ALTO                                                      M. Stiemerling
Internet-Draft                                           NEC Europe Ltd.
Intended status: Informational                                 S. Kiesel
Expires: January 4, 2015                         University of Stuttgart
                                                              S. Previdi
                                                               M. Scharf
                                                Alcatel-Lucent Bell Labs
                                                            July 3, 2014

                     ALTO Deployment Considerations


   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 and presents corresponding examples.  The document
   also includes recommendations for network administrators and
   application designers planning to deploy 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
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   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 4, 2015.

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Copyright Notice

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   Provisions Relating to IETF Documents
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   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  . . . . . . . . . . . . .   5
     2.2.  Classification of Deployment Scenarios  . . . . . . . . .   6
       2.2.1.  Deployment Degrees of Freedom . . . . . . . . . . . .   6
       2.2.2.  Information Exposure  . . . . . . . . . . . . . . . .   8
       2.2.3.  More Advanced Deployments . . . . . . . . . . . . . .   8
   3.  Deployment Considerations by ISPs . . . . . . . . . . . . . .  10
     3.1.  Objectives for the Guidance to Applications . . . . . . .  11
       3.1.1.  General Objectives for Traffic Optimization . . . . .  11
       3.1.2.  Inter-Network Traffic Localization  . . . . . . . . .  12
       3.1.3.  Intra-Network Traffic Localization  . . . . . . . . .  13
       3.1.4.  Network Off-Loading . . . . . . . . . . . . . . . . .  15
       3.1.5.  Application Tuning  . . . . . . . . . . . . . . . . .  16
     3.2.  Provisioning of ALTO Maps . . . . . . . . . . . . . . . .  16
       3.2.1.  Data Sources  . . . . . . . . . . . . . . . . . . . .  16
       3.2.2.  Privacy Requirements  . . . . . . . . . . . . . . . .  18
       3.2.3.  Partitioning and Grouping of IP Address Ranges  . . .  19
       3.2.4.  Rating Criteria and/or Cost Calculation . . . . . . .  19
     3.3.  Known Limitations of ALTO . . . . . . . . . . . . . . . .  22
       3.3.1.  Limitations of Map-based Approaches . . . . . . . . .  22
       3.3.2.  Limitiations of Non-Map-based Approaches  . . . . . .  24
     3.4.  Monitoring ALTO . . . . . . . . . . . . . . . . . . . . .  25
       3.4.1.  Impact and Observation on Network Operation . . . . .  25
       3.4.2.  Measurement of the Impact . . . . . . . . . . . . . .  26
       3.4.3.  System and Service Performance  . . . . . . . . . . .  27
       3.4.4.  Monitoring Infrastructures  . . . . . . . . . . . . .  27
     3.5.  Map Examples for Different Types of ISPs  . . . . . . . .  28
       3.5.1.  Small ISP with Single Internet Uplink . . . . . . . .  28

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       3.5.2.  ISP with Several Fixed Access Networks  . . . . . . .  31
       3.5.3.  ISP with Fixed and Mobile Network . . . . . . . . . .  32
     3.6.  Deployment Experiences  . . . . . . . . . . . . . . . . .  33
   4.  Using ALTO for P2P Traffic Optimization . . . . . . . . . . .  34
     4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  34
       4.1.1.  Usage Scenario  . . . . . . . . . . . . . . . . . . .  34
       4.1.2.  Applicability of ALTO . . . . . . . . . . . . . . . .  34
     4.2.  Deployment Recommendations  . . . . . . . . . . . . . . .  37
       4.2.1.  ALTO Services . . . . . . . . . . . . . . . . . . . .  37
       4.2.2.  Guidance Considerations . . . . . . . . . . . . . . .  38
   5.  Using ALTO for CDNs . . . . . . . . . . . . . . . . . . . . .  40
     5.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  40
       5.1.1.  Usage Scenario  . . . . . . . . . . . . . . . . . . .  40
       5.1.2.  Applicability of ALTO . . . . . . . . . . . . . . . .  42
     5.2.  Deployment Recommendations  . . . . . . . . . . . . . . .  43
       5.2.1.  ALTO Services . . . . . . . . . . . . . . . . . . . .  43
       5.2.2.  Guidance Considerations . . . . . . . . . . . . . . .  44
   6.  Other Use Cases . . . . . . . . . . . . . . . . . . . . . . .  45
     6.1.  Application Guidance in Virtual Private Networks (VPNs) .  45
     6.2.  In-Network Caching  . . . . . . . . . . . . . . . . . . .  48
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  49
     7.1.  Information Leakage from the ALTO Server  . . . . . . . .  49
     7.2.  ALTO Server Access  . . . . . . . . . . . . . . . . . . .  50
     7.3.  Faking ALTO Guidance  . . . . . . . . . . . . . . . . . .  51
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  51
   9.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .  51
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  51
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  51
     10.2.  Informative References . . . . . . . . . . . . . . . . .  52
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  54
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  54

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].  The ALTO protocol is
   specified in [I-D.ietf-alto-protocol].

   This document discusses use cases and operational issues that can be
   expected when ALTO gets deployed.  This includes, but is not limited

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   to, location of the ALTO server, imposed load to the ALTO server, or
   from whom the queries are performed.  The document also provides
   guidance which ALTO services to use, and it summarized known
   challenges.  It thereby complements the management considerations in
   the protocol specification [I-D.ietf-alto-protocol], which are
   independent of any specific use of ALTO.

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 |             )
          `.           |           ,'
            `--.       |       _.-'
    +----------+  +----------+   +----------+
    |  ALTO    |  |  ALTO    |...|  ALTO    |
    |  Client  |  |  Client  |   |  Client  |
    +----------+  +----------+   +----------+

        Figure 1: Baseline deployment scenario of the ALTO protocol

   This document uses the terminology introduced in [RFC5693].  In
   particular, the following terms are defined by [RFC5693]:

   o  ALTO Service: Several resource providers may be able to provide
      the same resource.  The ALTO service gives guidance to a resource
      consumer and/or resource directory about which resource
      provider(s) to select in order to optimize the client's
      performance or quality of experience, while improving resource
      consumption in the underlying network infrastructure.

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   o  ALTO Server: A logical entity that provides interfaces to the
      queries to the ALTO service.

   o  ALTO Client: The logical entity that sends ALTO queries.
      Depending on the architecture of the application, one may embed it
      in the resource consumer and/or in the resource directory.

   According to that definition, both an ALTO server and an ALTO client
   are logical entities.  An ALTO service may be offered by more than
   one ALTO servers.  In ALTO deployments, the functionality of an ALTO
   server can therefore be realized by several server instances, e.g.,
   by using load balancing between different physical servers.  The term
   ALTO server should not be confused with use of a single physical

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.

