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Versions: (draft-stiemerling-alto-deployments) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 7971

ALTO                                                 M. Stiemerling, Ed.
Internet-Draft                                           NEC Europe Ltd.
Intended status: Informational                            S. Kiesel, Ed.
Expires: April 24, 2014                          University of Stuttgart
                                                              S. Previdi
                                                                   Cisco
                                                               M. Scharf
                                                Alcatel-Lucent Bell Labs
                                                        October 21, 2013


                     ALTO Deployment Considerations
                     draft-ietf-alto-deployments-08

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, which are able to provide a desired
   resource.  This memo discusses deployment related issues of ALTO.  It
   addresses different use cases of ALTO such as peer-to-peer file
   sharing and CDNs, security considerations, recommendations for
   network administrators, and also guidance for application designers
   using ALTO.

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."

   This Internet-Draft will expire on April 24, 2014.

Copyright Notice




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   Copyright (c) 2013 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
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   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.  General Considerations  . . . . . . . . . . . . . . . . . . .   4
     2.1.  ALTO Entities . . . . . . . . . . . . . . . . . . . . . .   4
       2.1.1.  Baseline Scenario . . . . . . . . . . . . . . . . . .   4
       2.1.2.  Placement of ALTO Entities  . . . . . . . . . . . . .   4
     2.2.  Classification of Deployment Scenarios  . . . . . . . . .   6
       2.2.1.  Deployment Degrees of Freedom . . . . . . . . . . . .   6
       2.2.2.  Information Exposure Models . . . . . . . . . . . . .   7
       2.2.3.  More Advanced Deployments . . . . . . . . . . . . . .   7
   3.  Deployment Considerations by ISPs . . . . . . . . . . . . . .   9
     3.1.  Objectives for the Guidance to Applications . . . . . . .   9
       3.1.1.  General Objectives for Traffic Optimization . . . . .   9
       3.1.2.  Inter-Network Traffic Localization  . . . . . . . . .  10
       3.1.3.  Intra-Network Traffic Localization  . . . . . . . . .  11
       3.1.4.  Network Off-Loading . . . . . . . . . . . . . . . . .  13
       3.1.5.  Application Tuning  . . . . . . . . . . . . . . . . .  14
     3.2.  Provisioning of ALTO Maps . . . . . . . . . . . . . . . .  14
       3.2.1.  Data Sources  . . . . . . . . . . . . . . . . . . . .  14
       3.2.2.  Privacy Requirements  . . . . . . . . . . . . . . . .  14
       3.2.3.  Map Partitioning and Grouping . . . . . . . . . . . .  15
       3.2.4.  Rating Criteria and/or Cost Calculation . . . . . . .  15
     3.3.  Known Limitations of ALTO . . . . . . . . . . . . . . . .  18
       3.3.1.  Limitations of Map-based Approaches . . . . . . . . .  18
       3.3.2.  Limitiations of Non-Map-based Approaches  . . . . . .  20
     3.4.  Map Examples for Different Types of ISPs  . . . . . . . .  20
       3.4.1.  Small ISP with Single Internet Uplink . . . . . . . .  20
       3.4.2.  ISP with Several Fixed Access Networks  . . . . . . .  22
       3.4.3.  ISP with Fixed and Mobile Network . . . . . . . . . .  24
     3.5.  Deployment Experiences  . . . . . . . . . . . . . . . . .  25
   4.  Using ALTO for P2P Traffic Optimization . . . . . . . . . . .  25
     4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  26
       4.1.1.  Usage Scenario  . . . . . . . . . . . . . . . . . . .  26
       4.1.2.  Applicability of ALTO . . . . . . . . . . . . . . . .  29



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     4.2.  Deployment Recommendations  . . . . . . . . . . . . . . .  29
       4.2.1.  ALTO Services . . . . . . . . . . . . . . . . . . . .  29
       4.2.2.  Guidance Considerations . . . . . . . . . . . . . . .  29
   5.  Using ALTO for CDNs . . . . . . . . . . . . . . . . . . . . .  33
     5.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  33
       5.1.1.  Usage Scenario  . . . . . . . . . . . . . . . . . . .  33
       5.1.2.  Applicability of ALTO . . . . . . . . . . . . . . . .  33
     5.2.  Deployment Recommendations  . . . . . . . . . . . . . . .  34
       5.2.1.  ALTO Services . . . . . . . . . . . . . . . . . . . .  34
       5.2.2.  Guidance Considerations . . . . . . . . . . . . . . .  35
   6.  Other Use Cases . . . . . . . . . . . . . . . . . . . . . . .  36
     6.1.  Monitoring Data Reporting . . . . . . . . . . . . . . . .  36
     6.2.  Virtual Private Networks (VPNs) . . . . . . . . . . . . .  36
     6.3.  In-Network Caching  . . . . . . . . . . . . . . . . . . .  36
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  37
     7.1.  Information Leakage from the ALTO Server  . . . . . . . .  37
     7.2.  ALTO Server Access  . . . . . . . . . . . . . . . . . . .  38
     7.3.  Faking ALTO Guidance  . . . . . . . . . . . . . . . . . .  38
   8.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .  39
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  39
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  39
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  39
   Appendix A.  Appendix: Monitoring ALTO  . . . . . . . . . . . . .  41
     A.1.  Monitoring Metrics Definition . . . . . . . . . . . . . .  41
     A.2.  Monitoring Data Sources . . . . . . . . . . . . . . . . .  42
     A.3.  Monitoring Structure  . . . . . . . . . . . . . . . . . .  42
   Appendix B.  Appendix: API between ALTO Client and Application  .  43
   Appendix C.  Contributors List and Acknowledgments  . . . . . . .  43
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  44

1.  Introduction

   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 (P2P) file sharing applications and
   Content Delivery Networks (CDNs).  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,
   which are able to provide a desired resource.  The basic ideas and
   problem space of ALTO is described in [RFC5693] and the set of
   requirements is discussed in [RFC6708].

   However, there are no considerations about what operational issues
   are to be expected once ALTO will be deployed.  This includes, but is
   not limited to, location of the ALTO server, imposed load to the ALTO
   server, or from whom the queries are performed.




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   Comments and discussions about this memo should be directed to the
   ALTO working group: alto@ietf.org.

2.  General Considerations

2.1.  ALTO Entities

2.1.1.  Baseline Scenario

   The ALTO protocol [I-D.ietf-alto-protocol] is a client/server
   protocol, operating between a number of ALTO clients and an ALTO
   server, as sketched in Figure 1.

                +----------+
                |  ALTO    |
                |  Server  |
                +----------+
                      ^
               _.-----|------.
           ,-''       |       `--.
         ,'           |           `.
        (     Network |             )
         `.           |           ,'
           `--.       |       _.-'
               `------|-----''
                      v
   +----------+  +----------+   +----------+
   |  ALTO    |  |  ALTO    |...|  ALTO    |
   |  Client  |  |  Client  |   |  Client  |
   +----------+  +----------+   +----------+

        Figure 1: Baseline Deployment Scenario of the ALTO Protocol

2.1.2.  Placement of ALTO Entities

   The ALTO server and ALTO clients can be situated at various entities
   in a network deployment.  The first differentiation is whether the
   ALTO client is located on the actual host that runs the application,
   as shown in Figure 2, or if the ALTO client is located on a resource
   directory, as shown in Figure 3.

                                               +-----+
                                          =====|     |**
                                      ====     +-----+  *
                                  ====            *     *
                              ====                *     *
     +-----+     +------+=====                 +-----+  *
     |     |.....|      |======================|     |  *



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     +-----+     +------+=====                 +-----+  *
   Source of      ALTO        ====                *     *
   topological    service         ====            *     *
   information                        ====     +-----+  *
                                          =====|     |**
                                               +-----+
   Legend:
   === ALTO client protocol
   *** Application protocol
   ... Provisioning protocol

     Figure 2: Overview of protocol interaction between ALTO elements
                       without a resource directory

   Figure 2 shows the operational model for applications that do not use
   a resouce directory.  An example would be a peer-to-peer file sharing
   application that does not use a tracker, such as edonkey.

                                                  +-----+
                                                **|     |**
                                              **  +-----+  *
                                            **       *     *
                                          **         *     *
        +-----+     +------+     +-----+**        +-----+  *
        |     |.....|      |=====|     |**********|     |  *
        +-----+     +------+     +-----+**        +-----+  *
      Source of      ALTO        Resource **         *     *
      topological    service     directory  **       *     *
      information                             **  +-----+  *
                                                **|     |**
                                                  +-----+

      Legend:
      === ALTO client protocol
      *** Application protocol
      ... Provisioning protocol

   Figure 3: Overview of protocol interaction between ALTO elements with
                           a resource directory

   In Figure 3, a use case with a resource directory is illustrated,
   e.g., a tracker in peer-to-peer filesharing.  Both deployment
   scenarios may differ in the number of ALTO clients that access an
   ALTO service: If ALTO clients are implemented in a resource
   directory, ALTO servers may be accessed by a limited and less dynamic
   set of clients, whereas in the general case any host could be an ALTO
   client.  This use case is further detailed in Section 4.




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   Using ALTO in CDNs may be similar to a resource directory
   [I-D.jenkins-alto-cdn-use-cases].  The ALTO server can also be
   queried by CDN entities to get a guidance about where the a
   particular client accessing data in the CDN is exactly located in the
   ISP's network, as discussed in Section 5.

2.2.  Classification of Deployment Scenarios

2.2.1.  Deployment Degrees of Freedom

   ALTO is a general-purpose solution and it is intended to be used by a
   wide range of applications.  This implies that there are different
   possibilities where the ALTO entities are actually located, i.e., if
   the ALTO clients and the ALTO server are in the same ISP's domain, or
   if the clients and the ALTO server are managed/owned/located in
   different domains.

   ALTO deployments can be differentiated e.g. according to the
   following aspects:

   1.  Applicable trust model: The deployment of ALTO can differ
       depending on whether ALTO client and ALTO server are operated
       within the same organization and/or network, or not.  This
       affects a lot of constraints, because the trust model is very
       different.  For instance, as discussed later in this memo, the
       level-of-detail of maps can depend on who the involved parties
       actually are.

