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ALTO                                                           S. Kiesel
Internet-Draft                                   University of Stuttgart
Intended status: Informational                                  M. Tomsu
Expires: September 9, 2010                                     N. Schwan
                                                               M. Scharf
                                                Alcatel-Lucent Bell Labs
                                                           March 8, 2010


                   Third-party ALTO server discovery
                      draft-kiesel-alto-3pdisc-02

Abstract

   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.

   This document describes why a third-party ALTO server discovery
   mechanism is required for an important class of applications, namely
   tracker-based P2P applications.  Several solution approaches are
   classified and evaluated.  The conclusion is that further work is
   required to standardize a protocol and procedures that follow one
   specific approach.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   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
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   The list of current Internet-Drafts can be accessed at
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   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on September 9, 2010.



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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Problem statement  . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  The need for third-party ALTO queries  . . . . . . . . . .  4
     2.2.  The need for third-party ALTO server discovery . . . . . .  6
   3.  Peer-to-peer application scenario  . . . . . . . . . . . . . .  8
   4.  Classification of solution approaches  . . . . . . . . . . . . 10
     4.1.  Solutions that do not require an update of the
           application protocol . . . . . . . . . . . . . . . . . . . 10
     4.2.  Solutions that do require an update of the application
           protocol . . . . . . . . . . . . . . . . . . . . . . . . . 12
   5.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     5.1.  Approach #1  . . . . . . . . . . . . . . . . . . . . . . . 16
     5.2.  Approach #2  . . . . . . . . . . . . . . . . . . . . . . . 16
     5.3.  Approach #3  . . . . . . . . . . . . . . . . . . . . . . . 16
     5.4.  Approach #4  . . . . . . . . . . . . . . . . . . . . . . . 17
     5.5.  Approach #5  . . . . . . . . . . . . . . . . . . . . . . . 17
     5.6.  Approach #6  . . . . . . . . . . . . . . . . . . . . . . . 17
   6.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 23
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24










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1.  Introduction

   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.  ALTO is realized by a client-server protocol.  ALTO
   clients send queries to ALTO servers, in order to solicit guidance.
   The ALTO client can be embedded in the resource consumer, which will
   eventually access the desired resource.  As an alternative, the ALTO
   client can be embedded in a resource directory, which assists
   resource consumers in finding appropriate resource providers.  In
   some specific peer-to-peer application protocols these resource
   directories are called "trackers".  ALTO queries, which are issued by
   a resource directory on behalf of a resource consumer, will be
   referred to as third-party ALTO queries.

   The challenge for third-party ALTO queries is that they have to be
   answered by the "right" ALTO server, i.e., the ALTO server which has
   the knowledge to give guidance to the resource consumer on behalf of
   which the query is sent.

   This document uses the terminology introduced in [RFC5693] and it
   investigates solution approaches that fulfill the requirements for
   ALTO server discovery documented in [I-D.ietf-alto-reqs].

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
























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2.  Problem statement

2.1.  The need for third-party ALTO queries

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

   Several ALTO client protocol proposals exist (e.g.,
   [I-D.ietf-alto-protocol], [I-D.kiesel-alto-h12]), which specify how
   an ALTO client can query an ALTO server for guiding information and
   receive the corresponding replies.  However, in the considered
   scenario of a tracker-based P2P application, there are two
   fundamentally different possibilities where to place the ALTO client:

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

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

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

   In the first scenario (see Figure 1), the resource consumer queries
   the resource directory for the desired resource (F1).  The resource
   directory returns a list of potential resource providers without
   considering ALTO (F2).  It is then the duty of the resource consumer
   to invoke ALTO (F3/F4), in order to solicit guidance regarding this
   list.

   In the second scenario (see Figure 2), the resource directory has an
   embedded ALTO client, which we will refer to as RDAC in this
   document.  After receiving a query for a given resource (F1) the
   resource directory invokes the RDAC to evaluate all resource
   providers it knows (F2/F3).  Then it returns a, possibly shortened,
   list containing the "best" resource providers to the resource
   consumer (F4).









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


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

   Note: the message sequences depicted in Figure 1 and Figure 2 may
   occur both in the target-aware and the target-independent query mode
   (c.f.  [I-D.ietf-alto-reqs]).  In the target-independent query mode
   no message exchange with the ALTO server might be needed after the
   tracker query, because the candidate resource providers could be
   evaluated using a locally cached "map", which has been retrieved from
   the ALTO server some time ago.