                                             =====|     |**
                                         ====     +-----+  *
                                     ====            *     *
                                 ====                *     *
        +-----+     +------+=====                 +-----+  *
        |     |.....|      |======================|     |  *
        +-----+     +------+=====                 +-----+  *
      Source of      ALTO        ====                *     *
      topological    service         ====            *     *
      information                        ====     +-----+  *
                                             =====|     |**
      === 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 an ALTO client running at
   endpoints.  An example would be a peer-to-peer file sharing
   application that does not use a tracker, such as edonkey.  In

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   addition, ALTO clients at peers could also be used in a similar way
   even if there is a tracker, as further discussed in Section 4.1.2.

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

      === 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.

   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 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 protocol 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

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   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.

   The following sections enumerate different classes of use cases for
   ALTO, and they discuss deployment implications of each of them.  An
   ALTO server can in principle be operated by any organization, and
   there is no requirement that an ALTO server is deployed and operated
   by an Internet Service Provider (ISP).  Yet, since the ALTO solution
   is designed for ISPs, most examples in this document assume that the
   operator of an ALTO server is a network operator (e.g., an ISP or the
   network department in a large enterprise) that offers ALTO guidance
   in particular to users if this network.

   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]).

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2.2.2.  Information Exposure

   An ALTO server stores information about preferences (e.g., for IP
   address ranges) 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., IP
   ranges or topology information in general) to the ALTO client.  The
   ALTO protocol supports this scheme by the Network and Cost Map

   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 further detailed in
   Section 3.3.

2.2.3.  More Advanced Deployments

   From an ALTO client's perspective, there are different ways to use

   1.  Single service instance with single metric guidance: An ALTO
       client only obtains guidance regarding a single metric from a
       single ALTO service, e.g., an ALTO server that is offered by the
       network service provider of the corresponding access network.
       Corresponding ALTO server instances can be discovered e.g. by

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       ALTO server discovery [I-D.ietf-alto-server-discovery]
       [I-D.kist-alto-3pdisc].  Being a REST-ful protocol, an ALTO
       service can use known methods to balance the load between
       different server instances or between clusters of servers, i.e.,
       an ALTO server can be realized by many instances with a load
       balancing scheme.  The ALTO protocol also supports the use of
       different URIs for different ALTO features.

   2.  Single service instance with multiple metric guidance: An ALTO
       client could also query an ALTO service for different kinds of
       information, e.g., cost maps with different metrics.  The ALTO
       protocol is extensible and permits such operation.  However, ALTO
       does not define how a client shall deal with different forms of
       guidance, and it is up to the client to determine what provided
       information may indeed be useful.

   3.  Multiple service offers: An ALTO client can also decide to access
       multiple ALTO servers providing guidance, possibly from different
       operators or organisations.  Each of these services may only
       offer partial guidance, e.g., for a certain network partition.
       In that case, it may be difficult for an ALTO client to compare
       the guidance from different services.  Different organization may
       use different methods to determine maps, and they may also have
       different (possibly even contradicting or competing) guidance
       objectives.  How to discover multiple ALTO servers and how to
       deal with conflicting guidance is an open issue.

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

   1.  Authoritative 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 design is that ISP operate ALTO servers
   independently, irrespectively of other ISPs.  This may true for most
   envisioned deployments of ALTO but there may be certain deployments
   that may have different settings.  Figure 4 shows such setting with a
   university network that is connected to two upstream providers.  NREN
   is a National Research and Education Network and ISP is a commercial

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   upstream provider to this university network.  The university, as
   well as ISP, are operating their own ALTO server.  The ALTO clients,
   located on the peers will contact the ALTO server located at the

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

                Figure 4: Example of a cascaded ALTO server

   In this setting all "destinations" useful for the peers within NREN
   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 NREN will be handled by the
   ISP upstream provider.  Therefore, the ALTO server at the university
   may also include the guidance given by the ISP ALTO server in its
   replies to the ALTO clients.  This is an example for cascaded ALTO

3.  Deployment Considerations by ISPs

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3.1.  Objectives for the Guidance to Applications

3.1.1.  General Objectives for Traffic Optimization

   The Internet consists of many networks.  The networks are operated by
   Network Service Providers (NSP), Internet Service Providers (named
   ISP in this memo), which also include e.g. universities, enterprises,
   or other organizations.  The Internet provides network connectivity
   e.g. by access networks, such as cable networks, xDSL networks, 3G/4G
   mobile networks, etc.  Network operators need to manage, to control
   and to audit the traffic.  Therefore, it is important to understand
   how to deploy an ALTO service and its expected impact.

   The general objective of ALTO is to give guidance to applications on
   what endpoints (e.g., 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 support application-level 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 certain areas such as 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, wireless links
       should be used efficiently.  For example, in the case of a P2P
       service, it is likely that hosts in fixed networks should avoid
       retrieving data from hosts in mobile networks, and hosts in

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       mobile networks should prefer retrieval of data from hosts in
       fixed networks.

   4.  Application tuning: ALTO is also a tool to optimize the
       performance of applications that depend on the network and
       perform resource selection decisions among network endpoints.
       And example is the network-aware selection of Content Delivery
       Network (CDN) caches.

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

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
   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).

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

       ### preferred "connections"
       --- non-preferred "connections"

               Figure 5: Inter-network traffic localization

   Examples for corresponding ALTO maps can be found in Section 3.5.
   Depending on the application characteristics, it may not be possible
   or even not be desirable to completely localize all traffic.

3.1.3.  Intra-Network Traffic Localization

   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

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

          Figure 7: With intra-network ALTO traffic localization

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   The objective here is to avoid bottlenecks by optimized endpoint
   selection at application level.  ALTO is not a method to deal with
   the congestion at the bottleneck.

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 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|
                         `-.         ,-'      +-----------+

       === 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.

   A realization of this scenario may have certain limitations and may
   not be possible in all cases.  For instance, it may require that the
   ALTO server can distinguish mobile and non-mobile hosts, e.g., based
   on their IP address.  This may depend on mobility solutions and may
   not be possible or accurate.  In general, ALTO is not intended as a

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   fine-grained traffic engineering solution for individual hosts.
   Instead, it typically works on aggregates (e.g., if it is known that
   certain IP prefixes are often assigned to mobile users).