   2.  Size of user group: The main use case of ALTO is to provide
       guidance to any Internet application.  However, an operator of an
       ALTO server could also decide to only offer guidance to a set of
       well-known ALTO clients, e. g., after authentication and
       authorization.  In the peer-to-peer application use case, this
       could imply that only selected trackers are allowed to access the
       ALTO server.  The security implications of using ALTO in closed
       groups differ from the public Internet.

   3.  Covered destinations: In general, an ALTO server has to be able
       to provide guidance for all potential destinations.  Yet, in
       practice a given ALTO client may only be interested in a subset
       of destinations, e.g., only in the network cost between a limited
       set of resource providers.  For instance, CDN optimization may
       not need the full ALTO cost maps, because traffic between
       individual residential users is not in scope.  This may imply
       that an ALTO server only has to provide the costs that matter for
       a given user, e. g., by customized maps.





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   The following sections enumerate different classes of use cases for
   ALTO, and they discuss the deployment implications of each of them.

   However, it must be emphasized that any application using ALTO must
   also work if no ALTO servers can be found or if no responses to ALTO
   queries are received, e.g., due to connectivity problems or overload
   situations (see also [RFC6708]).

2.2.2.  Information Exposure Models

   An ALTO server stores information about preferences (e.g., a list of
   preferred autonomous systems, IP ranges, etc) and ALTO clients can
   retrieve these preferences.  There are basically two different
   approaches on where the preferences are actually processed:

   1.  The ALTO server has a list of preferences and clients can
       retrieve this list via the ALTO protocol.  This preference list
       can partially be updated by the server.  The actual processing of
       the data is done on the client and thus there is no data of the
       client's operation revealed to the ALTO server .

   2.  The ALTO server has a list of preferences or preferences
       calculated during runtime and the ALTO client is sending
       information of its operation (e.g., a list of IP addresses) to
       the server.  The server is using this operational information to
       determine its preferences and returns these preferences (e.g., a
       sorted list of the IP addresses) back to the ALTO client.

   Approach 1 has the advantage (seen from the client) that all
   operational information stays within the client and is not revealed
   to the provider of the server.  On the other hand, approach 1
   requires that the provider of the ALTO server, i.e., the network
   operator, reveals information about its network structure (e.g., AS
   numbers, IP ranges, topology information in general) to the ALTO
   client.  The ALTO protocol supports this scheme by the Network and
   Cost Map Service.

   Approach 2 has the advantage (seen from the operator) that all
   operational information stays with the ALTO server and is not
   revealed to the ALTO client.  On the other hand, approach 2 requires
   that the clients send their operational information to the server.
   This approach is realized by the ALTO Endpoint Cost Service (ECS).

   Both approaches have their pros and cons, as detailed in Section 3.3.

2.2.3.  More Advanced Deployments





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   From an ALTO client's perspective, there are two fundamental ways to
   use ALTO:

   1.  Single server: An ALTO client only obtains guidance from a single
       ALTO server instance, e.g., an ALTO server that is offered by the
       network service provider of the corresponding access network.
       This ALTO server can be discovered e.g. by ALTO server discovery
       [I-D.ietf-alto-server-discovery].

   2.  Multiple servers: An ALTO client is aware of more than one ALTO
       server.  This scenario is mostly identical to the former one if
       all those servers provide the same guidance (e.g., load
       balancing).  Yet, an ALTO client can also decide to access
       multiple servers providing different guidance, possibly from
       different operators.  In that case, it may be difficult for an
       ALTO client to compare the guidance from different servers.  How
       to discover multiple servers is an open issue.

   There are also different options regarding the guidance offered by an
   ALTO server:

   1.  Authorative servers: An ALTO server instance can provide guidance
       for all destinations for all kinds of ALTO clients.

   2.  Cascaded servers: An ALTO server may itself include an ALTO
       client and query other ALTO servers, e.g., for certain
       destinations.  This results is a cascaded deployment of ALTO
       servers, as further explained below.

   3.  Inter-server synchronization: Different ALTO servers my
       communicate by other means.  This approach is not further
       discussed in this document.

   An assumption of the ALTO solution is that ISP operate ALTO servers
   independently, irrespectively of other ISPs.  This may true for most
   envisioned deployments of ALTO but there are certain deployments that
   may have different settings.  Figure 4 shows such setting with a
   university network that is connected to two upstream providers.  ISP2
   is the national research network and ISP1 is a commercial upstream
   provider to this university network.  The university, as well as
   ISP1, are operating their own ALTO server.  The ALTO clients, located
   on the peers will contact the ALTO server located at the university.

      +-----------+
      |   ISP1    |
      |   ALTO    |
      |  Server   |
      +----------=+



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         ,-------=            ,------.
      ,-'        =`-.      ,-'         `-.
     /   Upstream=   \    /   Upstream    \
    (       ISP1 =    )  (       ISP2      )
     \           =   /    \               /
      `-.        =,-'      `-.         ,-'
         `---+---=            `+------'
             |   =             |
             |   =======================
             |,-------------.  |       =
           ,-+               `-+    +-----------+
         ,'      University     `.  |University |
        (        Network          ) |   ALTO    |
         `.  =======================|  Server   |
           `-=               +-'    +-----------+
             =`+------------'|
             = |             |
      +--------+-+         +-+--------+
      |   Peer1  |         |   PeerN  |
      +----------+         +----------+

                      Figure 4: Cascaded ALTO Server

   In this setting all "destinations" useful for the peers within ISP2
   are free-of-charge for the peers located in the university network
   (i.e., they are preferred in the rating of the ALTO server).
   However, all traffic that is not towards ISP2 will be handled by the
   ISP1 upstream provider.  Therefore, the ALTO server at the university
   has also to include the guidance given by the ISP1 ALTO server in its
   replies to the ALTO clients.  This is an example for cascaded ALTO
   servers.

3.  Deployment Considerations by ISPs

3.1.  Objectives for the Guidance to Applications

3.1.1.  General Objectives for Traffic Optimization

   The Internet is a large network consisting of many networks
   worldwide.  These networks are built by network operators or Internet
   Service Providers (named ISP in this memo), and these networks
   provide network connectivity to access networks, such as cable
   networks, xDSL networks, 3G/4G mobile networks, etc.  Some of these
   networks are also built by universities or big organizations.  These
   network providers need to manage, to control and to audit the
   traffic.  Thus, it's important for ISPs to understand the requirement
   of optimizing traffic, and how to deploy ALTO service in these
   manageability and controllability networks.



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   The objective of ALTO is to give guidance to applications on what IP
   addresses or IP prefixes are to be preferred according to the
   operator of the ALTO server.  The ALTO protocol gives means to let
   the ALTO server operator express its preference, whatever this
   preference is.

   ALTO enables ISPs to perform traffic engineering by influencing
   application resource selections.  This traffic engineering can have
   different objectives:

   1.  Inter-network traffic localization: ALTO can help to reduce
       inter-domain traffic.  The networks of ISPs are connected through
       peering points.  From a business view, the inter-network
       settlement is needed for exchanging traffic between these
       networks.  These peering agreements can be costly.  To reduce
       these costs, a simple objective is to decrease the traffic
       exchange across the peering points and thus keep the traffic in
       the own network or Autonomous System (AS) as far as possible.

   2.  Intra-network traffic localization: In case of large ISPs, the
       network may be grouped into several networks, domains, or
       Autonomous Systems (ASs).  The core network includes one or
       several backbone networks, which are connected to multiple
       aggregation, metro, and access networks.  If traffic can be
       limited to access networks, this decreases the usage of backbone
       and thus helps to save resources and costs.

   3.  Network off-loading: Compared to fixed networks, mobile networks
       have some special characteristics, including smaller link
       bandwidth, high cost, limited radio frequency resource, and
       limited terminal battery.  In mobile networks, the usage of
       wireless link should be decreased as far as possible and be used
       efficiently.  For example, in the case of a P2P service, the
       hosts in fixed networks should avoid retrieving data from hosts
       in the mobile networks, and hosts in mobile networks should
       prefer the data retrieval from hosts in fixed networks.

   4.  Application tuning: ALTO is also a powerful tool to optimize the
       performance of applications that depend on the network and
       perform resource selection decisions.

   In the following, these objectives are explained in more detail with
   deployment examples.

3.1.2.  Inter-Network Traffic Localization

   ALTO guidance can be used to keep traffic local in a network.  An
   ALTO server can let applications prefer other hosts within the same



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   network operator's network instead of randomly connecting to other
   hosts that are located in another operator's network.  Here, a
   network operator would always express its preference for hosts in its
   own network, while hosts located outside its own network are to be
   avoided (i.e., they are undesired to be considered by the
   applications).  Figure 5 shows such a scenario where hosts prefer
   hosts in the same network (e.g., Host 1 and Host 2 in ISP1 and Host 3
   and Host 4 in ISP2).

                            ,-------.         +-----------+
          ,---.          ,-'         `-.      |   Host 1  |
       ,-'     `-.      /     ISP 1   ########|ALTO Client|
      /           \    /              #  \    +-----------+
     /    ISP X    \   |              #  |    +-----------+
    /               \  \              ########|   Host 2  |
   ;             +----------------------------|ALTO Client|
   |             |   |   `-.         ,-'      +-----------+
   |             |   |      `-------'
   |             |   |      ,-------.         +-----------+
   :             |   ;   ,-'         `########|   Host 3  |
    \            |  /   /     ISP 2   # \     |ALTO Client|
     \           | /   /              #  \    +-----------+
      \          +---------+          #  |    +-----------+
       `-.     ,-'     \   |          ########|   Host 4  |
          `---'         \  +------------------|ALTO Client|
                         `-.         ,-'      +-----------+
                            `-------'

       Legend:
       ### preferred "connections"
       --- non-preferred "connections"

                Figure 5: ALTO Traffic Network Localization

   TBD: Describes limits of this approach (e.g., traffic localization
   guidance is of less use if the peers cannot upload); describe how
   maps would look like.

3.1.3.  Intra-Network Traffic Localization












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   The above sections described the results of the ALTO guidance on an
   inter-network level.  However, ALTO can also be used for intra-
   network localization.  In this case, ALTO provides guidance which
   internal hosts are to be preferred inside a single network or, e.g.,
   one AS.  Figure 6 shows such a scenario where Host 1 and Host 2 are
   located in Net 2 of ISP1 and connect via a low capacity link to the
   core (Net 1) of the same ISP1.  If Host 1 and Host 2 exchange their
   data with remote hosts, they would probably congest the bottleneck
   link.