   The problem with the first approach is, that while the resource
   directory might know thousands of peers taking part in a swarm, the
   list returned to the resource consumer is usually shortened for
   efficiency reasons.  Therefore, the "best" (in the sense of ALTO)



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   potential resource providers might not be contained in that list
   anymore, even before ALTO can consider them.

   For example, consider a swarm with 10,000 peers known to the tracker.
   A new peer wants to join the swarm and therefore asks the tracker for
   a list of peers.  For simplicity, we assume that 100 peers would be
   desirable neighbors for the new peer (in the sense of better-than-
   random peer selection) while the other 9,900 are less favorable.
   Assume that the tracker randomly selects 100 peers out of the 10,000
   known peers and returns them to the new peer.  With a probability of
   approx. 36% this list does not contain a single favorable peer, and
   with 99% probability there are only four or less of the favorable
   peers on the list.  Processing this list with the guiding ALTO
   information will ensure that the few favorable peers are ranked to
   the top of the list; however, the benefit is rather limited as the
   number of favorable peers in the list is just too small.  Much better
   traffic optimization could be achieved if the tracker would evaluate
   all 10,000 peers using ALTO, and return a list of 100 peers
   afterwards.  This list would then include a significantly higher
   fraction of favorable peers.  (Note, that if the tracker returned
   favorable peers only, there would be a risk that the swarm might
   disconnect and split into several 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.

2.2.  The need for third-party ALTO server discovery

   The previous section has shown why it is advantageous that entities
   such as resource directories can perform ALTO queries on behalf of
   resource consumers.  We will refer to this kind of ALTO query as
   "third-party ALTO query".  ALTO queries are sent to ALTO servers,
   which have knowledge of network topology and other information on
   which the ALTO guidance is based.

   The challenge for third-party ALTO queries is that they have to be
   answered by the "right" ALTO server, i.e., the ALTO server which has
   the knowledge to give guidance to the resource consumer on behalf of
   which the query is sent.

   One potential deployment scenario for ALTO is to establish a group of
   centralized ALTO servers which have complete knowledge and therefore
   can evaluate any pair of resource consumers and providers,
   respectively.  Directing a third-party ALTO query to one of these



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   servers would be a rather simple task.

   However, it is likely that there will be deployment scenarios with
   many ALTO servers, each having only partial knowledge and therefore
   being able to give guidance regarding only a defined group of
   resource consumers (e.g., those in its topological vicinity, or those
   connected to the same network operator).  The reasons for
   partitioning the overall knowledge include scalability and separate
   administrative responsibilities.  For the remainder of this document,
   we assume that the second scenario has to be supported.  The first
   scenario can be seen as special case of it, i.e., a solution that
   supports the second scenario will support the first scenario as well.
   We will identify and assess several approaches for finding the
   "right" ALTO server, which has the knowledge to give guidance to the
   resource consumer on behalf of which a third-party ALTO query is to
   be sent.



































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3.  Peer-to-peer application scenario

   For illustration purposes the following chapters provide several
   examples, which all refer to the scenario presented in Figure 3.
   However, the evaluations and conclusions presented in this document
   do not only consider this scenario, but they are much more general.


                        +------+       +---------+
                        | ALTO |---+---| Tracker |
                        | Srv3 |   |   +---------+
                        +------+   |
                                 |RTR| ISP 3
                                   |
               ****                |     ***********
            ***    **********************           *******
           *          Internet Backbone Networks           *
            ***************              ******************
                |          ******      **        |
              |RTR| ISP 1        ******        |RTR| ISP 2
                |                                |        +------+
   +-------+    |   +------+                     +--------| ALTO |
   |Peer 1a|----+---| ALTO |                     |        | Srv2 |
   +-------+    |   | Srv1 |                   |NAT|      +------+
                |   +------+                     |
   +-------+    |                   +-------+    |      +---------+
   |Peer 1b|----+                   |Peer 2a|----+------|ConfigSrv|
   +-------+    |                   +-------+    |      +---------+
              |RTR|                              |
   +-------+    |                   +-------+    |        +-------+
   |Peer 1c|----+                   |Peer 2b|----+-|RTR|--|Peer 2c|
   +-------+                        +-------+             +-------+

                          Figure 3: ALTO scenario

   Figure 3 shows three networks with connected end hosts, which are
   operated by different Internet Service Provides, identified as ISP1,
   ISP2, and ISP3, respectively.  These networks are interconnected by
   Internet backbone networks.