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

   An ALTO server collects topological information from a variety of
   sources in the network and provides a cohesive, abstracted view of
   the network topology to applications using an ALTO client.  The ALTO
   server builds an ALTO-specific network topology that represents the
   network as it should be understood and utilized by applications at

   ALTO abstract network topologies can be automatically generated from
   the physical or logical topology of the network.  The generation
   would typically be based on policies and rules set by the network
   operator.  The maps and the guidance can significantly differ
   depending on the use case, the network architecture, and the trust
   relationship between ALTO server and ALTO client, etc.  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 operators have to ensure that the ALTO
   topology does not contain any details that would endanger the network
   integrity and security.  For instance, ALTO is not intended to leak
   raw Interior Gateway Protocol (IGP) or Border gateway Protocol (BGP)
   databases to ALTO clients.

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          +--------+     +--------+
          | Client |     | Client |
          +--------+     +--------+
                  ^       ^
                  |       | ALTO protocol
                 |  ALTO   |
                 | Server  |
                  ^   ^   ^    Potential
                  |   |   |  data sources
         +--------+   |   +--------+
         |            |            |
    +---------+  +---------+  +---------+
    |   BGP   |  |   I2RS  |  |   NMS   |
    | Speaker |  |  Client |  |   OSS   |
    +---------+  +---------+  +---------+
         ^            ^            ^
         |            |            |
    Link-State      I2RS      SNMP/NETCONF,
     NLRI for       data      traffic statistics,
     IGP/BGP                  IPFIX, etc.

                 Figure 9: Potential data sources for ALTO

   As illustrated in Figure 9, the topology data used by an ALTO server
   can originate from different data sources:

   o  The document [I-D.ietf-idr-ls-distribution] describes a mechanism
      by which links state and traffic engineering information can be
      collected from networks and shared with external components using
      the BGP routing protocol.  This is achieved using a new BGP
      Network Layer Reachability Information (NLRI) encoding format.
      The mechanism is applicable to physical and virtual IGP links and
      can also include Traffic Engineering (TE) data.  For instance,
      prefix data can be carried and originated in BGP, while TE data is
      originated and carried in an IGP.  The mechanism described is
      subject to policy control.  An ALTO Server can also use other
      mechanisms to get network data, for example, peering with multiple
      IGP and BGP speakers.

   o  The Interface to the Routing System (I2RS) is a solution for state
      transfer in and out of the Internet's routing system
      [I-D.ietf-i2rs-architecture].  An ALTO server could use an I2RS
      client to observe routing-related information.

   o  An ALTO server can also leverage a Network Management System (NMS)
      or an Operations Support System (OSS) as data sources.  NMS or OSS

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      solutions are used to control, operate, and manage a network,
      e.g., using the Simple Network Management Protocol (SNMP) or
      NETCONF.  As explained for instance in
      [I-D.farrkingel-pce-abno-architecture], the NMS and OSS can be
      consumers of network events reported and can act on these reports
      as well as displaying them to users and raising alarms.  The NMS
      and OSS can also access the Traffic Engineering Database (TED) and
      Label Switched Path Database (LSP-DB) to show the users the
      current state of the network.  In addition, NMS and OSS systems
      may have access to IGP/BGP routing information, network inventory
      data (e.g., links, nodes, or link properties not visible to
      routing protocols, such as Shared Risk Link Groups), statistics
      collection system that provides traffic information, such as
      traffic demands or link utilizations obtained from IP Flow
      Information Export (IPFIX), as well as other Operations,
      Administration, and Maintenance (OAM) information (e.g., syslog).
      NMS or OSS systems also may have functions to correlate and
      orchestrate information originating from other data sources.  For
      instance, it could be required to correlate IP prefixes with
      routers (Provider, Provider Edge, Customer Edge, etc.), IGP areas,
      VLAN IDs, or policies.

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

   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

   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.

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3.2.3.  Partitioning and Grouping of IP Address Ranges

   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).  A PID is characterized by a string
   identifier.  If an ALTO server offers the Map Service, corresponding
   identifiers have to be configured.

   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.  This will typically be governed by policies that
   enforce a certain level of abstraction and prevent leakage of
   sensitive operational data.

   For instance, an ALTO server may leverage BGP information that is
   available in a networks service provider network layer and compute
   the group of prefix.  An example are BGP communities, which are used
   in MPLS/IP networks as a common mechanism to aggregate and group
   prefixes.  A 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).  These
   BGP communities could be used to map IP address ranges to PIDs.  By
   an additional policy, the ALTO server operator may decide an
   arbitrary cost defined between groups.  Alternatively, there are
   algorithms that allow a dynamic computation of cost between groups.
   The ALTO protocol itself is independent of such algorithms and

3.2.4.  Rating Criteria and/or Cost Calculation

   Rating criteria are used in the ALTO protocol to express topology- or
   connectivity-related properties, which are evaluated in order to
   generate the ALTO guidance.  The ALTO protocol specification defines
   as basic set of rating criteria the "routingcost" metric, which has
   to be supported by all implementations.  It is up to the ALTO server
   how that metric is calculated.

   There is also an extension procedure for adding new criteria and
   metrics.  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; a formal definition of metrics can be found in
   [I-D.wu-alto-te-metrics].  Instead, the only purpose of the following
   list is to document the rating criteria that have been proposed so

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   far.  It can also depend on the use case of ALTO whether such rating
   criteria are useful, and whether the corresponding information would
   indeed be made available by ISPs.

   Distance-related rating criteria:

   o  Relative topological distance: The term 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.  In addition to the AS path, a
      relative cost value could also be calculated taking into account
      other routing protocol parameters, such as BGP local preference or
      multi-exit discriminator (MED) attributes.

   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 (e.g., continent, country) of an IP address.

   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).

   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).

   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.

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   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.

   o  Other metrics representing an abstract cost, e.g., determined by
      policies that distinguish "cheap" from "expensive" IP subnet
      ranges, e.g., without detailing the cost function.

   These rating criteria are subject to the remarks below:

   The ALTO client must be aware that with high probability the actual
   performance values differs from whatever an ALTO server exposes.  In
   particular, an ALTO client must not consider a 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 have all information about the actual

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

   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.

   o  Performance metrics that raise privacy concerns.  For instance, it
      has been questioned whether an ALTO service could publicly expose
      the provisioned access bandwidth, e.g. of cable / DSL customers,
      because this could enables identification of "premium" customers.

3.3.  Known Limitations of ALTO

3.3.1.  Limitations of Map-based Approaches

   The specification of the Map Service in the ALTO protocol
   [I-D.ietf-alto-protocol] is based on the concept of 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.

   One main assumption for map-based approaches is that the information
   provided in these maps is static for a longer period of time.  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

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   the partitions).  However, there are several cases where this
   assumption is not valid:

   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.

   These effects can be explained as follows:

   Case 1: ISPs may 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.

