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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: May 17, 2012                                Cisco Systems, Inc.
                                                          M. Stiemerling
                                                         NEC Europe Ltd.
                                                               R. Woundy
                                                     Comcast Corporation
                                                               Y R. Yang
                                                         Yale University
                                                       November 14, 2011


       Application-Layer Traffic Optimization (ALTO) Requirements
                      draft-ietf-alto-reqs-12.txt

Abstract

   Many Internet applications are used to access resources, such as
   pieces of information or server processes, which are available in
   several equivalent replicas on different hosts.  This includes, but
   is not limited to, peer-to-peer file sharing applications.  The goal
   of Application-Layer Traffic Optimization (ALTO) is to provide
   guidance to applications, which have to select one or several hosts
   from a set of candidates 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 resource
   consumption in 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 May 17, 2012.

Copyright Notice

   Copyright (c) 2011 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  . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  ALTO Client Protocol . . . . . . . . . . . . . . . . . . .  7
       3.1.1.  General Requirements . . . . . . . . . . . . . . . . .  7
       3.1.2.  Host Group Descriptor Support  . . . . . . . . . . . .  7
       3.1.3.  Rating Criteria Support  . . . . . . . . . . . . . . .  8
       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  . . . . . . . . . . . . . . . . . . 13
     3.3.  Security and Privacy . . . . . . . . . . . . . . . . . . . 14
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
     5.1.  High-level security considerations . . . . . . . . . . . . 16
     5.2.  Information Disclosure Scenarios . . . . . . . . . . . . . 16
       5.2.1.  Classification of Information Disclosure Scenarios . . 16
       5.2.2.  Discussion of Information Disclosure Scenarios . . . . 17
     5.3.  Security Requirements  . . . . . . . . . . . . . . . . . . 18
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 19
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 19
   Appendix A.  Contributors List and Acknowledgments . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21






















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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 resource consumption in
   the underlying network infrastructure.

   Usually, it would be difficult or even impossible for application
   entities to acquire this information by other mechanisms, e.g., using
   measurements between the peers of a P2P overlay, because of
   complexity or because it is based on network topology information,
   network operational costs, or network policies, which the respective
   network provider does not want to disclose in detail.

   The 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 which 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, i. e.,
      "low topological distance from a given resource consumer". 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



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      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 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 ratring 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 enriced 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.

   The ALTO problem statement [RFC5693] defines a terminology (see
   Section 2 of [RFC5693] and Section 2.2 of this document), introduces
   several components.  It 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.







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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 ARv12-1 to AR-1, and so on.  Furthermore, remove this
   note. ***]

3.1.  ALTO Client Protocol

3.1.1.  General Requirements

   REQ.  ARv12-1: 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.  However, this document enumerates
   requirements for ALTO, to be considered when specifying, assessing,
   or comparing protocols and implementations.

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.  ARv12-2: The ALTO client protocol MUST support the usage of
   multiple host group descriptor types.

   REQ.  ARv12-3: ALTO clients and ALTO servers MUST clearly identify
   the type of each host group descriptor sent in ALTO queries or
   responses.

   REQ.  ARv12-4: 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).  Such an IP address range will
   contains all hosts in question.

   REQ.  ARv12-5: 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



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   IANA registry.

   REQ.  ARv12-6: 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, which can be used to translate host group descriptors of
   that type to IPv4/IPv6 address prefixes, e.g., by means of a mapping
   table or an algorithm.

   REQ.  ARv12-7: 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.

   REQ.  ARv12-8: 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.  ARv12-9: An ALTO client protocol specification MUST define
   mechanisms, which 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.  ARv12-10: 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 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.  ARv12-11: 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



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   registry.

   REQ.  ARv12-12: 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.

      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.  ARv12-13: 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.

   REQ.  ARv12-14: 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.  ARv12-15: In the target-aware query mode, the ALTO query
   message SHOULD allow the ALTO client to express which ratring
   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.  ARv12-16: An ALTO client protocol specification MUST define
   mechanisms, which 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



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      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.  ARv12-17: An ALTO client protocol MUST support the mode of
   operation in which the ALTO client is directly embedded in the
   resource consumer.

   REQ.  ARv12-18: 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.  ARv12-19: An ALTO client protocol MUST be designed in a way
   that the ALTO service can be provided by an entity which is not the
   operator of the underlying IP network.

   REQ.  ARv12-20: An ALTO client protocol MUST be designed in a way
   that different instances of the ALTO service operated by different
   providers can coexist.

   REQ.  ARv12-21: An ALTO client protocol MUST support at least one
   query mode, either the target-aware or the target-independent query
   mode.

   REQ.  ARv12-22: An ALTO client protocol SHOULD support both the
   target-aware and the target-independent query mode.

   REQ.  ARv12-23: 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.  ARv12-24: 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




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   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, which 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,
      which 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

   REQ.  ARv12-25: 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.  ARv12-26: An ALTO client protocol MUST include support for
   adding protocol extensions in a non-disruptive, backward-compatible
   way.

   REQ.  ARv12-27: 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.  ARv12-28: An ALTO client protocol MUST use TCP based transport.