   ISP1's network connects to (amongst others) three hosts that run the
   peers of a P2P application, identified as peers 1a, 1b, and 1c,
   respectively.  Peers 1a and 1b are in topological vicinity, while 1c
   is more distant from them, because of the additional router (RTR) in
   between.  ISP1 operates an ALTO server, identified as ALTO Srv1,
   which can give guidance to resource consumers in ISP1's network,
   i.e., to peers 1a, 1b, and 1c.




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   ISP2's network has a very similar structure as described above, and
   it contains peers 2a, 2b, and 2c, as well as ALTO Srv2.  The main
   difference to ISP1's network is that ISP2 uses a carrier grade NAT
   device, in order to masquerade peers 2a, 2b, and 2c "behind" one
   single "external" (globally unique) IPv4 address.  ALTO server 2 is
   assumed to have a globally unique IPv4 address, i.e., it can be
   queried from other hosts in the Internet without any special NAT
   traversal mechanisms.

   ISP3's network contains a resource directory ("tracker") for a
   tracker-based P2P application.  ISP3 operates ALTO Srv3, which is
   populated with information that could be used for giving guidance to
   resource consumers in ISP3's network.

   We assume that the tracker already knows that peers 1b, 1c, 2b, and
   2c are taking part in a specific P2P overlay.  If peer 1a wishes to
   join the overlay, it sends an application protocol specific message
   to the tracker, asking for other peer's addresses.  Because of the
   reasons outlined in Section 2.1 the tracker should ask ALTO for
   guidance prior to replying.  More specifically, it should query ALTO
   server 1, because this ALTO server can give guidance to peer 1a.
   Analogical to that, if peer 2a sends a query to the tracker, the
   tracker needs to ask ALTO server 2 for guidance.  The procedures for
   identifying this ALTO server and conveying the guiding information to
   the tracker are the scope of this document.


























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4.  Classification of solution approaches

   There are several approaches for directing a third-party ALTO query
   from the RDAC to the "right" ALTO server.  The selection of the
   "right" ALTO server needs to consider the resource consumer on behalf
   of which the query will be performed.  The set of available options
   therefore depends on the available information about the resource
   consumer,

   The primary criterion in the following classification is whether ALTO
   must work together with all existing (P2P) application protocols, or
   whether we can assume that these protocols can be augmented with new
   ALTO-specific information fields.

4.1.  Solutions that do not require an update of the application
      protocol

   If we do not want to make specific assumptions on the (P2P)
   application protocol, we cannot assume that there are any other peer
   identifiers apart from IP addresses.  Therefore, we assume that the
   only information identifying the resource consumer is the source IP
   address of messages sent from the resource consumer to the resource
   directory.  This address may be the (public) IP address of the
   resource consumer, or it may be the external address of the last NAT
   on the path between resource consumer and resource directory.

   The RDAC that wants to perform the third-party ALTO query has two
   options:

   o  Approach #1: The RDAC invokes a discovery mechanism external to
      the ALTO client protocol, in order to map from the resource
      consumer's IP address to the "right" ALTO server.  The ALTO query
      will then be sent there directly (see Figure 4).

   o  Approach #2: Independent of the resource consumer's identity, the
      RDAC uses the ALTO client protocol to send the ALTO query to one
      preconfigured ALTO server.  The resource consumer's IP address is
      included in the query message.  Based on this IP address and using
      mechanisms of the ALTO client protocol the first ALTO server
      forwards (see Figure 5) or redirects (see Figure 6) the query to
      the "right" ALTO server.  This implies that ALTO servers must know
      each other, based on some discovery mechanism or manual
      configuration.








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    Peer 1a         Tracker        ALTO Srv1          DNS
   ----+----       ----+----       ----+----       ----+----
       |               |               |               |
       | F1 Tracker Q  |               |               |
       |==============>| F2 ALTO disc. Q (peer1a)      |
       |               |******************************>|
       |               | F3 ALTO disc. R: ALTO Srv1    |
       |               |<******************************|
       |               | F4 ALTO cp Q  |               |
       |               |-------------->|               |
       |               | F5 ALTO cp R  |               |
       | F6 Tracker R  |<--------------|               |
       |<==============|               |               |
       |               |               |               |


   ====  Application protocol (i.e., tracker-based P2P app protocol)
   ----  ALTO client protocol
   ****  ALTO discovery protocol (e.g., based on DNS queries)