   It has been 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.

   Case 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

   Case 3: In residential access networks (e.g., DSL, cable), IP
   prefixes are assigned to broadband gateways, which are the first IP-
   hop in the access-network between the Customer Premises Equipment
   (CPE) and the Internet.  The access-network between CPE and broadband
   gateway (called aggregation network) can have varying characteristics
   (and thus associated costs), but still using the same IP prefix.  For

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   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., 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.

   Case 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] also
   includes the Endpoint Cost Service (ECS) mechanism.  ALTO clients can
   ask guidance for specific IP addresses to the ALTO server, thereby
   avoiding the need of processing maps.  This can mitigate some of the
   problems mentioned in the previous section.

   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.
   Therefore, the ALTO client may have to await the server response
   before starting a communication, which results in an additional

   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.

   When ALTO server receives an ECS request, it may not have the most
   appropriate topology information in order to accurately determine the
   ranking.  [I-D.ietf-alto-protocol] generally assumes that a server
   can always offer some guidance.  In such a case the ALTO server could
   adopt one of the following strategies:

   o  Reply with available information (best effort).

   o  Query another ALTO server presumed to have better topology
      information and return that response (cascaded servers).

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

   The protocol mechanisms and decision processes that would be used to
   determine if redirection is necessary and which mode to use is out of
   the scope of this document, since protocol extensions could be

3.4.  Monitoring ALTO

3.4.1.  Impact and Observation on Network Operation

   ALTO presents a new opportunity for managing network traffic by
   providing additional information to clients.  In particular, the
   deployment of an ALTO Server may shift network traffic patterns, and
   the potential impact to network operation can be large.  An ISP
   providing ALTO may want to assess the benefits of ALTO as part of the
   management and operations (cf.  [I-D.ietf-alto-protocol]).  For
   instance, the ISP might be interested in understanding whether the
   provided ALTO maps are effective, and in order to decide whether an
   adjustment of the ALTO configuration would be useful.  Such insight
   can be obtained from a monitoring infrastructure.  An NSP offering
   ALTO could consider the impact on (or integration with) traffic
   engineering and the deployment of a monitoring service to observe the
   effects of ALTO operations.  The measurement of impacts can be
   challenging because ALTO-enabled applications may not provide related
   information back to the ALTO Service Provider.

   To construct an effective monitoring infrastructure, the ALTO Service
   Provider should decide how to monitor the performance of ALTO and
   identify and deploy data sources to collect data to compute the
   performance metrics.  In certain trusted deployment environments, it
   may be possible to collect information directly from ALTO clients.
   It may also be possible to vary or selectively disable ALTO guidance
   for a portion of ALTO clients either by time, geographical region, or
   some other criteria to compare the network traffic characteristics
   with and without ALTO.  Monitoring an ALTO service could also be
   realized by third parties.  In this case, insight into ALTO data may
   require a trust relationship between the monitoring system operator
   and the network service provider offering an ALTO service.

   The required monitoring depends on the network infrastructure and the
   use of ALTO, and an exhaustive description is outside the scope of
   this document.

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3.4.2.  Measurement of the Impact

   ALTO realizes an interface between the network and applications.
   This implies that an effective monitoring infrastructure may have to
   deal with both network and application performance metrics.  This
   document does not comprehensively list all performance metrics that
   could be relevant, nor does it formally specify metrics.

   The impact of ALTO can be classified regarding a number of different

   o  Total amount and distribution of traffic: ALTO enables ISPs to
      influence and localize traffic of applications that use the ALTO
      service.  An ISP may therefore be interested in analyzing the
      impact on the traffic, i.e., whether network traffic patterns are
      shifted.  For instance, if ALTO shall be used to reduce the inter-
      domain P2P traffic, it makes sense to evaluate the total amount of
      inter-domain traffic of an ISP.  Then, one possibility is to study
      how the introduction of ALTO reduces the total inter-domain
      traffic (inbound and/our outbound).  If the ISPs intention is to
      localize the traffic inside his network, the network-internal
      traffic distribution will be of interest.  Effectiveness of
      localization can be quantified in different ways, e.g., by the
      load on core routers and backbone links, or by considering more
      advanced effects, such as the average number of hops that traffic
      traverses inside a domain.

   o  Application performance: The objective of ALTO is improve
      application performance.  ALTO can be used by very different types
      applications, with different communication characteristics and
      requirements.  For instance, if ALTO guidance achieves traffic
      localization, one would expect that applications achieve a higher
      throughput and/or smaller delays to retrieve data.  If
      application-specific performance characteristics (e.g., video or
      audio quality) can be monitored, such metrics related to user
      experience could also help to analyze the benefit of an ALTO
      deployment.  If available, selected statistics from the TCP/IP
      stack in hosts could be leveraged, too.

   Of potential interest can also be the share of applications or
   customers that actually use an offered ALTO service, i.e., the
   adoption of the service.

   Monitoring statistics can be aggregated, averaged, and normalized in
   different ways.  This document does not mandate specific ways how to
   calculate metrics.

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3.4.3.  System and Service Performance

   A number of interesting parameters can be measured at the ALTO
   server.  [I-D.ietf-alto-protocol] suggests certain ALTO-specific
   metrics to be monitored:

   o  Requests and responses for each service listed in a Information
      Directory (total counts and size in bytes).

   o  CPU and memory utilization

   o  ALTO map updates

   o  Number of PIDs

   o  ALTO map sizes (in-memory size, encoded size, number of entries)

   This data characterizes the workload, the system performance as well
   as the map data.  Obviously, such data will depend on the
   implementation and the actual deployment of the ALTO service.
   Logging is also recommended in [I-D.ietf-alto-protocol].

3.4.4.  Monitoring Infrastructures

   Understanding the impact of ALTO may require interaction between
   different systems, operating at different layers.  Some information
   discussed in the preceding sections is only visible to an ISP, while
   application-level performance can hardly be measured inside the
   network.  It is possible that not all information of potential
   interest can directly be measured, either because no corresponding
   monitoring infrastructure or measurement method exists, or because it
   is not easily accessible.

   One way to quantify the benefit of deploying ALTO is to measure
   before and after enabling the ALTO service.  In addition to passive
   monitoring, some data could also be obtained by active measurements,
   but due to the resulting overhead, the latter should be used with
   care.  Yet, in all monitoring activities an ALTO service provider has
   to take into account that ALTO clients are not bound to ALTO server
   guidance as ALTO is only one source of information, and any
   measurement result may thus be biased.

