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Versions: (draft-kiesel-alto-reqs) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 6708

Network Working Group                                     S. Kiesel, Ed.
Internet-Draft                                   University of Stuttgart
Intended status: Informational                                S. Previdi
Expires: November 30, 2012                           Cisco Systems, Inc.
                                                          M. Stiemerling
                                                         NEC Europe Ltd.
                                                               R. Woundy
                                                     Comcast Corporation
                                                               Y R. Yang
                                                         Yale University
                                                            May 29, 2012


       Application-Layer Traffic Optimization (ALTO) Requirements
                      draft-ietf-alto-reqs-15.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 November 30, 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 ARv15-1 to AR-1, and so on.  Furthermore, remove this
   note. ***]

3.1.  ALTO Client Protocol

3.1.1.  General Requirements

   REQ.  ARv15-1: 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.  ARv15-2: An ALTO 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.  ARv15-3: The ALTO client protocol MUST support the usage of
   multiple host group descriptor types.

   REQ.  ARv15-4: 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.  ARv15-5: 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.  ARv15-6: 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.  ARv15-7: 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.  ARv15-8: 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.  ARv15-9: 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.  ARv15-10: 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.  ARv15-11: 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.  ARv15-12: 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.  ARv15-13: 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.  ARv15-14: 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.  ARv15-15: 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.  ARv15-16: 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.  ARv15-17: 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.  ARv15-18: An ALTO client protocol MUST support the mode of
   operation in which the ALTO client is directly embedded in the
   resource consumer.

   REQ.  ARv15-19: 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.  ARv15-20: 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.  ARv15-21: An ALTO client protocol MUST be designed in a way
   that different instances of the ALTO service operated by different
   providers can coexist.

   REQ.  ARv15-22: 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.  ARv15-23: 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.  ARv15-24: 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.  ARv15-25: 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.  ARv15-26: 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.  ARv15-27: An ALTO client protocol MUST include support for
   adding protocol extensions in a non-disruptive, backward-compatible
   way.

   REQ.  ARv15-28: 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.  ARv15-29: An ALTO client protocol MUST use congestion-aware
   transport, e.g., by using TCP.

   REQ.  ARv15-30: 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.  ARv15-31: 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.  ARv15-32: 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.  ARv15-33: 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.  ARv15-34: 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.  ARv15-35: ALTO clients MUST be able to perform third-party ALTO
   server discovery, even if they are located behind a network address
   translator (NAT).

   REQ.  ARv15-36: 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.  ARv15-37: 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.  ARv15-38: Every ALTO server discovery mechanism SHOULD be able
   to return the respective contact information for multiple ALTO
   servers.

   REQ.  ARv15-39: 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.  ARv15-40: 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.  ARv15-41: 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.  ARv15-42: 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.  ARv15-43: 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.  ARv15-44: 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.  ARv15-45: 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.  ARv15-46: 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.  From a user privacy perspective, neither the ALTO server nor a
   third party using or misusing the ALTO service should be able to
   infer the application behavior, e.g., who is exchanging which files
   with whom using a P2P file sharing application.  Many network
   operators, in contrast, 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 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 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 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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