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ALTO                                                      M. Stiemerling
Internet-Draft                                           NEC Europe Ltd.
Intended status: Informational                                 S. Kiesel
Expires: January 12, 2012                        University of Stuttgart
                                                           July 11, 2011


                     ALTO Deployment Considerations
                     draft-ietf-alto-deployments-02

Abstract

   Many Internet applications are used to access resources, such as
   pieces of information or server processes, which are available in
   several equivalent replicas on different hosts.  This includes, but
   is not limited to, peer-to-peer file sharing applications.  The goal
   of Application-Layer Traffic Optimization (ALTO) is to provide
   guidance to these applications, which have to select one or several
   hosts from a set of candidates, that are able to provide a desired
   resource.  The protocol is under specification in the ALTO working
   group.  This memo discusses deployment related issues of ALTO for
   peer-to-peer and CDNs, some preliminary security considerations, and
   also initial guidance for application designers using ALTO.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on January 12, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents



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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
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   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  General Considerations . . . . . . . . . . . . . . . . . . . .  5
     2.1.  General Placement of ALTO  . . . . . . . . . . . . . . . .  5
     2.2.  Relationship between ALTO and Applications . . . . . . . .  7
     2.3.  Provided Guidance  . . . . . . . . . . . . . . . . . . . .  7
       2.3.1.  Keeping Traffic Local in Network . . . . . . . . . . .  8
       2.3.2.  Off-Loading Traffic from Network . . . . . . . . . . .  8
       2.3.3.  Intra-Network Localization/Bottleneck Off-Loading  . .  9
     2.4.  Provisiong ALTO Maps . . . . . . . . . . . . . . . . . . . 11
   3.  Deployment Considerations by ISPs  . . . . . . . . . . . . . . 12
     3.1.  Requirement for Traffic Optimization by ISPs . . . . . . . 12
     3.2.  Considerations for ISPs  . . . . . . . . . . . . . . . . . 13
       3.2.1.  Very small ISPs with simple Network Structure  . . . . 13
       3.2.2.  Large ISPs with layered fixed Network Structure  . . . 13
       3.2.3.  ISPs with Mobile Network . . . . . . . . . . . . . . . 15
   4.  Using ALTO for P2P . . . . . . . . . . . . . . . . . . . . . . 17
     4.1.  Using ALTO for Tracker-based Peer-to-Peer Applications . . 19
     4.2.  Expectations of ALTO . . . . . . . . . . . . . . . . . . . 21
   5.  Using ALTO for CDNs  . . . . . . . . . . . . . . . . . . . . . 22
   6.  Advanced Features  . . . . . . . . . . . . . . . . . . . . . . 23
     6.1.  Cascading ALTO Servers . . . . . . . . . . . . . . . . . . 23
     6.2.  ALTO for IPv4 and IPv6 . . . . . . . . . . . . . . . . . . 24
     6.3.  Monitoring ALTO  . . . . . . . . . . . . . . . . . . . . . 24
       6.3.1.  Monitoring Metrics Definition  . . . . . . . . . . . . 24
       6.3.2.  Monitoring Data Sources  . . . . . . . . . . . . . . . 25
       6.3.3.  Monitoring Structure . . . . . . . . . . . . . . . . . 25
   7.  Known Limitations of ALTO  . . . . . . . . . . . . . . . . . . 27
     7.1.  Limitations of Map-based Approaches  . . . . . . . . . . . 27
     7.2.  Limitiations of Non-Map-based Approaches . . . . . . . . . 28
     7.3.  General Challenges . . . . . . . . . . . . . . . . . . . . 28
   8.  Extensions to the ALTO Protocol  . . . . . . . . . . . . . . . 30
     8.1.  Host Group Descriptors . . . . . . . . . . . . . . . . . . 30
     8.2.  Rating Criteria  . . . . . . . . . . . . . . . . . . . . . 30
       8.2.1.  Distance-related Rating Criteria . . . . . . . . . . . 30
       8.2.2.  Charging-related Rating Criteria . . . . . . . . . . . 31
       8.2.3.  Performance-related Rating Criteria  . . . . . . . . . 31
       8.2.4.  Inappropriate Rating Criteria  . . . . . . . . . . . . 32