   REQ.  ARv12-29: An ALTO client protocol specification MUST specify
   mechanisms, or detail how to leverage appropriate mechanisms provided



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   by underlying protocol layers, which can be used by an ALTO server to
   inform clients about an impending or occurring overload situation,
   and request them to throttle their 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.  ARv12-30: An ALTO client protocol specification MUST specify
   mechanisms, or detail how to leverage appropriate mechanisms provided
   by underlying protocol layers, which can be used by an ALTO server to
   inform clients about an impending or occurring overload situation,
   and redirect them to another ALTO server.

   REQ.  ARv12-31: An ALTO client protocol specification MUST specify
   mechanisms, or detail how to leverage appropriate mechanisms provided
   by underlying protocol layers, which can be used by an ALTO server to
   inform clients about an impending or occurring overload situation,
   and terminate the conversation with the ALTO client.

   REQ.  ARv12-32: An ALTO client protocol specification MUST specify
   mechanisms, or detail how to leverage appropriate mechanisms provided
   by underlying protocol layers, which can be used by an ALTO server to
   inform clients about its inability to answer queries due to technical
   problems or system maintenance, and advise them to retry the query
   later.

   REQ.  ARv12-33: An ALTO client protocol specification MUST specify
   mechanisms, or detail how to leverage appropriate mechanisms provided
   by underlying protocol layers, which can be used by an ALTO server to
   inform clients about its inability to answer queries due to technical
   problems or system maintenance, and redirect them to another ALTO
   server.

   REQ.  ARv12-34: An ALTO client protocol specification MUST specify
   mechanisms, or detail how to leverage appropriate mechanisms provided
   by underlying protocol layers, which can be used by an ALTO server to
   inform clients about its inability to answer queries due to technical
   problems or system maintenance, and terminate the conversation with
   the ALTO client.

   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



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   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.  ARv12-35: ALTO clients which are embedded in the resource
   consumer MUST be able to use an ALTO server discovery mechanism, in
   order to find one or several ALTO servers that can provide ALTO
   guidance suitable for the resource consumer.  This mode of operation
   is called "resource consumer initiated ALTO server discovery".

   REQ.  ARv12-36: ALTO clients which are embedded in a resource
   directory and perform third-party ALTO queries on behalf of a remote
   resource consumer MUST be able to use an ALTO server discovery
   mechanism, in order to find one or several ALTO servers that can
   provide ALTO guidance suitable for the respective resource consumer.
   This mode of operation is called "third-party ALTO server discovery".

   REQ.  ARv12-37: 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.  ARv12-38: ALTO clients MUST be able to perform third-party ALTO
   server discovery, even if they are located behind a network address
   translator (NAT).

   REQ.  ARv12-39: 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
   translator (NAT).

   REQ.  ARv12-40: 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.  ARv12-41: Every ALTO server discovery mechanism SHOULD be able
   to return the respective contact information for multiple ALTO
   servers.



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   REQ.  ARv12-42: 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.

   REQ.  ARv12-43: 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.  ARv12-44: 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.  ARv12-45: 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.  ARv12-46: 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.  ARv12-47: 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.  ARv12-48: 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.

   REQ.  ARv12-49: 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

   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 the application behavior to the 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  (3) Disclosure of ALTO server operator's data (e.g., network
      topology information) to an unauthorized third party.  There are a
      three sub-cases here:

      *  (3a) An ALTO server sends the information directly to an
         unauthorized ALTO client.




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      *  (3b) An unauthorized party snoops on the data transmission from
         the ALTO server to an authorized ALTO client.

      *  (3c) An authorized ALTO client knowingly forwards the
         information it had received from the ALTO server to an
         unauthorized party.

   o  (4) Disclosure of the application behavior to an unauthorized
      third party.

   o  (5) Excess retrieval of ALTO server operator's data by
      collaborating ALTO clients.  Several authorized ALTO clients could
      ask an ALTO server for guidance, and redistribute the responses
      among each other (see also case 3c).  By correlating the ALTO
      responses they could find out more information than intended to be
      disclosed by the ALTO server operator.

5.2.2.  Discussion of Information Disclosure Scenarios

   Scenario (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 (3c) and (5),
   see below.

   (2) 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.

   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



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   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.

   (3a), (3b), and (4) 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 (3c) and (5),
   see below.

   Straightforward authentication and encryption schemes will not help
   solving (3c) and (5), 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
   (3c) and (5) cannot be prevented from happening, and therefore they
   should feed only such data into an ALTO server, which they do not
   consider sensitive with respect to (3c) and (5).

   These insights are reflected in the requirements in this document.

5.3.  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.

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.

   [RFC5693]  Seedorf, J. and E. Burger, "Application-Layer Traffic
              Optimization (ALTO) Problem Statement", RFC 5693,
              October 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  Zoran Despotovic <despotovic@docomolab-euro.com>

   o  Jason Livingood <Jason_Livingood@cable.comcast.com>

   o  Saverio Niccolini <saverio.niccolini@nw.neclab.eu>

   o  Michael Scharf <michael.scharf@alcatel-lucent.com>

   o  Nico Schwan <nico.schwan@alcatel-lucent.com>

   o  Jan Seedorf <jan.seedorf@nw.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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