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 CIDR notation) containing all hosts in question.
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REQ. ARv16-6: An ALTO client protocol MUST be extensible to enable
support of other host group descriptor types in future. 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. ARv16-7: 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.
REQ. ARv16-8: 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.,
Autonomous System (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 this 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. ARv16-9: 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. ARv16-10: 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. ARv16-11: 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.,
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it is not associated with any unit of measurement. A more-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.
REQ. ARv16-12: An ALTO client protocol MUST be extensible to enable
support of other rating criteria types in future. An ALTO client
protocol specification MUST define an appropriate procedure for
adding new rating criteria types, e.g., by establishing an IANA
registry.
REQ. ARv16-13: 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. ARv16-14: Applications using ALTO guidance MUST NOT rely solely
on the ALTO guidance to avoid causing network congestion. Instead,
applications 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 that the host characteristics
attributes, which are stored and processed in the ALTO servers for
giving the guidance, are 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. ARv16-15: 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. ARv16-16: 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.
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REQ. ARv16-17: 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.
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. ARv16-18: An ALTO client protocol MUST support the mode of
operation in which the ALTO client is directly embedded in the
resource consumer.
REQ. ARv16-19: 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. ARv16-20: 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. ARv16-21: An ALTO client protocol MUST be designed in a way
that different instances of the ALTO service operated by different
providers can coexist.
REQ. ARv16-22: 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.
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REQ. ARv16-23: 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 ALTO Clients and ALTO Servers. Furthermore, it MUST
specify an appropriate protocol mechanism for negotiating between
ALTO Client and ALTO Server, which query mode to use.
REQ. ARv16-24: 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. ARv16-25: An ALTO client protocol SHOULD allow the ALTO server
to add information about appropriate modes of re-use to its ALTO
responses. Re-use 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 re-use should occur
o re-use 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 re-use is appropriate for any resource consumer that would send
(or cause a third party sending 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 re-use is appropriate for any resource consumer that would send
(or cause a third party sending 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 re-use is appropriate for any resource consumer that would send
(or cause a third party sending 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
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REQ. ARv16-26: 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].
3.1.5. Protocol Extensibility
REQ. ARv16-27: An ALTO client protocol MUST include support for
adding protocol extensions in a non-disruptive, backward-compatible
way.
REQ. ARv16-28: 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. ARv16-29: An ALTO client protocol MUST use congestion-aware
transport, e.g., by using TCP.
REQ. ARv16-30: An ALTO client protocol specification MUST specify
mechanisms, or detail how to leverage appropriate mechanisms provided
by underlying protocol layers that can be used by an ALTO server to
inform clients about an impending or occurring overload situation,
and 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 the client to 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. ARv16-31: An ALTO client protocol specification MUST specify
mechanisms, or detail how to leverage appropriate mechanisms provided
by underlying protocol layers that can be used by an ALTO server to
inform clients about its inability to answer queries due to technical
problems or system maintenance, and provide all of the following
options to the server:
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o terminate the conversation with the client,
o redirect the client to another ALTO server, and
o request the client to retry the query later.
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 in order 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. ARv16-32: 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. ARv16-33: 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. ARv16-34: ALTO clients MUST be able to perform resource
consumer initiated ALTO server discovery, even if they are located
behind a network address translator (NAT).
REQ. ARv16-35: ALTO clients MUST be able to perform third-party ALTO
server discovery, even if they are located behind a network address
translator (NAT).
REQ. ARv16-36: 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 network address
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translator (NAT).
REQ. ARv16-37: 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.
REQ. ARv16-38: Every ALTO server discovery mechanism SHOULD be able
to return the respective contact information for multiple ALTO
servers.
REQ. ARv16-39: 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. ARv16-40: An ALTO client protocol specification MUST specify
mechanisms for the authentication of ALTO servers, or how to leverage
appropriate mechanisms provided by underlying protocol layers.
REQ. ARv16-41: An ALTO client protocol specification MUST specify
mechanisms for the authentication of ALTO clients, or how to leverage
appropriate mechanisms provided by underlying protocol layers.
REQ. ARv16-42: An ALTO client protocol specification MUST specify
mechanisms for the encryption of messages, or how to leverage
appropriate mechanisms provided by underlying protocol layers.
REQ. ARv16-43: 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. ARv16-44: 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. ARv16-45: An ALTO client protocol MAY include mechanisms that
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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.
REQ. ARv16-46: An ALTO client protocol specification MUST specify
appropriate mechanisms for protecting the ALTO service against DoS
attacks, or how to leverage appropriate mechanisms provided by
underlying protocol layers.
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4. 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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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:
o (1) Excess disclosure of 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 somehow obfuscated information
to an ALTO client.
o (2) Disclosure of ALTO server operator's data (e.g., network
topology information) to an unauthorized third party. There are a
three sub-cases 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.
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* (2c) An authorized ALTO client knowingly forwards the
information it had received from the ALTO server to an
unauthorized party.
o (3) Excess retrieval of 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:
o (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.
o (5) Disclosure of the application behavior or other user private
data to an unauthorized third party. There are a three sub-cases
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 had received from the ALTO client to an
unauthorized party.
o (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:
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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, and 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, and 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,
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.
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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 archtectural options, e.g.,
have an insecure discovery mechanism and use server certificates
to later verify its response (c.f. 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.
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.
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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 List and Acknowledgments
The initial version of this document was co-authored by Laird Popkin.
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.
The authors were supported by the following people, who have
contributed to this document:
o Richard Alimi <ralimi@google.com>
o Jason Livingood <Jason_Livingood@cable.comcast.com>
o Michael Scharf <michael.scharf@alcatel-lucent.com>
o Nico Schwan <nico.schwan@alcatel-lucent.com>
o Jan Seedorf <jan.seedorf@neclab.eu>
The authors would like to thank the members of the P2PI and ALTO
mailing lists for their feedback.
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: ietf-alto@skiesel.de
URI: http://www.rus.uni-stuttgart.de/nks/
Stefano Previdi
Cisco Systems, Inc.
Email: sprevidi@cisco.com
Martin Stiemerling
NEC Laboratories Europe
Email: martin.stiemerling@neclab.eu
URI: http://ietf.stiemerling.org
Richard Woundy
Comcast Corporation
Email: Richard_Woundy@cable.comcast.com
Yang Richard Yang
Yale University
Email: yry@cs.yale.edu
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