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   9.  API between ALTO Client and Application  . . . . . . . . . . . 33
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 34
     10.1. Information Leakage from the ALTO Server . . . . . . . . . 34
     10.2. ALTO Server Access . . . . . . . . . . . . . . . . . . . . 34
     10.3. Faking ALTO Guidance . . . . . . . . . . . . . . . . . . . 35
   11. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 36
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 37
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 37
     12.2. Informative References . . . . . . . . . . . . . . . . . . 37
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 39
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 40








































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1.  Introduction

   Many Internet applications are used to access resources, such as
   pieces of information or server processes, which are available in
   several equivalent replicas on different hosts.  This includes, but
   is not limited to, peer-to-peer file sharing applications and Content
   Delivery Networks (CDNs).  The goal of Application-Layer Traffic
   Optimization (ALTO) is to provide guidance to applications, which
   have to select one or several hosts from a set of candidates, that
   are able to provide a desired resource.  The basic ideas of ALTO are
   described in the problem space of ALTO is described in [RFC5693] and
   the set of requirements is discussed in [I-D.ietf-alto-reqs].

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

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































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2.  General Considerations

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

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

                Figure 1: Network Overview of ALTO Protocol

2.1.  General Placement of ALTO

   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, (e.g., peer-to-peer filesharing application) or
   if the ALTO client is located on resource directory, as shown in
   Figure 3 (e.g., a tracker in peer-to-peer filesharing).
















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

          Figure 2: Overview of protocol interaction between ALTO
                     elements,scenario without tracker

   Figure 2 shows the operational model for applications that do not use
   a tracker, such as, edonky, or in if the tracker should be the
   querying party.  This use case also holds true for CDNs.  The ALTO
   server can also be queried by CDNs to get a guidance about where the
   a particular client accessing data in the CDN is exactly located in
   the ISP's network.
























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                                                  +-----+
                                                **|     |**
                                              **  +-----+  *
                                            **       *     *
                                          **         *     *
        +-----+     +------+     +-----+**        +-----+  *
        |     |.....|      |=====|     |**********|     |  *
        +-----+     +------+     +-----+**        +-----+  *
      Source of      ALTO        Resource **         *     *
      topological    service     directory  **       *     *
      information               ("tracker")   **  +-----+  *
                                                **|     |**
                                                  +-----+
                                                   Peers
      Legend:
      === ALTO client protocol
      *** Application protocol
      ... Provisioning protocol

     Figure 3: Overview of protocol interaction between ALTO elements,
                           scenario with tracker

   However, Figure 3 does not denote where the ALTO elements are
   actually located, i.e., if the tracker and the ALTO server are in the
   same ISP's domain, or if the tracker and the ALTO server are managed/
   owned/located in different domains.  The latter is the typical use
   case, e.g., taking Pirate Bay as example that serves Bittorrent peers
   world-wide.

2.2.  Relationship between ALTO and Applications

   ALTO is intended to be used by a wide-range of applications.
   However, 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 situation (see also
   [I-D.ietf-alto-reqs]).  (Editor's note: better text needed here!)

2.3.  Provided Guidance

   ALTO gives guidance to applications on what IP addresses or IP
   prefixes, and such which hosts are to be preferred according to the
   operator of the ALTO server.  The general assumption of the ALTO WG
   is that a network operator would always express to prefer hosts in
   its own network while hosts located outside its own network are to be
   avoided (are undesired to be considered by the applications).  This
   might be applicable in some cases but may not be applicable in the
   general case.  The ALTO protocol gives only the means to let the ALTO
   server operator to express is preference, whatever this preference



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   is.  This section explores this space.

