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INFORMATIONAL

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: 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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