   Potential sources for monitoring the use of ALTO include:

   o  Network Operations, Administration, and Maintenance (OAM) systems:
      Many ISPs deploy OAM systems to monitor the network traffic, which
      may have insight into traffic volumes, network topology, and
      bandwidth information inside the management area.  Data can be

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      obtained by SNMP, NETCONF, IP Flow Information Export (IPFIX),
      syslog, etc.

   o  Applications/clients: Relevant data could be obtained by
      instrumentation of applications.

   o  ALTO server: If available, log files or other statistics data
      could be analyzed.

   o  Other application entities: In several use cases, there are other
      application entities that could provide data as well.  For
      instance, there may be centralized log servers that collect data.

   In many ALTO use cases some data sources are located within an ISP
   network while some other data is gathered at application level.
   Correlation of data could require a collaboration agreement between
   the ISP and an application owner, including agreements of data
   interchange formats, methods of delivery, etc.  In practice, such a
   collaboration may not be possible in all use cases of ALTO, because
   the monitoring data can be sensitive, and because the interacting
   entities may have different priorities.  Details of how to build an
   over-arching monitoring system for evaluating the benefits of ALTO
   are outside the scope of this memo.

3.5.  Map Examples for Different Types of ISPs

3.5.1.  Small ISP with Single Internet Uplink

   The ALTO protocol does not mandate how to determine costs between
   endpoints and/or determine map data.  In complex usage scenarios this
   can be a non-trivial problem.  In order to show the basic principle,
   this and the following section explain for different deployment
   scenarios how ALTO maps could be structured.

   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 10.  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,

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   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           |----------->|                     |
     |                           |  C3 (=C2)   \\\\             ////
      \\                       //                  \-----------/
        \\                   //
          \\\\           ////

             Figure 10: Example ALTO deployment in small ISPs

   A simplified extract of the corresponding ALTO network and cost maps
   is listed in Figure 11 and Figure 12, assuming that the network of
   ISP A has the IPv4 address ranges and
   In this example, the cost values C1 and C2 can be set to any number

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

        "network-map" : {
          "PID1" : {
            "ipv4" : [
          "PID2" : {
            "ipv4" : [
            "ipv6" : [

                    Figure 11: 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 12: Example ALTO cost map

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3.5.2.  ISP with Several Fixed Access Networks

   This example discusses a P2P traffic optimization use case for a
   lager 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.

   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 13.

   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 13.  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 corresponding 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.

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

    Figure 13: ALTO deployment in large ISPs with layered fixed network

3.5.3.  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 14 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 14.

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

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   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

   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.

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

          Figure 14: ALTO deployment in ISPs with mobile network

   These examples show that for ALTO in particular the relations between
   different costs matter; the operator of the server has several
   degrees of freedom how to set the absolute values.

3.6.  Deployment Experiences

   The examples in the previous section are simple and do not consider
   specific requirements inside access networks, such as different link
   types.  Deploying an ALTO service in real network may require dealing
   with further network conditions and requirements.  One real example

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   is described in greater detail in reference

   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

4.  Using ALTO for P2P Traffic Optimization

4.1.  Overview

4.1.1.  Usage Scenario

   Originally, peer-to-peer (P2P) applications have been the main driver
   for the development of ALTO.  P2P systems can be build without and
   with use of a centralized resource directory ("tracker").  The scope
   of this section is the interaction of P2P applications with the ALTO
   service, focusing on the use case with a centralized resource
   directory.  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.

4.1.2.  Applicability of ALTO

   A tracker-based P2P application can leverage ALTO in different ways.
   In the following, the different alternatives and their pros and cons
   are discussed.

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

      Figure 15: Global tracker accessing ALTO server at various ISPs

   Figure 15 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] [I-D.kist-alto-3pdisc].
   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.  For instance, this is called peer exchange (PEX) in

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                            ,-------.         +-----------+
          ,---.          ,-'         `-.  +==>|   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| ****
            *            `-.         ,-'      +-----------+ ****
            *               `-------'                       ****
            *                                               ****
       === ALTO client protocol
       *** Application protocol

             Figure 16: Global tracker and local ALTO servers

   The scenario in Figure 16 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 |/     +-----------+ *
               *          +-------------+                    *
               *             `-------'                       *
        === ALTO client protocol
        *** Application protocol

     Figure 17: Local trackers and local ALTO servers (P4P approaach)

   There are some attempts to let ISP's to deploy their own trackers, as
   shown in Figure 17.  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.

4.2.  Deployment Recommendations

4.2.1.  ALTO Services

   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.  In case of peer-to-peer networks,
   two different ALTO services can be used: The Cost Map Service is
   often preferred as solution by peer-to-peer software implementors and
   users, since it avoids disclosing peer IP addresses to a centralized
   entity.  Different to that, network operators may have a preference
   for the Endpoint Cost Service, since it does not require exposure of
   the network topology.

   For actual use of ALTO in P2P applications, both software vendors and
   network operators have to agree which ALTO services to use.  The ALTO

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   protocol is flexible and supports both services.  Note that for other
   use cases of ALTO, in particular in more controlled environments,
   both the Cost Map Service as well as Endpoint Cost Service might be
   feasible and it is more an engineering trade-off whether to use a
   map-based or query-based ALTO service.

4.2.2.  Guidance Considerations

   As explained in Section 4.1.2, for 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")

   Both approaches have advantages and drawbacks that have to be
   considered.  If the ALTO client is in the resource consumer
   (Figure 16), a potentially very large number of clients has to be
   deployed.  Instead, when using an ALTO client in the resource
   directory (Figure 15 and Figure 17), ostensibly peers do not have to
   directly query the ALTO server.  In this case, an ALTO server could
   even not permit access to peers.

   However, it seems to be beneficial for all participants to let the
   peers directly query the ALTO server.  Considering the plethora of
   different applications that could use ALTO, e.g. multiple tracker or
   non-tracker based P2P systems or other applications searching for
   relays, this renders the ALTO service more useful.  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.
   For a given peer one can also expect that an ALTO server of the
   corresponding ISP provides useful guidance and can be discovered.

   Yet, ALTO clients in the resource consumer also have drawbacks
   compared to use in the resource directory.  In the following, both
   scenarios are compared more in detail in order to explain the impact
   on ALTO guidance and 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

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   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

   In the second scenario (see Figure 19), the resource directory has an
   embedded ALTO client, which we will refer to as Resource Directory
   ALTO Client (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).

         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 19: Basic message sequence chart for third-party ALTO query

   Note: The message sequences depicted in Figure 18 and Figure 19 may
   occur both in the target-aware and the target-independent query mode
   (cf.  [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

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   using a locally cached "map", which has been retrieved from the ALTO
   server some time ago.