2.3.1.  Keeping Traffic Local in Network

   ALTO guidance can be used to let applications prefer other peers
   within the same network operator's network instead of randomly
   connecting to other peers which are located in another operator's
   network.  Figure 4 shows such a scenario where peers prefer peers in
   the same network (e.g., Peer 1 and Peer 2 in ISP1 and Peer 3 and Peer
   4 in ISP2).


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

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

                Figure 4: ALTO Traffic Network Localization

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

2.3.2.  Off-Loading Traffic from Network

   Another scenario where the use of ALTO can be beneficial is in mobile
   broadband networks, e.g., CDMA200 or UMTS, but where the network
   operator may have the desire to guide peers in its own network to use
   peers in remote networks.  One reason can be that the wireless
   network is not made for the load cause by, e.g., peer-to-peer



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   applications, and the operator has the need that peers fetch their
   data from remote peers in other parts of the Internet.


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

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

              Figure 5: ALTO Traffic Network De-Localization

   Figure 5 shows the result of such a guidance process where Peer 2
   prefers a connection with Peer4 instead of Peer 1, as shown in
   Figure 4.

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

2.3.3.  Intra-Network Localization/Bottleneck Off-Loading

   The above sections described the results of the ALTO guidance on an
   inter-network level.  However, ALTO can also be used to guide peers
   on which internal peers are to be preferred.  For instance, to guide
   Peers on a remote network side to prefer to connect to each other,
   instead of crossing a bottleneck link, a backhaul link to connect the
   side to the network core.  Figure 6 shows such a scenario where Peer
   1 and Peer 2 are located in Net 2 of ISP1 and connect via a low
   capacity link to the core (Net 1) of the same ISP1.  Peer1 and Peer 2
   would both exchange their data with remote peers, probably clogging
   the bottleneck link.



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

         Figure 6: Without Intra-Network ALTO Traffic Localization

   The operator can guide the peers in such a situation to try first
   local peers 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
     \             /
      \           /
       `-.     ,-'
          `---'
       Legend:
       ### peer "connections"
       ~~~ bottleneck link

          Figure 7: With Intra-Network ALTO Traffic Localization



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   TBD: describe how maps would look like.

2.4.  Provisiong ALTO Maps

   This section will describe how ALTO maps in the protocol can be
   populated before using them.













































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3.  Deployment Considerations by ISPs

   The Internet is a large network constituted of multiple networks
   worldwide.  Numerous of these networks are built by telecom operators
   or network operators (named ISP in this memo), and these networks
   provide network connectivity, such as cable networks, 3G and so on.
   As well as some of networks are built by universities or big
   organizations themselves, and these networks are used to provide
   connectivity for research and work.  The essence of Internet is its
   connectivity and sharing capability.  However, ISPs emphasize
   network's manageability and controllability, because ISPs provide
   public network access service for most person and families, they need
   to manage, to control and to audit the traffic.  Thus, it's important
   for ISPs to understand the requirement of optimizing traffic, and how
   to deploy ALTO service in these manageability and controllability
   networks.

3.1.  Requirement for Traffic Optimization by ISPs

   All networks of ISPs are connected to each other through peering
   points.  From view of business mode, the inter-network settlement is
   needed in traffic exchanging between these ISP's networks.  The
   current settlement can be costly.  So to save these cost, the simple
   and basic method is to decrease the traffic exchange across the
   peering points and keep the traffic in own network area.

   For some large ISPs, their whole network is layered.  The upper layer
   network includes one or several backbone networks, and the lower
   layer network includes multiple access networks.  These access
   networks are connected to backbone networks, and the exchange traffic
   with others through backbone network.  In this kind of layered
   network, the bandwidth of backbone network is important and may be
   scarce.  Traffic should be limited to the access networks, so to
   decrease the usage of backbone as far as possible.

   Compared to fixed networks, mobile networks have some special
   characters, including small link bandwidth, high cost, limited radio
   frequency resource, and terminal battery.  In mobile network, the
   usage of wireless link should be decreased as far as possible and be
   high-efficient.  For example, in the case of a P2P service, the
   clients in the fixed network should decrease the data transport from
   the clients in the mobile networks, as well as the clients in the
   mobile networks should prefer the data transmission from the clients
   in the fixed networks.