                               ,-------.         +-----------+
          ,---.             ,-'         `-.      |   Host 1  |
       ,-'     `-.         /     ISP 1  #########|ALTO Client|
      /           \       /      Net 2  #   \    +-----------+
     /    ISP 1    \      |     #########   |    +-----------+
    /     Net 1     \     \     #           /    |   Host 2  |
   ;             ###;      \    #      ##########|ALTO Client|
   |               X~~~~~~~~~~~~X#######,-'      +-----------+
   |             ### |  ^      `-------'
   |                 |  |
   :                 ;  |
    \               /  Bottleneck
     \             /
      \           /
       `-.     ,-'
          `---'
       Legend:
       ### peer "connections"
       ~~~ bottleneck link

         Figure 6: Without Intra-Network ALTO Traffic Localization

   The operator can guide the hosts in such a situation to try first
   local hosts in the same network islands, avoiding or at least
   lowering the effect on the bottleneck link, as shown in Figure 7.

                               ,-------.         +-----------+
          ,---.             ,-'         `-.      |   Peer 1  |
       ,-'     `-.         /     ISP 1  #########|ALTO Client|
      /           \       /      Net 2  #   \    +-----------+
     /    ISP 1    \      |             #   |    +-----------+
    /     Net 1     \     \             #########|   Peer 2  |
   ;                ;      \           ##########|ALTO Client|
   |                #~~~~~~~~~~~########,-'      +-----------+
   |             ### |  ^      `-------'
   |                 |  |
   :                 ;  |
    \               /  Bottleneck



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     \             /
      \           /
       `-.     ,-'
          `---'
       Legend:
       ### peer "connections"
       ~~~ bottleneck link

          Figure 7: With Intra-Network ALTO Traffic Localization

3.1.4.  Network Off-Loading

   Another scenario is off-loading traffic from networks.  This use of
   ALTO can be beneficial in particular in mobile broadband networks.
   The network operator may have the desire to guide hosts in its own
   network to use hosts in remote networks.  One reason can be that the
   wireless network is not made for the load cause by, e.g., peer-to-
   peer applications, and the operator has the need that peers fetch
   their data from remote peers in other parts of the Internet.

                            ,-------.         +-----------+
          ,---.          ,-'         `-.      |   Host 1  |
       ,-'     `-.      /     ISP 1   +-------|ALTO Client|
      /           \    /              |  \    +-----------+
     /    ISP X    \   |              |  |    +-----------+
    /               \  \              +-------|   Host 2  |
   ;             #-###########################|ALTO Client|
   |             #   |   `-.         ,-'      +-----------+
   |             #   |      `-------'
   |             #   |      ,-------.         +-----------+
   :             #   ;   ,-'         `+-------|   Host 3  |
    \            #  /   /     ISP 2   | \     |ALTO Client|
     \           # /   /              |  \    +-----------+
      \          ###########          |  |    +-----------+
       `-.     ,-'     \   #          +-------|   Host 4  |
          `---'         \  ###################|ALTO Client|
                         `-.         ,-'      +-----------+
                            `-------'

       Legend:
       === preferred "connections"
       --- non-preferred "connections"

              Figure 8: ALTO Traffic Network De-Localization

   Figure 8 shows the result of such a guidance process where Host 2
   prefers a connection with Host 4 instead of Host 1, as shown in
   Figure 5.



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   TBD: Limits of this approach in general and with respect to p2p.
   describe how maps would look like.

3.1.5.  Application Tuning

   ALTO can also provide guidance to optimize the application-level
   topology of networked applications, e.g., by exposing network
   performance information.  Applications can often run own measurements
   to determine network performance, e.g., by active delay measurements
   or bandwidth probing, but such measurements result in overhead and
   complexity.  Accessing an ALTO server can be a simpler alternative.
   In addition, an ALTO server may also expose network information that
   applications cannot easily measure or reverse-engineer.

3.2.  Provisioning of ALTO Maps

3.2.1.  Data Sources

   TBD: This section will describe how ALTO maps in the protocol can be
   populated before using them.  The maps can significantly differ
   depending on the use case, the network architecture, and the trust
   relationship between ALTO server and ALTO client, etc.

   The ALTO server builds an ALTO-specific network topology that
   represents the network as it should be understood and utilized by the
   application.  Besides the security requirements that consist of not
   delivering any confidential or critical information about the
   infrastructure, there are efficiency requirements in terms of what
   aspects of the network are visible and required by the given use case
   and/or application.

   The ALTO server builds topology (for either Map and ECS services)
   based on multiple sources that may include routing protocols, network
   policies, state and performance information, geo-location, etc.  The
   network topology information is controlled and managed by the ALTO
   server.  In all cases, the ALTO topology will not contain any details
   that would endanger the network integrity and security, e.g., there
   will be no leaking of OSPF/ISIS/BGP databases to ALTO clients.

3.2.2.  Privacy Requirements

   Providing ALTO guidance results in a win-win situation both for
   network providers and users of the ALTO information.  Applications
   possibly get a better performance, while the the network provider has
   means to optimize the traffic engineering and thus its costs.

   Still, ISPs may have other important requirements when deploying
   ALTO: In particular, an ISP may not be willing to expose sensitive



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   operational details of its network.  The topology abstraction of ALTO
   enables an ISP to expose the network topology at a desired
   granularity only.

   With the ALTO Endpoint Cost Service, the ALTO client does not to have
   to implement any specific algorithm or mechanism in order to
   retrieve, maintain and process network topology information (of any
   kind).  The complexity of the network topology (computation,
   maintenance and distribution) is kept in the ALTO server and ECS is
   delivered on demand.  This allows the ALTO server to enhance and
   modify the way the topology information sources are used and
   combined.  This simplifies the enforcement of privacy policies of the
   ISP.

   The ALTO Network Map and Cost Map service expose an abstracted view
   on the ISP network topology.  Therefore, in this case care is needed
   when constructing those maps, as further discussed in Section 3.2.3.

3.2.3.  Map Partitioning and Grouping

   Host group descriptors are used in the ALTO client protocol to
   describe the location of a host in the network topology.  These
   identifiers are called Partition ID (PID) and e.g. expand to a set of
   IP address ranges (CIDR).

   An automated ALTO implementation may use dynamic algorithms to
   aggregate network topology.  However, it is often desirable to have a
   mechanism through which the network operator can control the level
   and details of network aggregation based on a set of requirements and
   constraints.

   IP/MPLS networks make use of a common mechanism to aggregate and
   group prefixes that is called BGP Communities.  BGP is the protocol
   all ISP networks use in order to exchange information about their
   prefix reachability.  BGP Community is an attribute used to tag a
   prefix to group prefixes based on mostly any criteria (as an example,
   most ISP networks originate BGP prefixes with communities identifying
   the Point of Presence (PoP) where the prefix has been originated).

   The ALTO server may leverage the BGP information that is available in
   the SP network layer and compute group of prefixes.  By policy, the
   ALTO server operator may decide an arbitrary cost defined between
   groups.  Alternatively, there are algorithms that allows a dynamic
   computation of cost between groups.

3.2.4.  Rating Criteria and/or Cost Calculation





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   Rating criteria are used in the ALTO client protocol to express
   topology- or connectivity-related properties, which are evaluated in
   order to generate the ALTO guidance.  The ALTO client protocol
   specification defines a basic set of rating criteria, which have to
   be supported by all implementations, and an extension procedure for
   adding new criteria.

   The following list gives an overview on further rating criteria that
   have been proposed or which are in use by ALTO-related prototype
   implementations.  This list is not intended as normative text.
   Instead, the only purpose of the following list is to document the
   rating criteria that have been proposed so far, and to solicit
   further feedback and discussion.

   Distance-related rating criteria:

   o  Relative topological distance: relative means that a larger
      numerical value means greater distance, but it is up to the ALTO
      service how to compute the values, and the ALTO client will not be
      informed about the nature of the information.  One way of
      generating this kind of information MAY be counting AS hops, but
      when querying this parameter, the ALTO client MUST NOT assume that
      the numbers actually are AS hops.

   o  Absolute topological distance, expressed in the number of
      traversed autonomous systems (AS).

   o  Absolute topological distance, expressed in the number of router
      hops (i.e., how much the TTL value of an IP packet will be
      decreased during transit).

   o  Absolute physical distance, based on knowledge of the approximate
      geolocation (continent, country) of an IP address.

   Charging-related rating criteria:

   o  Traffic volume caps, in case the Internet access of the resource
      consumer is not charged by "flat rate".  For each candidate
      resource provider, the ALTO service could indicate the amount of
      data that may be transferred from/to this resource provider until
      a given point in time, and how much of this amount has already
      been consumed.  Furthermore, it would have to be indicated how
      excess traffic would be handled (e.g., blocked, throttled, or
      charged separately at an indicated price).  The interaction of
      several applications running on a host, out of which some use this
      criterion while others don't, as well as the evaluation of this
      criterion in resource directories, which issue ALTO queries on
      behalf of other peers, are for further study.



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   Performance-related rating criteria:

   o  The minimum achievable throughput between the resource consumer
      and the candidate resource provider, which is considered useful by
      the application (only in ALTO queries), or

   o  An arbitrary upper bound for the throughput from/to the candidate
      resource provider (only in ALTO responses).  This may be, but is
      not necessarily the provisioned access bandwidth of the candidate
      resource provider.

   o  The maximum round-trip time (RTT) between resource consumer and
      the candidate resource provider, which is acceptable for the
      application for useful communication with the candidate resource
      provider (only in ALTO queries), or

   o  An arbitrary lower bound for the RTT between resource consumer and
      the candidate resource provider (only in ALTO responses).  This
      may be, for example, based on measurements of the propagation
      delay in a completely unloaded network.

   These rating criteria are subject to the remarks below:

   The ALTO client MUST be aware, that with high probability, the actual
   performance values differ significantly from these upper and lower
   bounds.  In particular, an ALTO client MUST NOT consider the "upper
   bound for throughput" parameter as a permission to send data at the
   indicated rate without using congestion control mechanisms.