   The first approach has the following problem: 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.

   Much better traffic optimization could be achieved if the tracker
   would evaluate all known peers using ALTO.  This list would then
   include a significantly higher fraction of "good" peers.  (If the
   tracker returned "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 (see

5.  Using ALTO for CDNs

5.1.  Overview

5.1.1.  Usage Scenario

   This section briefly introduces the usage of ALTO for Content
   Delivery Networks (CDNs), as explained e.g. in
   [I-D.jenkins-alto-cdn-use-cases].  CDNs are used in the delivery of
   some Internet services (e.g. delivery of websites, software updates
   and video delivery) from a location closer to the location of the
   user.  A CDN typically consists of a network of servers often
   attached to Network Service Provider (NSP) networks.  The point of
   attachment is often as close to content consumers and peering points
   as economically or operationally feasible in order to decrease
   traffic load on the NSP backbone and to provide better user
   experience measured by reduced latency and higher throughput.

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   CDNs use several techniques to redirect a client to a server
   (surrogate).  A request routing function within a CDN is responsible
   for receiving content requests from user agents, obtaining and
   maintaining necessary information about a set of candidate
   surrogates, and for selecting and redirecting the user agent to the
   appropriate surrogate.  One common way is relying on the DNS system,
   but there are many other ways, see [RFC3568].

   In order to derive the optimal benefit from a CDN it is preferable to
   deliver content from the servers (caches) that are "closest" to the
   end user requesting the content. "closest" may be as simple as
   geographical or IP topology distance, but it may also consider other
   combinations of metrics and CDN or Network Service Provider (NSP)

   User Agent                  Request Router                 Surrogate
        |                             |                           |
        |     F1 Initial Request      |                           |
        +---------------------------->|                           |
        |                             +--+                        |
        |                             |  | F2 Surrogate Selection |
        |                             |<-+       (using ALTO)     |
        |   F3 Redirection Response   |                           |
        |<----------------------------+                           |
        |                             |                           |
        |     F4 Content Request      |                           |
        |                             |                           |
        |                             |          F5 Content       |
        |                             |                           |

               Figure 20: Example of CDN surrogate selection

   Figure 20 illustrates the interaction between a user agent, a request
   router, and a surrogate for the delivery of content in a single CDN.
   As explained in [I-D.jenkins-alto-cdn-use-cases], the user agent
   makes an initial request to the CDN (F1).  This may be an
   application-level request (e.g., HTTP) or a DNS request.  In the
   second step (F2), the request router selects an appropriate surrogate
   (or set of surrogates) based on the user agent's (or its proxy's) IP
   address, the request router's knowledge of the network topology
   (which can be obtained by ALTO) and reachability cost between CDN
   caches and end users, and any additional CDN policies.  Then (F3),
   the request router responds to the initial request with an
   appropriate response containing a redirection to the selected cache,
   for example by returning an appropriate DNS A/AAAA record, a HTTP 302
   redirect, etc.  The user agent uses this information to connect

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   directly to the surrogate and request the desired content (F4), which
   is then delivered (F5).

5.1.2.  Applicability of ALTO

   The most simple use case for ALTO in a CDN context is to improve the
   selection of a CDN surrogate or origin.  In this case, the CDN makes
   use of an ALTO server to choose a better CDN surrogate or origin than
   would otherwise be the case.  Although it is possible to obtain raw
   network map and cost information in other ways, for example passively
   listening to the NSP's routing protocols or use of active probing,
   the use of an ALTO service to expose that information may provide
   additional control to the NSP over how their network map/cost is
   exposed.  Additionally it may enable the NSP to maintain a functional
   separation between their routing plane and network map computation
   functions.  This may be attractive for a number of reasons, for

   o  The ALTO service could provide a filtered view of the network and/
      or cost map that relates to CDN locations and their proximity to
      end users, for example to allow the NSP to control the level of
      topology detail they are willing to share with the CDN.

   o  The ALTO service could apply additional policies to the network
      map and cost information to provide a CDN-specific view of the
      network map/cost, for example to allow the NSP to encourage the
      CDN to use network links that would not ordinarily be preferred by
      a Shortest Path First routing calculation.

   o  The routing plane may be operated and controlled by a different
      operational entity (even within a single NSP) to the CDN.
      Therefore, the CDN may not be able to passively listen to routing
      protocols, nor may it have access to other network topology data
      (e.g., inventory databases).

   When CDN servers are deployed outside of an NSP's network or in a
   small number of central locations within an NSP's network, a
   simplified view of the NSP's topology or an approximation of
   proximity is typically sufficient to enable the CDN to serve end
   users from the optimal server/location.  As CDN servers are deployed
   deeper within NSP networks it becomes necessary for the CDN to have
   more detailed knowledge of the underlying network topology and costs
   between network locations in order to enable the CDN to serve end
   users from the most optimal servers for the NSP.

   The request router in a CDN will typically also take into account
   criteria and constraints that are not related to network topology,
   such as the current load of CDN surrogates, content owner policies,

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   end user subscriptions, etc.  This document only discusses use of
   ALTO for network information.

   A general issue for CDNs is that the CDN logic has to match the
   client's IP address with the closest CDN surrogate, both for DNS or
   HTTP redirect based approaches (see, for instance,
   [I-D.penno-alto-cdn]).  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).

   In addition to use by a single CDN, ALTO can also be used in
   scenarios that interconnect several CDNs.  This use case is detailed
   in [I-D.seedorf-cdni-request-routing-alto].

5.2.  Deployment Recommendations

5.2.1.  ALTO Services

   In its simplest form an ALTO server would provide an NSP with the
   capability to offer a service to a CDN that provides network map and
   cost information.  The CDN can use that data to enhance its surrogate
   and/or origin selection.  If an NSP offers an ALTO network and cost
   map service to expose a cost mapping/ranking between end user IP
   subnets (within that NSP's network) and CDN surrogate IP subnets/
   locations, periodic updates of the maps may be needed.  As introduced
   in Section 3.3), it is common for broadband subscribers to obtain
   their IP addresses dynamically and in many deployments the IP subnets
   allocated to a particular network region can change relatively
   frequently, even if the network topology itself is reasonably static.

   An alternative would be to use the ALTO Endpoint Cost Service (ECS):
   When an end user request a given content, the CDN request router
   issues an ECS request with the endpoint address (IPv4/IPv6) of the
   end user (content requester) and the set of endpoint addresses of the
   surrogate (content targets).  The ALTO server receives the request
   and ranks the list of content targets addresses based on their
   distance from the content requester.  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 surrogate.