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3.2.  Considerations for ISPs

3.2.1.  Very small ISPs with simple Network Structure

   For very small ISPs, the traffic optimizing problem they focus is
   that how to decrease the traffic exchanging with other ISPs, because
   of high settlement costs.  To use the ALTO service to optimize
   traffic, small ISPs can define two optimization areas: one is their
   own network; the other is all outer networks connected with their
   network.  The cost map can be defined like this: the cost of link
   between clients of inner ISP's networks is lower than from clients of
   outer ISP's networks to clients of inner ISP's networks.  So the
   client of this ISP will prefer to require data from the clients in
   the same ISP with high priority.

   One example is given as below in Figure 8.  ISP A is one small ISP,
   only having one access network.  In ALTO service deploying, we can
   define ISP A to be one optimization area, named as PID1, and define
   other networks to be the other optimization area, named as PID2.  C1
   is denoted as the link cost in inner ISP A. C2 is denoted as the link
   cost from PID2 to PID1.  We define the cost map as:

   C1<C2

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


                  Figure 8: ALTO deployment in small ISPs

3.2.2.  Large ISPs with layered fixed Network Structure

   For large ISPs with layered fixed network structure, the traffic
   optimizing problems they focus will include that: using backbone
   network by high-efficiency, adjusting traffic balance in different
   access networks according to traffic conditions and management
   policies, and considering settlement cost with other ISPs.  So in



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   ALTO service deploying to this kind of large ISP, first the
   optimization area can be defined according to real network condition.
   For example, each access network can be defined to be one
   optimization area.  Then cost can be defined according to the
   optimizing requirement by ISPs.  There is one example described below
   and also shown in Figure 9.

   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 of backbone network in inner ISP A; and
   the second requirement is to decrease the P2P traffic to outer ISPs.
   Always, the second requirement is prior to the first one.  Also, we
   assume that the settlement rate with ISP B is lower than with other
   ISPs.  Then ISP A can deploy ALTO service to meet the need of traffic
   optimization.  We will give the detail example of ALTO service
   definition and configuration according to requirements above.

   In inner network of ISP A, we can define each access network to be
   one optimization area, and assign one PID to every access network,
   such as PID1, PID2, and PID 3.  Because of different settlement with
   different outer ISPs, we define ISP B to be one optimization area,
   and assign PID 4 to it, as well as define all other networks to be
   one optimization area and PID 5.

   We assign cost names (C1, C2, C3, C4, C5, C6, C7) as the figure
   below.  C1 is denoted as the link cost in inner AN A, the same as C2
   and C3.  C4 is denoted as the link cost from PID 1 to PID 2, the same
   as C5.  C6 is denoted as the link cost from the ISP B to ISP A. C7 is
   denoted as the link cost from other networks to ISP A.

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

   This is one very simple example above, in which we do not consider
   the different link type of access network.  In deploying ALTO service
   in real network, we must consider more real network conditions and
   requirements.  One real example is described in greater detail in
   [I-D.lee-alto-chinatelecom-trial].











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    +------------------------------------+         +----------------+
    | ISP A  +---------------+           |         |                |
    |        |    Backbone   |           |   C6    |      ISP B     |
    |     +--+    Network    +---+       |<--------+      PID 4     |
    |     |  +-------+-------+   |       |         |                |
    |     |          |           |       |         |                |
    |     |          |           |       |         +----------------+
    | +---+--+    +--+---+     +-+----+  |
    | |AN A  | C4 |AN B  |  C5 |AN C  |  |
    | |PID 1 +--->|PID 2 |<----+PID 3 |  |
    | |C1    |    |C2    |     |C3    |  |         +----------------+
    | +---+--+    +---+--+     +-+----+  |         |                |
    |     |           |          |       |         |                |
    |     |           |          |       |   C7    | Other Networks |
    |     |  +--------+--+       |       |<--------+ PID 5          |
    |     +--|  ALTO     +-------+       |         |                |
    |        |  Service  |               |         |                |
    |        +-----------+               |         |                |
    +------------------------------------+         +----------------+