   The discrepancies are due to various reasons, including, but not
   limited to the facts that

   o  the ALTO service is not an admission control system

   o  the ALTO service may not know the instantaneous congestion status
      of the network

   o  the ALTO service may not know all link bandwidths, i.e., where the
      bottleneck really is, and there may be shared bottlenecks

   o  the ALTO service may not know whether the candidate peer itself is
      overloaded

   o  the ALTO service may not know whether the candidate peer throttles
      the bandwidth it devotes for the considered application






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   o  the ALTO service may not know whether the candidate peer will
      throttle the data it sends to us (e.g., because of some fairness
      algorithm, such as tit-for-tat)

   Because of these inaccuracies and the lack of complete, instantaneous
   state information, which are inherent to the ALTO service, the
   application must use other mechanisms (such as passive measurements
   on actual data transmissions) to assess the currently achievable
   throughput, and it MUST use appropriate congestion control mechanisms
   in order to avoid a congestion collapse.  Nevertheless, these rating
   criteria may provide a useful shortcut for quickly excluding
   candidate resource providers from such probing, if it is known in
   advance that connectivity is in any case worse than what is
   considered the minimum useful value by the respective application.

   Rating criteria that SHOULD NOT be defined for and used by the ALTO
   service include:

   o  Performance metrics that are closely related to the instantaneous
      congestion status.  The definition of alternate approaches for
      congestion control is explicitly out of the scope of ALTO.
      Instead, other appropriate means, such as using TCP based
      transport, have to be used to avoid congestion.

3.3.  Known Limitations of ALTO

   This section describes some known limitations of ALTO in general or
   specific mechanisms in ALTO.

3.3.1.  Limitations of Map-based Approaches

   The specification of the ALTO protocol [I-D.ietf-alto-protocol] uses
   so-called network maps.  The network map approach uses host group
   descriptors that group one or multiple subnetworks (i.e., IP
   prefixes) to a single aggregate.  A set of IP prefixes is called
   partition and the associated Host Group Descriptor is called
   Partition ID (PID).  The "costs" between the various partition IDs is
   stored in a second map, the cost map.  Map-based approaches lower the
   signaling load on the server as maps have to be retrieved only if
   they change.











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   One main assumption for map-based approaches is that the information
   provided in these maps is static for a longer period of time, where
   this period of time refers to days, but not hours or even minutes.
   This assumption is fine as long as the network operator does not
   change any parameter, e.g., routing within the network and to the
   upstream peers, IP address assignment stays stable (and thus the
   mapping to the partitions).  However, there are several cases where
   this assumption is not valid, as:

   1.  ISPs reallocate IP subnets from time to time;

   2.  ISPs reallocate IP subnets on short notice;

   3.  IP prefix blocks may be assigned to a router that serves a
       variety of access networks;

   4.  Network costs between IP prefixes may change depending on the
       ISP's routing and traffic engineering.

   For 1): ISPs reallocate IPv4 subnets within their infrastructure from
   time to time, partly to ensure the efficient usage of IPv4 addresses
   (a scarce resource), and partly to enable efficient route tables
   within their network routers.  The frequency of these "renumbering
   events" depend on the growth in number of subscribers and the
   availability of address space within the ISP.  As a result, a
   subscriber's household device could retain an IPv4 address for as
   short as a few minutes, or for months at a time or even longer.

   Some folks have suggested that ISPs providing ALTO services could
   sub-divide their subscribers' devices into different IPv4 subnets (or
   certain IPv4 address ranges) based on the purchased service tier, as
   well as based on the location in the network topology.  The problem
   is that this sub-allocation of IPv4 subnets tends to decrease the
   efficiency of IPv4 address allocation.  A growing ISP that needs to
   maintain high efficiency of IPv4 address utilization may be reluctant
   to jeopardize their future acquisition of IPv4 address space.

   However, this is not an issue for map-based approaches if changes are
   applied in the order of days.

   For 2): ISPs can use techniques that allow the reallocation of IP
   prefixes on very short notice, i.e., within minutes.  An IP prefix
   that has no IP address assignment to a host anymore can be
   reallocated to areas where there is currently a high demand for IP
   addresses.

   For 3): In DSL-based access networks, IP prefixes are assigned to
   DSLAMs which are the first IP-hop in the access-network between the



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   CPE and the Internet.  The access-network between CPE and DSLAM
   (called aggregation network) can have varying characteristics (and
   thus associated costs), but still using the same IP prefix.  For
   instance one IP addresses IP11 out of a IP prefix IP1 can be assigned
   to a VDSL (e.g., 2 MBit/s uplink) access line while the subsequent IP
   address IP12 is assigned to a slow ADSL line (e.g., 128 kbit/s
   uplink).  These IP addresses are assigned on a first come first
   served basis, i.e., the a single IP address out of the same IP prefix
   can change its associated costs quite fast.  This may not be an issue
   with respect to the used upstream provider (thus the cross ISP
   traffic) but depending on the capacity of the aggregation-network
   this may raise to an issue.

   For 4): The routing and traffic engineering inside an ISP network, as
   well as the peering with other autonomous systems, can change
   dynamically and affect the information exposed by an ALTO server.  As
   a result, cost map and possibly also network maps can change.

3.3.2.  Limitiations of Non-Map-based Approaches

   The specification of the ALTO protocol [I-D.ietf-alto-protocol] uses,
   amongst others mechanism, a mechanism called Endpoint Cost Service
   (ECS).  ALTO clients can ask guidance for specific IP addresses to
   the ALTO server.  However, asking for IP addresses, asking with long
   lists of IP addresses, and asking quite frequently may overload the
   ALTO server.  The server has to rank each received IP address, which
   causes load at the server.  This may be amplified by the fact that
   not only a single ALTO client is asking for guidance, but a larger
   number of them.  The results of the ECS are also more difficult to
   cache than ALTO maps.

   Caching of IP addresses at the ALTO client or the usage of the H12
   approach [I-D.kiesel-alto-h12] in conjunction with caching may lower
   the query load on the ALTO server.

3.4.  Map Examples for Different Types of ISPs

3.4.1.  Small ISP with Single Internet Uplink













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   For a small ISP, the inter-domain traffic optimizing problem is how
   to decrease the traffic exchanged with other ISPs, because of high
   settlement costs.  By using the ALTO service to optimize traffic, a
   small ISP can define two "optimization areas": one is its own
   network; the other one consists of all other network destinations.
   The cost map can be defined as follows: the cost of link between
   clients of inner ISP's networks is lower than between clients of
   outer ISP's networks and clients of inner ISP's network.  As a
   result, a host with ALTO client inside the network of this ISP will
   prefer retrieving data from hosts connected to the same ISP.

   An example is given in Figure 9.  It is assumed that ISP A is a small
   ISP only having one access network.  As operator of the ALTO service,
   ISP A can define its network to be one optimization area, named as
   PID1, and define other networks to be the other optimization area,
   named as PID2.  C1 is denoted as the cost inside the network of ISP
   A. C2 is denoted as the cost from PID2 to PID1, and C3 from PID1 to
   PID2.  For the sake of simplifity, in the following C2=C3 is assumed.
   In order to keep traffic local inside ISP A, it makes sense to
   define: C1<C2

             -----------
         ////           \\\\
       //                   \\
     //                       \\                  /-----------\
    | +---------+               |             ////             \\\\
    | | ALTO    |  ISP A        |    C2      |    Other Networks   |
   |  | Service |  PID 1         <-----------     PID 2
    | +---------+  C1           |            |                     |
    |                           |             \\\\             ////
     \\                       //                  \-----------/
       \\                   //
         \\\\           ////
             -----------

              Figure 9: Example ALTO deployment in small ISPs

   A simplified extract of the corresponding ALTO network and cost maps
   is listed in Figure 10 and Figure 11, assuming that the network of
   ISP A has the IPv4 address ranges 192.0.2.0/24 and 198.51.100.0/25.
   In this example, the cost values C1 and C2 can be set to any number
   C1<C2.

      HTTP/1.1 200 OK
      ...
      Content-Type: application/alto-networkmap+json

      {



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       ...
        "network-map" : {
          "PID1" : {
            "ipv4" : [
              "192.0.2.0/24",
              "198.51.100.0/25"
            ]
          },
          "PID2" : {
            "ipv4" : [
              "0.0.0.0/0"
            ],
            "ipv6" : [
              "::/0"
            ]
          }
        }
      }

                    Figure 10: Example ALTO network map

      HTTP/1.1 200 OK
      ...
      Content-Type: application/alto-costmap+json

      {
          ...
          "cost-type" : {"cost-mode"  : "numerical",
                         "cost-metric": "routingcost"
          }
        },
        "cost-map" : {
          "PID1": { "PID1": C1,  "PID2": C2 },
          "PID2": { "PID1": C2,  "PID2": 0 },
        }
      }

                     Figure 11: Example ALTO cost map

3.4.2.  ISP with Several Fixed Access Networks

   For a large ISP with a fixed network comprising several access
   networks and a core network, the traffic optimizing problems will
   include (1) using the backbone network efficiently, (2) adjusting the
   traffic balance in different access networks according to traffic
   conditions and management policies, and (3) achieving a reduction of
   settlement costs with other ISPs.




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   Such a large ISP deploying an ALTO service may want to optimize its
   traffic according to the network topology of its access networks.
   For example, each access network could be defined to be one
   optimization area, i.e., traffic should be kept locally withing that
   area if possible.  Then the costs between those access networks can
   be defined according to a corresponding traffic optimizing
   requirement by this ISP.  One example setup is further described
   below and also shown in Figure 12.

   In this example, ISP A has one backbone network and three access
   networks, named as AN A, AN B, and AN C. A P2P application is used in
   this example.  For the traffic optimization, the first requirement is
   to decrease the P2P traffic on the backbone network inside the
   Autonomous System of ISP A; and the second requirement is to decrease
   the P2P traffic to other ISPs, i.e., other Autonomous Systems.  The
   second requirement can be assumed to have priority over the first
   one.  Also, we assume that the settlement rate with ISP B is lower
   than with other ISPs.  Then ISP A can deploy an ALTO service to meet
   these traffic optimization requirements.  In the following, we will
   give an example of an ALTO setting and configuration according to
   these requirements.