   Since CDNs operate in a controlled environment, the ALTO network/cost
   map service and ECS have a similar level of security and
   confidentiality of network-internal information.  However, the
   network/cost map service and ECS differ in the way the ALTO service

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   is delivered and address a different set of requirements in terms of
   topology information and network operations.

   If a CDN already has means to model connectivity policies, the map-
   based approaches could possibly be integrated into that.  If the ECS
   service is preferred, a request router that uses ECS could cache the
   results of ECS queries for later usage in order to address the
   scalability limitations of ECS and to reduce 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

5.2.2.  Guidance Considerations

   In the following it is discussed how a CDN could make use of ALTO

   In one deployment scenario, ALTO could expose NSP end user
   reachability to a CDN.  The request router needs to have information
   which end user IP subnets are reachable via which networks or network
   locations.  The network map services offered by ALTO could be used to
   expose this topology information while avoiding routing plane peering
   between the NSP and the CDN.  For example, if CDN surrogates are
   deployed within the access or aggregation network, the NSP is likely
   to want to utilize the surrogates deployed in the same access/
   aggregation region in preference to surrogates deployed elsewhere, in
   order to alleviate the cost and/or improve the user experience.

   In addition, CDN surrogates could also use ALTO guidance, e.g., if
   there is more than one upstream source of content or several origins.
   In this case, ALTO could help a surrogate with the decision which
   upstream source to use.  This specific variant of using ALTO is not
   further detailed in this document.

   If content can be provided by several CDNs, there may be a need to
   interconnect these CDNs.  In this case, ALTO can be uses as interface
   [I-D.seedorf-cdni-request-routing-alto], in particular for footprint
   and capabilities advertisement interface.

   Other and more advanced scenarios of deploying ALTO are also listed
   in [I-D.jenkins-alto-cdn-use-cases] and [I-D.penno-alto-cdn].

   The granularity of ALTO information required depends on the specific
   deployment of the CDN.  For example, an over-the-top CDN whose
   surrogates are deployed only within the Internet "backbone" may only
   require knowledge of which end user IP subnets are reachable via

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   which NSPs' networks, whereas a CDN deployed within a particular
   NSP's network requires a finer granularity of knowledge.

   ALTO server ranks addresses based on topology information it acquires
   from the network.  By default, according to [I-D.ietf-alto-protocol],
   distance in ALTO represents an abstract routing cost that can be
   computed from routing protocol information (e.g., OSPF, ISIS, BGP).
   But an ALTO server may also take into consideration other routing
   criteria such as MPLS-VPN (MP-BGP) and MPLS-TE (RSVP) information, or
   other information sources for policy, state, and performance
   information (e.g., geo-location), as explained in Section 3.2.1.

   The different methods and algorithms through which the ALTO server
   computes topology information and rankings is out of the scope of
   this document.  However, if rankings are based on routing protocol
   information, it is obvious that network events may impact the ranking
   computation.  Due to internal redundancy and resilience mechanisms
   inside current networks, most of the network events happening in the
   infrastructure will be handled internally in the network, and they
   should have limited impact on a CDN.  However, catastrophic events
   such as main trunks failures or backbone partitioning will have to
   take into account by the ALTO server to redirect traffic away from
   the impacted area.

   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.  In a CDN/ALTO interworking architecture with few CDN
   components interacting with the ALTO server there are less
   scalability issues in maintaining state about clients in the ALTO
   server, compared to ALTO guidance to any Internet user.  However,
   such a notification mechanism requires a corresponding notification
   mechanism in the ALTO protocol.

6.  Other Use Cases

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

6.1.  Application Guidance in Virtual Private Networks (VPNs)

   Virtual Private Network (VPN) technology is widely used in public and
   private networks to create groups of users that are separated from
   other users of the network and allows these users to communicate
   among them as if they were on a private network.  Network Service
   Providers (NSPs) offer different types of VPNs.  [RFC4026]
   distinguishes between Layer 2 VPN (L2VPN) and Layer 3 VPN (L3VPN)
   using different sub-types.  In the following, the term "VPN" is used
   to refer to provider supplied virtual private networking.

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   From the perspective of an application at an endpoint, a VPN may not
   be very different to any other IP connectivity solution, but there
   are a number of specific applications that could benefit from ALTO
   topology exposure and guidance in VPNs.  Similar like in the general
   Internet, one advantage is that applications do not have to perform
   excessive measurements on their own.  For instance, potential use
   cases for ALTO application guidance in VPNs environments are:

   o  Enterprise application optimization: Enterprise customers often
      run distributed applications that exchange large amounts of data,
      e.g., for synchronization of replicated data bases.  Both for
      placement of replicas as well as for the scheduling of transfers
      insight into network topology information could be useful.

   o  Private cloud computing solution: An enterprise customer could run
      own data centers at the four sites.  The cloud management system
      could want to understand the network costs between different sites
      for intelligent routing and placement decisions of Virtual
      Machines (VMs) among the VPN sites.

   o  Cloud-bursting: One or more VPN endpoints could be located in a
      public cloud.  If an enterprise customer needs additional
      resources, they could be provided by a public cloud, which is
      accessed through the VPN.  Network topology awareness would help
      to decide in which data center of the public cloud those resources
      should be allocated.

   These examples focus on enterprises, which are typical users of VPNs.
   VPN customers typically have no insight into the network topology
   that transports the VPN.  Similar like in other ALTO use cases,
   better-than-random application-level decisions would be enabled by an
   ALTO server offered by the NSP, as illustrated in Figure Figure 21.

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                       |  Customer's   |
                       |   management  |
                       |  application  |.
                       | (ALTO client) |  .
                       +---------------+    .  VPN provisioning
                               ^              . (out-of-scope)
                               | ALTO           .
                               V                  .
                    +---------------------+       +----------------+
                    |     ALTO server     |       | VPN portal/OSS |
                    |   provided by NSP   |       | (out-of-scope) |
                    +---------------------+       +----------------+
                               ^ VPN network
                               * and cost maps
                     /---------*---------\ Network service provider
                     |         *         |
        +-------+   _______________________   +-------+
        | App a | ()_____. .________. .____() | App d |
        +-------+    |   | |        | |  |    +-------+
                     \---| |--------| |--/
                         | |        | |
                         |^|        |^| Customer VPN
                          V          V
                      +-------+  +-------+
                      | App b |  | App c |
                      +-------+  +-------+

                       Figure 21: Using ALTO in VPNs

   A common characteristic of these use cases is that applications will
   not necessarily run in the public Internet, and that the relationship
   between the provider and customer of the VPN is rather well-defined.
   Since VPNs run often in a managed environment, an ALTO server may
   have access to topology information (e.g., traffic engineering data)
   that would not be available for the public Internet, and it may
   expose it to the customer of the VPN only.