    Figure 9: ALTO deployment in large ISPs with layered  fixed network
                                structures

3.2.3.  ISPs with Mobile Network

   For ISPs with mobile network and fixed network, the traffic
   optimizing problems they focus will be optimizing the mobile traffic,
   except problems on last hop section.  Wireless radio frequency
   resource is scarce and costly in mobile network.  The requirement of
   traffic optimization in mobile network is mainly decreasing the usage
   of radio resource.  The ALTO service can be deployed to meet these
   needs.

   For example in one ISP A as below in Figure 10, there is one mobile
   network is connected to backbone network.  In this kind of network
   structure, mobile network can be defined as one optimization area,
   and assigned PID 1.  We also define other PID and cost as figure
   below.

   To decrease the usage of wireless link, the relationship of these
   costs will be defined to:

   From view of mobile network:(C4 < C1).  This means that, the clients
   in mobile network requiring data resource from clients of the other
   access networks is prior to clients of mobile network.  This policy
   can decrease the usage of wireless link and power consumption in
   terminal.



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   From view of AN A:(C2 < C6, C5 = maximum cost).  This means that, to
   other optimization area, requiring data from mobile network should be
   avoided.

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


          Figure 10: ALTO deployment in ISPs with mobile network




















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4.  Using ALTO for P2P
                                 ,-------.
          ,---.               ,-'         `-.   +-----------+
       ,-'     `-.           /     ISP 1     \  |   Peer 1  |*****
      /           \         / +-------------+ \ |           |    *
     /    ISP X    \   +=====>+ ALTO Server |  )+-----------+    *
    /               \  =    \ +-------------+ / +-----------+    *
   ; +-----------+   : =     \               /  |   Peer 2  |    *
   | |  Tracker  |<====+      `-.         ,-'   |           |*****
   | |ALTO Client|<====+         `-------'      +-----------+   **
   | +-----------+   | =         ,-------.                      **
   :        *        ; =      ,-'         `-.   +-----------+   **
    \       *       /  =     /     ISP 2     \  |   Peer 3  |   **
     \      *      /   =    / +-------------+ \ |           |*****
      \     *     /    +=====>| ALTO Server |  )+-----------+  ***
       `-.  *  ,-'          \ +-------------+ / +-----------+  ***
          `-*-'              \               /  |   Peer 4  |*****
            *                 `-.         ,-'   |           | ****
            *                    `-------'      +-----------+ ****
            *                                                 ****
            *                                                 ****
            ***********************************************<******
       Legend:
       === ALTO client protocol
       *** Application protocol

      Figure 11: Global tracker accessing ALTO server at various ISPs

   Figure 11 depicts a tracker-based system, where 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.
   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.kiesel-alto-3pdisc].  However, the peers do not have any way to
   query the server themselves.  This setting allows to give 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, AKA peer exchange in various peer-to-peer
   protocols.











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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| ****
            *            `-.         ,-'      +-----------+ ****
            *               `-------'                       ****
            *                                               ****
            ***********************************************<****
       Legend:
       === ALTO client protocol
       *** Application protocol



              Figure 12: Global Tracker - Local ALTO Servers

   The scenario in Figure 12 lets the peers directly communicate with
   their ISP's ALTO server (i.e., ALTO client embedded in the peers),
   giving thus the peers the most control on which information they
   query for, as they can integrate information received from trackers
   and through direct peer-to-peer knowledge exchange.


















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

     Figure 13: P4P approach with local tracker and local ALTO server

   There are some attempts to let ISP's to deploy their own trackers, as
   shown in Figure 13.  In this case, the client has no chance to get
   guidance from the ALTO server, other than talking to the ISP's
   tracker.  However, the peers would have still chance the contact
   other trackers, deployed by entities other than the peer's ISP.