   In inner network of ISP A, we can define each access network to be
   one optimization area, and assign one PID to each access network,
   such as PID 1, PID 2, and PID 3.  Because of different peerings with
   different outer ISPs, we define ISP B to be one optimization area,
   and we assign PID 4 to it.  We define all other networks to be one
   optimization area and assign PID 5 to it.

   We assign costs (C1, C2, C3, C4, C5, C6, C7, C8) as shown in Figure
   12.  Cost C1 is denoted as the link cost in inner AN A (PID 1), and
   C2 and C3 are defined accordingly.  C4 is denoted as the link cost
   from PID 1 to PID 2, and C5 is the correspinding cost from PID 3,
   which is assumed to have a similar value.  C6 is the cost between PID
   1 and PID 3.  For simplicity, we assume symmetrical costs between the
   AN this example.  C7 is denoted as the link cost from the ISP B to
   ISP A. C8 is the link cost from other networks to ISP A.

   According to previous discussion of the first requirement and the
   second requirement, the relationship of these costs will be defined
   as: (C1, C2, C3) < (C4, C5, C6) < (C7) < (C8)

    +------------------------------------+         +----------------+
    | ISP A  +---------------+           |         |                |
    |        |    Backbone   |           |   C7    |      ISP B     |
    |     +--+    Network    +---+       |<--------+      PID 4     |
    |     |  +-------+-------+   |       |         |                |
    |     |          |           |       |         |                |



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    |     |          |           |       |         +----------------+
    | +---+--+    +--+---+     +-+----+  |
    | |AN A  | C4 |AN B  |  C5 |AN C  |  |
    | |PID 1 +<-->|PID 2 |<--->+PID 3 |  |
    | |C1    |    |C2    |     |C3    |  |         +----------------+
    | +---+--+    +------+     +-+----+  |         |                |
    |     |                      |       |   C8    | Other Networks |
    |     +----------------------+       |<--------+ PID 5          |
    |                 C6                 |         |                |
    |                                    |         |                |
    +------------------------------------+         +----------------+


    Figure 12: ALTO deployment in large ISPs with layered fixed network
                                structures

3.4.3.  ISP with Fixed and Mobile Network

   An ISP with both mobile network and fixed network my focus on
   optimizing the mobile traffic by keeping traffic in the fixed network
   as far as possible, because wireless bandwidth is a scarce resource
   and traffic is costly in mobile network.  In such a case, the main
   requirement of traffic optimization could be decreasing the usage of
   radio resources in the mobile network.  An ALTO service can be
   deployed to meet these needs.

   Figure 13 shows an example: ISP A operates one mobile network, which
   is connected to a backbone network.  The ISP also runs two fixed
   access networks AN A and AN B, which are also connected to the
   backbone network.  In this network structure, the mobile network can
   be defined as one optimization area, and PID 1 can be assigned to it.
   Access networks AN A and B can also be defined as optimization areas,
   and PID 2 and PID 3 can be assigned, respectively.  The cost values
   are then defined as shown in Figure 13.

   To decrease the usage of wireless link, the relationship of these
   costs can be defined as follows:

   From view of mobile network: C4 < C1.  This means that clients in
   mobile network requiring data resource from other clients will prefer
   clients in AN A to clients in the mobile network.  This policy can
   decrease the usage of wireless link and power consumption in
   terminals.

   From view of AN A: C2 < C6, C5 = maximum cost.  This means that
   clients in other optimization area will avoid retrieving data from
   the mobile network.




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    +-----------------------------------------------------------------+
    |                                                                 |
    |  ISP A                 +-------------+                          |
    |               +--------+   ALTO      +---------+                |
    |               |        |   Service   |         |                |
    |               |        +------+------+         |                |
    |               |               |                |                |
    |               |               |                |                |
    |               |               |                |                |
    |       +-------+-------+       | C6    +--------+------+         |
    |       |     AN A      |<--------------|      AN B     |         |
    |       |     PID 2     |   C7  |       |      PID 3    |         |
    |       |     C2        |-------------->|      C3       |         |
    |       +---------------+       |       +---------------+         |
    |             ^    |            |              |     ^            |
    |             |    |            |              |     |            |
    |             |    |C4          |              |     |            |
    |          C5 |    |            |              |     |            |
    |             |    |   +--------+---------+    |     |            |
    |             |    +-->|  Mobile Network  |<---+     |            |
    |             |        |  PID 1           |          |            |
    |             +------- |  C1              |----------+            |
    |                      +------------------+                       |
    +-----------------------------------------------------------------+


          Figure 13: ALTO deployment in ISPs with mobile network

3.5.  Deployment Experiences

   The examples in the previous section are simple and do not consider
   specific requirements inside access networks, uch as different link
   types.  Deploying an ALTO service in real network will have to
   require further network conditions and requirements.  One real
   example is described in greater detail in reference
   [I-D.lee-alto-chinatelecom-trial].

   Also, experiments have been conducted with ALTO-like deployments in
   Internet Service Provider (ISP) networks.  For instance, NTT
   performed tests with their HINT server implementation and dummy nodes
   to gain insight on how an ALTO-like service influence peer-to-peer
   systems [I-D.kamei-p2p-experiments-japan].  The results of an early
   experiment conducted in the Comcast network are documented in
   [RFC5632].

4.  Using ALTO for P2P Traffic Optimization





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4.1.  Overview

4.1.1.  Usage Scenario

   The scope of this section is the interaction of peer-to-peer
   applications that use a centralized resource directory ("tracker"),
   with the ALTO service.  In this scenario, the resource consumer
   ("peer") asks the resource directory for a list of candidate resource
   providers, which can provide the desired resource.

   For efficiency reasons (i.e., message size), usually only a subset of
   all resource providers known to the resource directory will be
   returned to the resource consumer.  Some or all of these resource
   providers, plus further resource providers learned by other means
   such as direct communication between peers, will be contacted by the
   resource consumer for accessing the resource.  The purpose of ALTO is
   giving guidance on this peer selection, which is supposed to yield
   better-than-random results.  The tracker response as well as the ALTO
   guidance are most beneficial in the initial phase after the resource
   consumer has decided to access a resource, as long as only few
   resource providers are known.  Later, when the resource consumer has
   already exchanged some data with other peers and measured the
   transmission speed, the relative importance of ALTO may dwindle.

                                 ,-------.
          ,---.               ,-'         `-.   +-----------+
       ,-'     `-.           /     ISP 1     \  |   Peer 1  |*****
      /           \         / +-------------+ \ |           |    *
     /    ISP X    \   +=====>+ ALTO Server |  )+-----------+    *
    /               \  =    \ +-------------+ / +-----------+    *
   ; +-----------+   : =     \               /  |   Peer 2  |    *
   | |  Tracker  |<====+      `-.         ,-'   |           |*****
   | |ALTO Client|<====+         `-------'      +-----------+   **
   | +-----------+   | =         ,-------.                      **
   :        *        ; =      ,-'         `-.   +-----------+   **
    \       *       /  =     /     ISP 2     \  |   Peer 3  |   **
     \      *      /   =    / +-------------+ \ |           |*****
      \     *     /    +=====>| ALTO Server |  )+-----------+  ***
       `-.  *  ,-'          \ +-------------+ / +-----------+  ***
          `-*-'              \               /  |   Peer 4  |*****
            *                 `-.         ,-'   |           | ****
            *                    `-------'      +-----------+ ****
            *                                                 ****
            *                                                 ****
            ***********************************************<******
       Legend:
       === ALTO client protocol
       *** Application protocol



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      Figure 14: Global tracker accessing ALTO server at various ISPs

   Figure 14 depicts a tracker-based system, in which the tracker embeds
   the ALTO client.  The tracker itself is hosted and operated by an
   entity different than the ISP hosting and operating the ALTO server.
   A tracker outside the network of the ISP is the typical use case.
   For instance, a tracker like Pirate Bay can serve Bittorrent peers
   world-wide.  Initially, the tracker has to look-up the ALTO server in
   charge for each peer where it receives a ALTO query for.  Therefore,
   the ALTO server has to discover the handling ALTO server, as
   described in [I-D.ietf-alto-server-discovery].  However, the peers do
   not have any way to query the server themselves.  This setting allows
   giving the peers a better selection of candidate peers for their
   operation at an initial time, but does not consider peers learned
   through direct peer-to-peer knowledge exchange.  This is called peer
   exchange (PEX) in bittorent, for instance.

                            ,-------.         +-----------+
          ,---.          ,-'         `-.  +==>|   Peer 1  |*****
       ,-'     `-.      /     ISP 1     \ =   |ALTO Client|    *
      /           \    / +-------------+<=+   +-----------+    *
     /    ISP X    \   | + ALTO Server |<=+   +-----------+    *
    /               \  \ +-------------+ /=   |   Peer 2  |    *
   ;   +---------+   :  \               / +==>|ALTO Client|*****
   |   | Global  |   |   `-.         ,-'      +-----------+   **
   |   | Tracker |   |      `-------'                         **
   |   +---------+   |      ,-------.         +-----------+   **
   :        *        ;   ,-'         `-.  +==>|   Peer 3  |   **
    \       *       /   /     ISP 2     \ =   |ALTO Client|*****
     \      *      /   / +-------------+<=+   +-----------+  ***
      \     *     /    | | ALTO Server |<=+   +-----------+  ***
       `-.  *  ,-'     \ +-------------+ /=   |   Peer 4  |*****
          `-*-'         \               / +==>|ALTO Client| ****
            *            `-.         ,-'      +-----------+ ****
            *               `-------'                       ****
            *                                               ****
            ***********************************************<****
       Legend:
       === ALTO client protocol
       *** Application protocol

              Figure 15: Global Tracker - Local ALTO Servers

   The scenario in Figure 15 lets the peers directly communicate with
   their ISP's ALTO server (i.e., ALTO client embedded in the peers),
   giving thus the peers the most control on which information they
   query for, as they can integrate information received from trackers
   and through direct peer-to-peer knowledge exchange.