   Also, a VPN will not necessarily be static.  The customer could
   possibly modify the VPN and add new VPN sites by a Web portal,
   network management systems, or other Operation Support Systems (OSS)
   solutions.  Prior to adding a new VPN site, an application will not
   be have connectivity to that site, i.e., an ALTO server could offer
   access to information that an application cannot measure on its own
   (e.g., expected delay to a new VPN site).

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   The VPN use cases, requirements, and solutions are further detailed
   in [I-D.scharf-alto-vpn-service].

6.2.  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 bandwidth 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 | |      |              |       |              |  |
   | | +---------+ |      |              |       |              |  |
   | +------+------+      +------+-------+       +------+-------+  |
   |        |                    |                      |          |
            |                    |                      |
        +---+---+            +---+---+                  |
        |       |            |       |                  |
     +--+--+ +--+--+      +--+--+ +--+--+            +--+--+
     |Peer2| |Peer3|      |Peer4| |Peer5|            |Peer6|
     +-----+ +-----+      +-----+ +-----+            +-----+

            Figure 22: General architecture of intra-ISP caches

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   Figure 22 depicts 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

   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

7.  Security Considerations

   The ALTO protocol specification [I-D.ietf-alto-protocol] discusses
   risk and protection strategies for the authenticity and integrity of
   ALTO Information, a potential undesirable guidance from authenticated
   ALTO information, the confidentiality of ALTO information, the
   privacy of ALTO users, and the availability of the ALTO service.  All
   those issues and potential countermeasures have to be taken into
   account when deploying an ALTO service.

   The following subsection further detail security issues resulting
   from specific uses of ALTO as discussed in this document.

7.1.  Information Leakage from the ALTO Server

   The ALTO server will be provisioned with information about the ISP's
   network and very likely also with information about neighboring ISPs.
   This information (e.g., network topology, business relations, etc.)
   is considered to be confidential to the ISP and can include very
   sensitive information.

   The ALTO server will naturally reveal parts of that information in
   small doses to clients, as the guidance given will depend on the
   above mentioned information.  This is seen beneficial for both
   parties, i.e., the ISPs and the clients.  However, there is the
   chance that one or multiple clients 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

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   more a perceived security risk.  In controlled environments (e.g., in
   the CDN use case) bilateral agreements could be used to reduce the
   risk of abuse.

   ALTO does not require any particular level of details of information
   disclosure, and hence the provider should evaluate how much
   information is revealed and the associated risks.

7.2.  ALTO Server Access

   Depending on the use case of ALTO, it may be desired to apply access
   restrictions to an ALTO server, i.e., by requiring client
   authentication.  According to [I-D.ietf-alto-protocol], ALTO requires
   that HTTP Digestion Authentication is supported, in order to achieve
   client authentication and possibly to limit the number of parties
   with whom ALTO information is directly shared.  TLS Client
   Authentication may also be supported.

   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 16).  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 15), 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 the server load could
   increase, since possibly more ALTO clients have to be served.

   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.

   Access control can also help to prevent Denial-of-Service attacks by
   arbitrary hosts from the Internet.  Denial of Service (DoS) can both
   affect an ALTO server and an ALTO client.  A server can get
   overloaded if too many 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

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   intentionally or due to an implementation mistake, too large to be
   handled by that particular client.

7.3.  Faking ALTO Guidance

   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 applications, e.g., a peer-to-peer applications.

   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.  IANA Considerations

   This document makes no specific request to IANA.

9.  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.

10.  References

10.1.  Normative References

              Alimi, R., Penno, R., and Y. Yang, "ALTO Protocol", draft-
              ietf-alto-protocol-27 (work in progress), March 2014.

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

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

10.2.  Informative References

              Lingli, D., Chen, W., Yi, Q., and Y. Zhang,
              "Considerations for ALTO with network-deployed P2P
              caches", draft-deng-alto-p2pcache-03 (work in progress),
              February 2014.

              King, D. and A. Farrel, "A PCE-based Architecture for
              Application-based Network Operations", draft-farrkingel-
              pce-abno-architecture-07 (work in progress), February

              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.

              Atlas, A., Halpern, J., Hares, S., Ward, D., and T.
              Nadeau, "An Architecture for the Interface to the Routing
              System", draft-ietf-i2rs-architecture-04 (work in
              progress), June 2014.

              Gredler, H., Medved, J., Previdi, S., Farrel, A., and S.
              Ray, "North-Bound Distribution of Link-State and TE
              Information using BGP", draft-ietf-idr-ls-distribution-05
              (work in progress), May 2014.

              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

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              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.

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

              Kiesel, S., Krause, K., and M. Stiemerling, "Third-Party
              ALTO Server Discovery (3pdisc)", draft-kist-alto-3pdisc-05
              (work in progress), January 2014.

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

              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.

              Scharf, M., Gurbani, V., Soprovich, G., and V. Hilt, "The
              Virtual Private Network (VPN) Service in ALTO: Use Cases,
              Requirements and Extensions", draft-scharf-alto-vpn-
              service-02 (work in progress), February 2014.

              Seedorf, J., Yang, Y., and J. Peterson, "CDNI Footprint
              and Capabilities Advertisement using ALTO", draft-seedorf-
              cdni-request-routing-alto-07 (work in progress), June

              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

              Wu, W., Yang, Y., Lee, Y., Dhody, D., and S. Randriamasy,
              "ALTO Traffic Engineering Cost Metrics", draft-wu-alto-te-
              metrics-03 (work in progress), June 2014.

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

   [RFC4026]  Andersson, L. and T. Madsen, "Provider Provisioned Virtual
              Private Network (VPN) Terminology", RFC 4026, March 2005.

   [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.

Appendix A.  Acknowledgments

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

   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, and Yan Zhang contributed
      Section 6.2.

   Thomas-Rolf Banniza, Vinayak Hegde, and Qin Wu provided very useful
   comments and reviewed the document.

   Martin Stiemerling is partially supported by the CHANGE project (, 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.

Authors' Addresses

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Internet-Draft          Deployment Considerations              July 2014

   Martin Stiemerling
   NEC Laboratories Europe
   Kurfuerstenanlage 36
   Heidelberg  69115

   Phone: +49 6221 4342 113
   Fax:   +49 6221 4342 155

   Sebastian Kiesel
   University of Stuttgart, Computing Center
   Allmandring 30
   Stuttgart  70550


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


   Michael Scharf
   Alcatel-Lucent Bell Labs
   Lorenzstrasse 10
   Stuttgart  70435


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