   Figure 13 and Figure 11 ostensibly take peers the possibility to
   directly query the ALTO server, if the communication with the ALTO
   server is not permitted for any reason.  However, considering the
   plethora of different applications of ALTO, e.g., multiple tracker
   and non-tracker based P2P systems and or applications searching for
   relays, it seems to be beneficial for all participants to let the
   peers directly query the ALTO server.  The peers are also the single
   point having all operational knowledge to decide whether to use the
   ALTO guidance and how to use the ALTO guidance.  This is a preference
   for the scenario depicted in Figure Figure 12.

4.1.  Using ALTO for Tracker-based Peer-to-Peer Applications







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

         Figure 14: Tracker-based P2P Application with random peer
                               preselection


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

   Figure 15: Tracker-based P2P Application with ALTO client in tracker




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   TBD: explain why Figure 15 usually will yield better results wrt.
   peer selection than Figure 14.

4.2.  Expectations of ALTO

   This section hints to some recent experiments conducted with ALTO-
   like deployments in Internet Service Provider (ISP) network's.  NTT
   performed tests with their HINT server implementation and dummy nodes
   to gain insight on how an ALTO-like service influence a peer-to-peer
   systems [I-D.kamei-p2p-experiments-japan].  The results of an early
   experiment conducted in the Comcast network are documented
   here[RFC5632]







































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5.  Using ALTO for CDNs

   Section 2 discussed the placement and usage of ALTO for P2P systems,
   but not beyond.  This section discuss the usage of ALTO for Content
   Delivery Networks (CDNs).  CDNs are used to bring a service (e.g., a
   web page, videos, etc) closer to the location of the user - where
   close refers to shorten the distance between the client and the
   server in the IP topology.  CDNs use several techniques to decide
   which server is closest to a client requesting a service.  One common
   way to do so, is relying on the DNS system, but there are many other
   ways, see [RFC3568].

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
































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6.  Advanced Features

6.1.  Cascading ALTO Servers

   The main assumptions of ALTO seems to be each ISP operates its own
   ALTO server independently, irrespectively of the ISP's situation.
   This may true for most envisioned deployments of ALTO but there are
   certain deployments that may have different settings.  Figure 16
   shows such setting, were for example, a university network is
   connected to two upstream providers.  ISP2 if the national research
   network and ISP1 is a commercial upstream provider to this university
   network.  The university, as well as ISP1, are operating their own
   ALTO server.  The ALTO clients, located on the peers will contact the
   ALTO server located at the university.


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

                      Figure 16: Cascaded ALTO Server

   In this setting all "destinations" useful for the peers within ISP2
   are free-of-charge for the peers located in the university network
   (i.e., they are preferred in the rating of the ALTO server).
   However, all traffic that is not towards ISP2 will be handled by the



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   ISP1 upstream provider.  Therefore, the ALTO server at the university
   has also to include the guidance given by the ISP1 ALTO server in its
   replies to the ALTO clients.  This can be called cascaded ALTO
   servers.

6.2.  ALTO for IPv4 and IPv6

   TBD

6.3.  Monitoring ALTO

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

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

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

6.3.1.  Monitoring Metrics Definition

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

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

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

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




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

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

6.3.2.  Monitoring Data Sources

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

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

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

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

6.3.3.  Monitoring Structure

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
















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

                      Figure 17: Monitoring Structure

























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7.  Known Limitations of ALTO

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

7.1.  Limitations of Map-based Approaches

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

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

   1.  ISPs reallocate IPv4 subnets from time to time;

   2.  ISPs reallocate IPv4 subnets on short notice;

   3.  IP prefix blocks may be assigned to a single DSLAM which serves a
       variety of access networks.

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

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



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      that needs to maintain high efficiency of IPv4 address utilization
      may be reluctant to jeopardize their future acquisition of IPv4
      address space.