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                            ,-------.         +-----------+
          ,---.          ,-'  ISP 1  `-.  ***>|   Peer 1  |
       ,-'     `-.      /+-------------+\ *   |           |
      /           \    / +   Tracker   |<**   +-----------+
     /    ISP X    \   | +-----===-----+<**   +-----------+
    /               \  \ +-----===-----+ /*   |   Peer 2  |
   ;   +---------+   :  \+ ALTO Server |/ ***>|           |
   |   | Global  |   |   +-------------+      +-----------+
   |   | Tracker |   |      `-------'
   |   +---------+   |                        +-----------+
   :          ^      ;      ,-------.         |   Peer 3  |
    \         *     /    ,-'  ISP 2  `-.  ***>|           |
     \        *    /    /+-------------+\ *   +-----------+
      \       *   /    / +   Tracker   |<**   +-----------+
       `-.    *,-'     | +-----===-----+ |    |   Peer 4  |<*
          `---*        \ +-----===-----+ /    |           | *
              *         \+ ALTO Server |/     +-----------+ *
              *          +-------------+                    *
              *             `-------'                       *
              ***********************************************
       Legend:
       === ALTO client protocol
       *** Application protocol

     Figure 16: P4P approach with local tracker and local ALTO server

   There are some attempts to let ISP's to deploy their own trackers, as
   shown in Figure 16.  In this case, the client has no chance to get
   guidance from the ALTO server, other than talking to the ISP's
   tracker.  However, the peers would have still chance the contact
   other trackers, deployed by entities other than the peer's ISP.

   Figure 16 and Figure 14 ostensibly take peers the possibility to
   directly query the ALTO server, if the communication with the ALTO
   server is not permitted for any reason.  However, considering the
   plethora of different applications of ALTO, e.g., multiple tracker
   and non-tracker based P2P systems and or applications searching for
   relays, it seems to be beneficial for all participants to let the
   peers directly query the ALTO server.  The peers are also the single
   point having all operational knowledge to decide whether to use the
   ALTO guidance and how to use the ALTO guidance.  This is a preference
   for the scenario depicted in Figure Figure 15.









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4.1.2.  Applicability of ALTO

   TODO

4.2.  Deployment Recommendations

4.2.1.  ALTO Services

   In case of peer-to-peer networks, there is basically a dilemma which
   ALTO service to use: The Cost Map Service is seen as the only working
   solution by peer-to-peer software vendors and the Endpoint Cost
   Service is seen as the only working by the network operators.  But
   neither the software vendors nor the operators seem to willing to
   change their position.  However, there is the need to get both sides
   on board, to come to a solution.  For other use cases of ALTO, in
   particular in more controlled environments, both approaches might be
   feasible and it is more an engineering tradeoff whether to use a map-
   based or query-based ALTO service.

4.2.2.  Guidance Considerations

   The ALTO protocol specification [I-D.ietf-alto-protocol] details how
   an ALTO client can query an ALTO server for guiding information and
   receive the corresponding replies.  However, in the considered
   scenario of a tracker-based P2P application, there are two
   fundamentally different possibilities where to place the ALTO client:

   1.  ALTO client in the resource consumer ("peer")

   2.  ALTO client in the resource directory ("tracker")

   In the following, both scenarios are compared in order to explain the
   need for third-party ALTO queries.

   In the first scenario (see Figure 18), the resource consumer queries
   the resource directory for the desired resource (F1).  The resource
   directory returns a list of potential resource providers without
   considering ALTO (F2).  It is then the duty of the resource consumer
   to invoke ALTO (F3/F4), in order to solicit guidance regarding this
   list.

   In the second scenario (see Figure 20), the resource directory has an
   embedded ALTO client, which we will refer to as RDAC in this
   document.  After receiving a query for a given resource (F1) the
   resource directory invokes the RDAC to evaluate all resource
   providers it knows (F2/F3).  Then it returns a, possibly shortened,
   list containing the "best" resource providers to the resource
   consumer (F4).



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   .............................          .............................
   : Tracker                   :          : Peer                      :
   :   ______                  :          :                           :
   : +-______-+                :          :            k good         :
   : |        |     +--------+ : P2P App. : +--------+ peers +------+ :
   : |   N    |     | random | : Protocol : | ALTO-  |------>| data | :
   : | known  |====>| pre-   |*************>| biased |       | ex-  | :
   : | peers, |     | selec- | : transmit : | peer   |------>| cha- | :
   : | M good |     | tion   | : n peer   : | select | n-k   | nge  | :
   : +-______-+     +--------+ : IDs      : +--------+ bad p.+------+ :
   :...........................:          :.....^.....................:
                                                |
                                                | ALTO
                                                | client protocol
                                              __|___
                                            +-______-+
                                            |        |
                                            | ALTO   |
                                            | server |
                                            +-______-+

         Figure 17: Tracker-based P2P Application with random peer
                               preselection

   Peer w. ALTO cli.            Tracker               ALTO Server
   --------+--------       --------+--------       --------+--------
           | F1 Tracker query      |                       |
           |======================>|                       |
           | F2 Tracker reply      |                       |
           |<======================|                       |
           | F3 ALTO client protocol query                 |
           |---------------------------------------------->|
           | F4 ALTO client protocol reply                 |
           |<----------------------------------------------|
           |                       |                       |

   ====  Application protocol (i.e., tracker-based P2P app protocol)
   ----  ALTO client protocol

      Figure 18: Basic message sequence chart for resource consumer-
                           initiated ALTO query

   .............................          .............................
   : Tracker                   :          : Peer                      :
   :   ______                  :          :                           :
   : +-______-+                :          :                           :
   : |        |     +--------+ : P2P App. :  k good peers &  +------+ :
   : |   N    |     | ALTO-  | : Protocol :  n-k bad peers   | data | :



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   : | known  |====>| biased |******************************>| ex-  | :
   : | peers, |     | peer   | : transmit :                  | cha- | :
   : | M good |     | select | : n peer   :                  | nge  | :
   : +-______-+     +--------+ : IDs      :                  +------+ :
   :.....................^.....:          :...........................:
                         |
                         | ALTO
                         | client protocol
                       __|___
                     +-______-+
                     |        |
                     | ALTO   |
                     | server |
                     +-______-+

   Figure 19: Tracker-based P2P Application with ALTO client in tracker

         Peer               Tracker w. RDAC           ALTO Server
   --------+--------       --------+--------       --------+--------
           | F1 Tracker query      |                       |
           |======================>|                       |
           |                       | F2 ALTO cli. p. query |
           |                       |---------------------->|
           |                       | F3 ALTO cli. p. reply |
           |                       |<----------------------|
           | F4 Tracker reply      |                       |
           |<======================|                       |
           |                       |                       |

   ====  Application protocol (i.e., tracker-based P2P app protocol)
   ----  ALTO client protocol

    Figure 20: Basic message sequence chart for third-party ALTO query

   Note: the message sequences depicted in Figure 18 and Figure 20 may
   occur both in the target-aware and the target-independent query mode
   (c.f. [RFC6708]).  In the target-independent query mode no message
   exchange with the ALTO server might be needed after the tracker
   query, because the candidate resource providers could be evaluated
   using a locally cached "map", which has been retrieved from the ALTO
   server some time ago.

   The problem with the first approach is, that while the resource
   directory might know thousands of peers taking part in a swarm, the
   list returned to the resource consumer is usually shortened for
   efficiency reasons.  Therefore, the "best" (in the sense of ALTO)
   potential resource providers might not be contained in that list
   anymore, even before ALTO can consider them.



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   For illustration, consider a simple model of a swarm, in which all
   peers fall into one of only two categories: assume that there are
   "good" ("good" in the sense of ALTO's better-than-random peer
   selection, based on an arbitrary desired rating criterion) and "bad'
   peers only.  Having more different categories makes the maths more
   complex but does not change anything to the basic outcome of this
   analysis.  Assume that the swarm has a total number of N peers, out
   of which are M "good" and N-M "bad" peers, which are all known to the
   tracker.  A new peer wants to join the swarm and therefore asks the
   tracker for a list of peers.

   If, according to the first approach, the tracker randomly picks n
   peers from the N known peers, the result can be described with the
   hypergeometric distribution.  The probability that the tracker reply
   contains exactly k "good" peers (and n-k "bad" peers) is:


               / m \   / N - m \
               \ k /   \ n - k /
   P(X=k) =  ---------------------
                     / N \
                     \ n /


           / n \        n!
   with    \ k /  = -----------    and   n! = n * (n-1) * (n-2) * .. * 1
                     k! (n-k)!



   The probability that the reply contains at most k "good" peers is:
   P(X<=k)=P(X=0)+P(X=1)+..+P(X=k).

   For example, consider a swarm with N=10,000 peers known to the
   tracker, out of which M=100 are "good" peers.  If the tracker
   randomly selects n=100 peers, the formula yields for the reply:
   P(X=0)=36%, P(X<=4)=99%. That is, with a probability of approx. 36%
   this list does not contain a single "good" peer, and with 99%
   probability there are only four or less of the "good" peers on the
   list.  Processing this list with the guiding ALTO information will
   ensure that the few favorable peers are ranked to the top of the
   list; however, the benefit is rather limited as the number of
   favorable peers in the list is just too small.

   Much better traffic optimization could be achieved if the tracker
   would evaluate all known peers using ALTO, and return a list of 100
   peers afterwards.  This list would then include a significantly
   higher fraction of "good" peers.  (Note, that if the tracker returned



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   "good" peers only, there might be a risk that the swarm might
   disconnect and split into several disjunct partitions.  However,
   finding the right mix of ALTO-biased and random peer selection is out
   of the scope of this document.)

   Therefore, from an overall optimization perspective, the second
   scenario with the ALTO client embedded in the resource directory is
   advantageous, because it is ensured that the addresses of the "best"
   resource providers are actually delivered to the resource consumer.
   An architectural implication of this insight is that the ALTO server
   discovery procedures must support third-party discovery.  That is, as
   the tracker issues ALTO queries on behalf of the peer which contacted
   the tracker, the tracker must be able to discover an ALTO server that
   can give guidance suitable for that respective peer.

5.  Using ALTO for CDNs

5.1.  Overview

5.1.1.  Usage Scenario

   This section discuss the usage of ALTO for Content Delivery Networks
   (CDNs) [I-D.jenkins-alto-cdn-use-cases].  CDNs are used to bring a
   service (e.g., a web page, videos, etc) closer to the location of the
   user - where close refers to shorten the distance between the client
   and the server in the IP topology.  CDNs use several techniques to
   decide which server is closest to a client requesting a service.  One
   common way to do so, is relying on the DNS system, but there are many
   other ways, see [RFC3568].

   The general issue for CDNs, independent of DNS or HTTP Redirect based
   approaches (see, for instance, [I-D.penno-alto-cdn]), is that the CDN
   logic has to match the client's IP address with the closest CDN
   cache.  This matching is not trivial, for instance, in DNS based
   approaches, where the IP address of the DNS original requester is
   unknown (see [I-D.vandergaast-edns-client-ip] for a discussion of
   this and a solution approach).

5.1.2.  Applicability of ALTO

   TODO: Rewording required

   When a user request a given content, the CDN locates the content in
   one or more caches and executes a selection algorithms in order to
   redirect the user to the 'best' cache.  In order to achieve that, the
   CDN issues an ECS request with the endpoint address (IPv4/IPv6) of
   the user (content requester) and the set of endpoint addresses of the
   content caches (content targets).  The ALTO server, receives the



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   request and ranks the list of content targets addresses based on
   their distance from the content requester.  By default, according to
   [I-D.ietf-alto-protocol], the distance represents the routing cost as
   computed by the routing layer (OSPF, ISIS, BGP) and may take into
   consideration other routing criteria such as MPLS-VPN (MP-BGP) and
   MPLS-TE (RSVP), policy and state and performance information in
   addition to other information sources (policy, geo-location, state
   and performance).

   Once the ALTO server computed the distance it replies with the ranked
   list of content target addresses.  The list being ranked by distance,
   the CDN is capable of integrating the rankings into its selection
   process (that will also incorporate other criteria) and redirect the
   user accordingly.

   The Request Router may request the Endpoint service from the ALTO
   client.

   Specifically, the Request Router requests the Endpoint Cost Service
   in order to rank/rate the content locations (i.e., IP addresses of
   CDN nodes) based on their distance/cost (by default the Endpoint Cost
   Service operates based on Routing Distance) from/to the user address.

   Once the Request Router obtained from the ALTO Server the ranked list
   of locations (for the specific user) it can incorporate this
   information into its selection mechanisms in order to point the user
   to the most appropriate location.

   A Request Router that uses the Endpoint Cost Service may query the
   ALTO Server for rankings of CDN Node IP addresses for each
   interesting host and cache the results for later usage.

   Maps Services and ECS deliver similar ALTO service by allowing the
   CDN to optimize internal selection mechanisms.  Both services deliver
   similar level of security, confidentiality of layer-specific
   information (i.e.: application and network) however, Maps and ECS
   differ in the way the ALTO service is delivered and address a
   different set of requirements in terms of topology information and
   network operations.

5.2.  Deployment Recommendations

5.2.1.  ALTO Services

   When ALTO server receives an ECS request, it may not have the most
   appropriate topology information in order to accurately determine the
   ranking.  In such case, the ALTO server, may want to adopt the
   following strategies:



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   o  Reply with available information (best effort).

   o  Redirect the request to another ALTO server presumed to have
      better topology information (redirection).

   o  Doing both (best effort and redirection).  In this case, the reply
      message contains both the rankings and the indication of another
      ALTO server where more accurate rankings may be delivered.

   The decision process that is used to determine if redirection is
   necessary (and which mode to use) is out of the scope of this
   document.  As an example, an ALTO server may decide to redirect any
   request having addresses that are located into a remote Autonomous
   System.  In such case the redirection message includes the ALTO
   server to be used and that resides in the remote AS.  Redirection
   implies communication between ALTO servers so to be able to signal
   their identity, location and type of visibility (AS number).

5.2.2.  Guidance Considerations

   Each reply sent back by the ALTO server to the ALTO client running in
   the CDN has a validity in time so that the CDN can cache the results
   in order to re-use it and hence reducing the number of transactions
   between CDN and ALTO server.  The ALTO server may indicate in the
   reply message how long the content of the message is to be considered
   reliable and insert a lifetime value that will be used by the CDN in
   order to cache (and then flush or refresh) the entry.

   An ALTO server implementation may want to keep state about ALTO
   clients so to inform and signal to these clients when a major network
   event happened so to clear the ALTO cache in the client.  In a CDN/
   ALTO interworking architecture where there's a few CDN component
   interacting with the ALTO server there are no scalability issues in
   maintaining state about clients in the ALTO server.

   ALTO server ranks addresses based on topology information it acquires
   from the network.  The different methods and algorithms through which
   the ALTO server computes topology information and rankings is out of
   the scope of this document.  However, and in the case the rankings
   are based on routing (IP/MPLS) topology, it is obvious that network
   events may impact the ranking computation.  The scope of the ECS
   service delivered to a CDN is not to maintain the CDN aware of any
   possible network topology changes since, due to redundancy of current
   networks, most of the network events happening in the infrastructure
   will have limited impact on the CDN.  However, catastrophic events
   such as main trunks failures or backbone partition will have to take
   into account by the ALTO server so to redirect traffic away from the
   failure impacted area.



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6.  Other Use Cases

   This section briefly surveys and references other use cases that have
   been suggested for ALTO.

6.1.  Monitoring Data Reporting

   TODO

6.2.  Virtual Private Networks (VPNs)

   TODO

6.3.  In-Network Caching

   Deployment of intra-domain P2P caches has been proposed for a
   cooperations between the network operator and the P2P service
   providers, e.g., to reduce the bandwith consumption in access
   networks [I-D.deng-alto-p2pcache].

      +--------------+                +------+
      | ISP 1 network+----------------+Peer 1|
      +-----+--------+                +------+
      |
   +--------+------------------------------------------------------+
   |        |                                      ISP 2 network   |
   |  +---------+                                                  |
   |  |L1 Cache |                                                  |
   |  +-----+---+                                                  |
   |        +--------------------+----------------------+          |
   |        |                    |                      |          |
   | +------+------+      +------+-------+       +------+-------+  |
   | | AN1         |      | AN2          |       | AN3          |  |
   | | +---------+ |      | +----------+ |       |              |  |
   | | |L2 Cache | |      | |L2 Cache  | |       |              |  |
   | | +---------+ |      | +----------+ |       |              |  |
   | +------+------+      +------+-------+       +------+-------+  |
   |        |                                           |          |
   |        +--------------------+                      |          |
   |        |                    |                      |          |
   | +------+------+      +------+-------+       +------+-------+  |
   | | SUB-AN11    |      | SUB-AN12     |       | SUB-AN31     |  |
   | | +---------+ |      |              |       |              |  |
   | | |L3 Cache | |      |              |       |              |  |
   | | +---------+ |      |              |       |              |  |
   | +------+------+      +------+-------+       +------+-------+  |
   |        |                    |                      |          |
   +--------+--------------------+----------------------+----------+



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            |                    |                      |
        +---+---+            +---+---+                  |
        |       |            |       |                  |
     +--+--+ +--+--+      +--+--+ +--+--+            +--+--+
     |Peer2| |Peer3|      |Peer4| |Peer5|            |Peer6|
     +-----+ +-----+      +-----+ +-----+            +-----+

            Figure 21: General architecture of intra-ISP caches

   Figure 21 depics the overall architecture of a potential P2P cache
   deployments inside an ISP 2 with various access network types.  As
   shown in the figure, P2P caches may be deployed at various levels,
   including the interworking gateway linking with other ISPs, internal
   access network gateways linking with different types of accessing
   networks (e.g. WLAN, cellular and wired), and even within an
   accessing network at the entries of individual WLAN sub-networks.
   Moreover, depending on the network context and the operator's policy,
   each cache can be a Forwarding Cache or a Bidirectional Cache
   [I-D.deng-alto-p2pcache].

   In such a cache architecture, the locations of caches could be used
   as dividers of different PIDs to guide intra-ISP network abstraction
   and mark costs among them according to the location and type of
   relevant caches.

   Further details and deployment considerations can be found in
   [I-D.deng-alto-p2pcache].

7.  Security Considerations

   The ALTO protocol itself as well as the ALTO client and server raise
   new security issues beyond the ones mentioned in
   [I-D.ietf-alto-protocol] and issues related to message transport over
   the Internet.  For instance, Denial of Service (DoS) is of interest
   for the ALTO server and also for the ALTO client.  A server can get
   overloaded if too many TCP requests hit the server, or if the query
   load of the server surpasses the maximum computing capacity.  An ALTO
   client can get overloaded if the responses from the sever are, either
   intentionally or due to an implementation mistake, too large to be
   handled by that particular client.

   This section is solely giving a first shot on security issues related
   to ALTO deployments.

7.1.  Information Leakage from the ALTO Server

   The ALTO server will be provisioned with information about the owning
   ISP's network and very likely also with information about neighboring



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   ISPs.  This information (e.g., network topology, business relations,
   etc.) is considered to be confidential to the ISP and must not be
   revealed.

   The ALTO server will naturally reveal parts of that information in
   small doses to peers, as the guidance given will depend on the above
   mentioned information.  This is seen beneficial for both parties,
   i.e., the ISP's and the peer's. However, there is the chance that one
   or multiple peers are querying an ALTO server with the goal to gather
   information about network topology or any other data considered
   confidential or at least sensitive.  It is unclear whether this is a
   real technical security risk or whether this is more a perceived
   security risk.

7.2.  ALTO Server Access

   Depending on the use case of ALTO, several access restrictions to an
   ALTO server may or may not apply.

   For peer-to-peer applications, a potential deployment scenario is
   that an ALTO server is solely accessible by peers from the ISP
   network (as shown in Figure 15).  For instance, the source IP address
   can be used to grant only access from that ISP network to the server.
   This will "limit" the number of peers able to attack the server to
   the user's of the ISP (however, including botnet computers).

   If the ALTO server has to be accessible by parties not located in the
   ISP's network (see Figure Figure 14), e.g., by a third-party tracker
   or by a CDN system outside the ISP's network, the access restrictions
   have to be looser.  In the extreme case, i.e., no access
   restrictions, each and every host in the Internet can access the ALTO
   server.  This might no be the intention of the ISP, as the server is
   not only subject to more possible attacks, but also on the load
   imposed to the server, i.e., possibly more ALTO clients to serve and
   thus more work load.