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

   For 2): ISPs can use techniques, such as ODAP (XXX) 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 reallocate to areas where there is currently a high
   demand for IP addresses.

   For 3): In DSL-based access networks, IP prefixes are assigned to
   DSLAMs which are the first IP-hop in the access-network between the
   CPE and the Internet.  The access-network between CPE and DSLAM
   (called aggregation network) can have varying characteristics (and
   thus associated costs), but still using the same IP prefix.  For
   instance one IP addresses IP11 out of a IP prefix IP1 can be assigned
   to a VDSL (e.g., 2 MBit/s uplink) access-line while the subsequent IP
   address IP12 is assigned to a slow ADSL line (e.g., 128 kbit/s
   uplink).  These IP addresses are assigned on a first come first
   served basis, i.e., the a single IP address out of the same IP prefix
   can change its associated costs quite fast.  This may not be an issue
   with respect to the used upstream provider (thus the cross ISP
   traffic) but depending on the capacity of the aggregation-network
   this may raise to an issue.

7.2.  Limitiations of Non-Map-based Approaches

   The specification of the ALTO protocol [I-D.ietf-alto-protocol] uses,
   amongst others mechanism, a mechanism called Endpoint Cost Service.
   ALTO clients can ask guidance for specific IP addresses to the ALTO
   server.  However, asking for IP addresses, asking with long lists of
   IP addresses, and asking quite frequent 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.

   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.

7.3.  General Challenges

   An ALTO server stores information about preferences (e.g., a list of
   preferred autonomous systems, IP ranges, etc) and ALTO clients can
   retrieve these preferences.  However, there are basically two



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   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 be partially 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 (we call it H1) 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, does
   approach 1 require that the provider of the ALTO server, i.e., the
   network operator, reveals information about its network structure
   (e.g., AS numbers, IP ranges, topology information in general) to the
   ALTO client.

   Approach 2 (we call it H2) 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, does approach 2
   require that the clients send their operational information to the
   server.

   Both approaches have their pros and cons and are extensively
   discussed on the ALTO mailing list.  But there is basically a
   dilemma: Approach 1 is seen as the only working solution by peer-to-
   peer software vendors and approach 2 is seen as the only working by
   the network operators.  But neither the software vendors nor the
   operators seem to willing to change their position.  However, there
   is the need to get both sides on board, to come to a solution.















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8.  Extensions to the ALTO Protocol

8.1.  Host Group Descriptors

   Host group descriptors are used in the ALTO client protocol to
   describe the location of a host in the network topology.  The ALTO
   client protocol specification defines a basic set of host group
   descriptor types, which have to be supported by all implementations,
   and an extension procedure for adding new descriptor types .  The
   following list gives an overview on further host group descriptor
   types that have been proposed in the past, or which are in use by
   ALTO-related prototype implementations.  This list is not intended as
   normative text.  Instead, the only purpose of the following list is
   to document the descriptor types that have been proposed so far, and
   to solicit further feedback and discussion:

   o  Autonomous System (AS) number

   o  Protocol-specific group identifiers, which expand to a set of IP
      address ranges (CIDR) and/or AS numbers.  In one specific solution
      proposal, these are called Partition ID (PID).

8.2.  Rating Criteria

   Rating criteria are used in the ALTO client protocol to express
   topology- or connectivity-related properties, which are evaluated in
   order to generate the ALTO guidance.  The ALTO client protocol
   specification defines a basic set of rating criteria, which have to
   be supported by all implementations, and an extension procedure for
   adding new criteria .  The following list gives an overview on
   further rating criteria that have been proposed in the past, or which
   are in use by ALTO-related prototype implementations.  This list is
   not intended as normative text.  Instead, the only purpose of the
   following list is to document the rating criteria that have been
   proposed so far, and to solicit further feedback and discussion:

8.2.1.  Distance-related Rating Criteria

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

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



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

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

8.2.2.  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.

8.2.3.  Performance-related Rating Criteria

   The following rating criteria are subject to the remarks below.