   There are also use cases where the access to the ALTO server has to
   be much more strictly controlled, i. e., where an authentication and
   authorization of the ALTO client to the server may be needed.  For
   instance, in case of CDN optimization the provider of an ALTO service
   as well as potential users are possibly well-known.  Only CDN
   entities may need ALTO access; access to the ALTO servers by
   residential users may neither be necessary nor be desired.

7.3.  Faking ALTO Guidance






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   It has not yet been investigated how a faked or wrong ALTO guidance
   by an ALTO server can impact the operation of the network and also
   the peers.

   Here is a list of examples how the ALTO guidance could be faked and
   what possible consequences may arise:

   Sorting  An attacker could change to sorting order of the ALTO
      guidance (given that the order is of importance, otherwise the
      ranking mechanism is of interest), i.e., declaring peers located
      outside the ISP as peers to be preferred.  This will not pose a
      big risk to the network or peers, as it would mimic the "regular"
      peer operation without traffic localization, apart from the
      communication/processing overhead for ALTO.  However, it could
      mean that ALTO is reaching the opposite goal of shuffling more
      data across ISP boundaries, incurring more costs for the ISP.

   Preference of a single peer  A single IP address (thus a peer) could
      be marked as to be preferred all over other peers.  This peer can
      be located within the local ISP or also in other parts of the
      Internet (e.g., a web server).  This could lead to the case that
      quite a number of peers to trying to contact this IP address,
      possibly causing a Denial of Service (DoS) attack.

8.  Conclusion

   This document discusses how the ALTO protocol can be deployed in
   different use cases and provides corresponding guidance and
   recommendations to network administrators and application developers.

9.  References

9.1.  Normative References

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

   [RFC3568]  Barbir, A., Cain, B., Nair, R., and O. Spatscheck, "Known
              Content Network (CN) Request-Routing Mechanisms", RFC
              3568, July 2003.

9.2.  Informative References

   [I-D.deng-alto-p2pcache]
              Lingli, D., Chen, W., Yi, Q., and Y. Zhang,
              "Considerations for ALTO with network-deployed P2P
              caches", draft-deng-alto-p2pcache-02 (work in progress),
              July 2013.



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   [I-D.ietf-alto-protocol]
              Alimi, R., Penno, R., and Y. Yang, "ALTO Protocol", draft-
              ietf-alto-protocol-20 (work in progress), October 2013.

   [I-D.ietf-alto-server-discovery]
              Kiesel, S., Stiemerling, M., Schwan, N., Scharf, M., and
              S. Yongchao, "ALTO Server Discovery", draft-ietf-alto-
              server-discovery-10 (work in progress), September 2013.

   [I-D.jenkins-alto-cdn-use-cases]
              Niven-Jenkins, B., Watson, G., Bitar, N., Medved, J., and
              S. Previdi, "Use Cases for ALTO within CDNs", draft-
              jenkins-alto-cdn-use-cases-03 (work in progress), June
              2012.

   [I-D.kamei-p2p-experiments-japan]
              Kamei, S., Momose, T., Inoue, T., and T. Nishitani, "ALTO-
              Like Activities and Experiments in P2P Network Experiment
              Council", draft-kamei-p2p-experiments-japan-09 (work in
              progress), October 2012.

   [I-D.kiesel-alto-h12]
              Kiesel, S. and M. Stiemerling, "ALTO H12", draft-kiesel-
              alto-h12-02 (work in progress), March 2010.

   [I-D.lee-alto-chinatelecom-trial]
              Li, K. and G. Jian, "ALTO and DECADE service trial within
              China Telecom", draft-lee-alto-chinatelecom-trial-04 (work
              in progress), March 2012.

   [I-D.penno-alto-cdn]
              Penno, R., Medved, J., Alimi, R., Yang, R., and S.
              Previdi, "ALTO and Content Delivery Networks", draft-
              penno-alto-cdn-03 (work in progress), March 2011.

   [I-D.vandergaast-edns-client-ip]
              Contavalli, C., Gaast, W., Leach, S., and D. Rodden,
              "Client IP information in DNS requests", draft-
              vandergaast-edns-client-ip-01 (work in progress), May
              2010.

   [RFC5632]  Griffiths, C., Livingood, J., Popkin, L., Woundy, R., and
              Y. Yang, "Comcast's ISP Experiences in a Proactive Network
              Provider Participation for P2P (P4P) Technical Trial", RFC
              5632, September 2009.






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

   [RFC6708]  Kiesel, S., Previdi, S., Stiemerling, M., Woundy, R., and
              Y. Yang, "Application-Layer Traffic Optimization (ALTO)
              Requirements", RFC 6708, September 2012.

Appendix A.  Appendix: Monitoring ALTO

   In addition to providing configuration, an ISP providing ALTO may
   want to deploy a monitoring infrastructure to assess the benefits of
   ALTO and adjust its ALTO configuration according to the results of
   the monitoring.

   To construct an effective monitoring infrastructure, the ISP should
   (1) define the performance metrics to be monitored; (2) and identify
   and deploy data sources to collect data to compute the performance
   metrics.  We discuss both below.

   [Editor's note: Is there a relationship to the IPPM working group at
   the IETF?]

A.1.  Monitoring Metrics Definition

   o  Inter-domain ALTO-Integrated Application Traffic (Network metric):
      This metric includes total cross domain traffic generated by
      applications that utilize ALTO guidance.  This metric evaluates
      the impacts of ALTO on the inbound and outbound traffic of a
      domain.

   o  Total Inter-domain Traffic (Network metric): This is similar to
      the preceding but focuses on all of the traffic, ALTO aware or
      not.  One possibility is that some of the reduction of interdomain
      traffic by ALTO aware applications may (XXX missing words?).  This
      metric is always used with the preceding and the following
      metrics.

   o  Intra-domain ALTO-Integrated Application Traffic (Network metric).
      (XXX description missing)

   o  Network hop count (Network metric): This metric provides the
      average number of hops that traffic traverses inside a domain.
      ALTO may reduce not only traffic volume but also the hops.  The
      metric can also indirectly reflect some application performance
      (e.g., latency).





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   o  Application download rate (Application metric): This metric
      measures application performance directly.  Download means inbound
      traffic to one user.  Global average means the average value of
      all users' download rates in one or more domains.

   o  Application Client type audit(Application metric): this metric
      gives the audit of client types in ALTO service.  The current
      types include fixed network client and mobile network client.

A.2.  Monitoring Data Sources

   The preceding metrics are derived from data sources.  We identify
   three data sources.

   1.  Application Log Server: Many application systems deploy Log
       Servers to collect data.

   2.  P2P Clients: Some P2P applications may not have Log Servers.
       When available, P2P client logs can provide data.  This is for
       P2P application

   3.  OAM: Many ISPs deploy OAM systems to monitor IP layer traffic.
       An OAM provides traffic monitoring of every network device in its
       management area.  It provides data such as link physical
       bandwidth and traffic volumes.

A.3.  Monitoring Structure

   As discussed in the preceding section, some data sources are from ISP
   while some others are from application.  When there is a
   collaboration agreement between the ISP and an application, there can
   be an integrated monitoring system as shown in the figure below.  In
   particular, an application developer may deploy Monitor Clients to
   communicate with Monitor Server of the ISP to transmit raw data from
   the Log Server or P2P clients of the application to the ISP.

   +------------------------------------------------+
   |                                                |
   |  New Entities            +--------------------------------------+
   |                          |                Service Provider      |
   |                          |                (P2P/CDN Operator etc)|
   |    +-----------+         |   +-----------+     |                |
   |    |ALTO Server|-------------|ALTO Client|     |                |
   |    +-----------+         |   +-----------+     |                |
   |                          |                     |  +----------+  |
   |                          |                     |  |Log Server|  |
   |                          |                     |  +----------+  |
   |   +--------------+       |  +--------------+   |  +----------+  |



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   |   |Monitor Server|----------|Monitor Client|   |  |P2P Client|  |
   |   +--------------+       |  +--------------+   |  +----------+  |
   |          |               |                     |                |
   | +--------|--------+      +--------------------------------------+
   +-|--------|--------|----------------------------+
     |        |        |
     |        |        |
     |      +---+      |
     |      |OAM|      |
     |      +---+      |
     |             ISP |
      -----------------

                      Figure 22: Monitoring Structure

Appendix B.  Appendix: API between ALTO Client and Application

   This section gives some informational guidance on how the interface
   between the actual application using the ALTO guidance and the ALTO
   client can look like.

   This is still TBD.

Appendix C.  Contributors List and Acknowledgments

   This memo is the result of contributions made by several people, such
   as:

   o  Xianghue Sun, Lee Kai, and Richard Yang contributed text on ISP
      deployment requirements and monitoring.

   o  Stefano Previdi contributed parts of the Section 5 on "Using ALTO
      for CDNs".

   o  Rich Woundy contributed text to Section 3.3.

   o  Lingli Deng, Wei Chen, Qiuchao Yi, Yan Zhang contributed
      Section 6.3.

   o  Thomas-Rolf Banniza carefully reviewed the document.

   Martin Stiemerling is partially supported by the CHANGE project (
   http://www.change-project.eu), a research project supported by the
   European Commission under its 7th Framework Program (contract no.
   257422).  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 CHANGE project or the European Commission.



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Authors' Addresses

   Martin Stiemerling (editor)
   NEC Laboratories Europe
   Kurfuerstenanlage 36
   Heidelberg  69115
   Germany

   Phone: +49 6221 4342 113
   Fax:   +49 6221 4342 155
   Email: martin.stiemerling@neclab.eu
   URI:   http://ietf.stiemerling.org


   Sebastian Kiesel (editor)
   University of Stuttgart, Computing Center
   Allmandring 30
   Stuttgart  70550
   Germany

   Email: ietf-alto@skiesel.de


   Stefano Previdi
   Cisco Systems, Inc.
   Via Del Serafico 200
   Rome  00191
   Italy

   Email: sprevidi@cisco.com


   Michael Scharf
   Alcatel-Lucent Bell Labs
   Lorenzstrasse 10
   Stuttgart  70435
   Germany

   Email: michael.scharf@alcatel-lucent.com












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