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

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

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

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

   The ALTO client MUST be aware, that with high probability, the actual
   performance values differ significantly from these upper and lower
   bounds.  In particular, an ALTO client MUST NOT consider the "upper



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   bound for throughput" parameter as a permission to send data at the
   indicated rate without using congestion control mechanisms.

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

   o  the ALTO service is not an admission control system

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

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

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

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

   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.

8.2.4.  Inappropriate Rating Criteria

   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.






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9.  API between ALTO Client and Application

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

   This is still TBD.












































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10.  Security Considerations

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

10.1.  Information Leakage from the ALTO Server

   The ALTO server will be provisioned with information about the owning
   ISP's network and very likely also with information about neighboring
   ISPs.  This information (e.g., network topology, business relations,
   etc) is consider to be confidential to the ISP and must not be
   revealed.

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

10.2.  ALTO Server Access

   Depending on the use case of ALTO, several access restrictions to an
   ALTO server may or may not apply.  For an ALTO server that is solely
   accessible by peers from the ISP network (as shown in Figure 12), 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).

   On the other hand, if the ALTO server has to be accessible by parties
   not located in the ISP's network (see Figure Figure 11), e.g., by a
   third-party tracker or by a CDN system outside the ISP's network, the
   access restrictions have to be more loose.  In the extreme case,
   i.e., no access restrictions, each and every host in the Internet can
   access the ALTO server.  This might no the intention of the ISP, as
   the server is not only subject to more possible attacks, but also on



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   the load imposed to the server, i.e., possibly more ALTO clients to
   serve and thus more work load.

10.3.  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 peers.

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

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

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

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




















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11.  Conclusion

   This is the first version of the deployment considerations and for
   sure the considerations are yet incomplete and imprecise.















































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12.  References

12.1.  Normative References

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

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

12.2.  Informative References

   [I-D.ietf-alto-protocol]
              Alimi, R., Penno, R., and Y. Yang, "ALTO Protocol",
              draft-ietf-alto-protocol-09 (work in progress), June 2011.

   [I-D.ietf-alto-reqs]
              Kiesel, S., Previdi, S., Stiemerling, M., Woundy, R., and
              Y. Yang, "Application-Layer Traffic Optimization (ALTO)
              Requirements", draft-ietf-alto-reqs-10 (work in progress),
              June 2011.

   [I-D.kamei-p2p-experiments-japan]
              Kamei, S., Momose, T., and T. Inoue, "ALTO-Like Activities
              and Experiments in P2P Network Experiment Council",
              draft-kamei-p2p-experiments-japan-05 (work in progress),
              March 2011.

   [I-D.kiesel-alto-3pdisc]
              Kiesel, S., Stiemerling, M., Schwan, N., Scharf, M.,
              Tomsu, M., and S. Yongchao, "ALTO Server Discovery
              Protocol", draft-kiesel-alto-3pdisc-05 (work in progress),
              March 2011.

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

   [I-D.lee-alto-chinatelecom-trial]
              Li, K. and G. Jian, "ALTO and DECADE service trial within
              China Telecom", draft-lee-alto-chinatelecom-trial-02 (work
              in progress), April 2011.

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



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

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

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





































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Appendix A.  Acknowledgments

   Xianghui Sun, Lee Kai, and Richard Yang contributed Section 3 and
   Section 6.3

   Martin Stiemerling is partially supported by the COAST project
   (COntent Aware Searching, retrieval and sTreaming,
   http://www.coast-fp7.eu), a research project supported by the
   European Commission under its 7th Framework Program (contract no.
   248036).  The views and conclusions contained herein are those of the
   authors and should not be interpreted as necessarily representing the
   official policies or endorsements, either expressed or implied, of
   the COAST project or the European Commission.






































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Authors' Addresses

   Martin Stiemerling
   NEC Laboratories Europe
   Kurfuerstenanlage 36
   Heidelberg  69115
   Germany

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


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

   Email: ietf-alto@skiesel.de






























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