              Figure 4: Message sequence chart for Approach 1


    Peer 1a         Tracker        ALTO Srv1     ALTO Srv2     ALTO Srv3
   ----+----       ----+----       ----+----     ----+----     ----+----
       |               |               | F1/2 HELLO  |             |
       |               |               |<###########>| F3/4 HELLO  |
       |               |               | F5/6 HELLO  |<###########>|
       |               |               |<#########################>|
       | F7 Tracker Q  |               |             |             |
       |==============>|               |             |             |
       |               | F8 ALTO cp Q  |             |             |
       |               |------------------------------------------>|
       |               |               |  F9 ALTO cp Q             |
       |               |               |<--------------------------|
       |               |               | F10 ALTO cp R             |
       |               |               |-------------------------->|
       |               | F11 ALTO cp R |             |             |
       | F12 Tracker R |<------------------------------------------|
       |<==============|               |             |             |
       |               |               |             |             |


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

    Figure 5: Message sequence chart for Approach 2 (query forwarding)



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    Peer 1a         Tracker        ALTO Srv1     ALTO Srv2     ALTO Srv3
   ----+----       ----+----       ----+----     ----+----     ----+----
       |               |               | F1/2 HELLO  |             |
       |               |               |<###########>| F3/4 HELLO  |
       |               |               | F5/6 HELLO  |<###########>|
       |               |               |<#########################>|
       | F7 Tracker Q  |               |             |             |
       |==============>|               |             |             |
       |               |  F8 ALTO cp Q |             |             |
       |               |------------------------------------------>|
       |               |  F9 ALTO cp redirect        |             |
       |               |<------------------------------------------|
       |               | F10 ALTO cp Q |             |             |
       |               |-------------->|             |             |
       |               | F11 ALTO cp R |             |             |
       | F12 Tracker R |<--------------|             |             |
       |<==============|               |             |             |
       |               |               |             |             |


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

    Figure 6: Message sequence chart for Approach 2 (query redirection)

4.2.  Solutions that do require an update of the application protocol

   If we assume that applications can be upgraded in order to support
   ALTO, the resource consumer can provide additional information to the
   RDAC in order to assist the process of ALTO server discovery.

   o  Approach #3: Using the extended application protocol, the resource
      consumer sends an additional peer-ID, which can be understood by
      ALTO, to the resource directory.  This peer-ID could be used to
      uniquely identify resource consumers and providers located behind
      NATs.  The RDAC uses this peer-ID in addition to or instead of the
      resource consumer's IP address (see Figure 7).  In all other
      aspects this approach is identical to approach #1.

   o  Approach #4: This approach is identical to approach #2, except
      that the peer-ID is used instead of the IP address, as described
      in approach #3.

   o  Approach #5: The resource consumer discovers its ALTO server on
      its own (i.e., not a third-party discovery).  Using the extended
      application protocol it sends the ALTO server's address to the
      RDAC.  The RDAC can use it for sending third-party ALTO queries



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      there.

   o  Approach #6: The resource consumer retrieves guiding information
      on its own, e.g., by discovering and querying an ALTO server or by
      doing measurements.  Using the extended application protocol it
      sends this information to the tracker, which can perform peer
      selection based on it.


    Peer 2a        ConfigSrv     Tracker        ALTO Srv2          DNS
   ----+----       ----+----    ----+----       ----+----       ----+---
       | F1 Config Q   |            |               |               |
       |;;;;;;;;;;;;;;>|            |               |               |
       |               |            |               |               |
       | F2 Config R   |            |               |               |
       | + LocalID LID |            |               |               |
       |<;;;;;;;;;;;;;;|            |               |               |
       |               |            |               |               |
       | F3 Tracker Q (LID)         |               |               |
       |===========================>| F4 ALTO disc. Q (ext.IP+LID)  |
       |               |            |******************************>|
       |               |            | F5 ALTO disc. R: ALTO Srv2    |
       |               |            |<******************************|
       |               |            | F6 ALTO cp Q  |               |
       |               |            |-------------->|               |
       |               |            | F7 ALTO cp R  |               |
       | F8 Tracker R  |            |<--------------|               |
       |<===========================|               |               |
       |               |            |               |               |


   ;;;;  Configuration protocol (e.g., DHCP)
   ====  Application protocol (i.e., tracker-based P2P app protocol)
   ----  ALTO client protocol
   ****  ALTO discovery protocol (e.g., based on DNS queries)

              Figure 7: Message sequence chart for Approach 3














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    Peer 2a        ConfigSrv        Tracker        ALTO Srv2
   ----+----       ----+----       ----+----       ----+----
       | F1 Config Q   |               |               |
       |;;;;;;;;;;;;;;>|               |               |
       |               |               |               |
       | F2 Config R   |               |               |
       | + Addr. of ALTO Srv2          |               |
       |<;;;;;;;;;;;;;;|               |               |
       |               |               |               |
       | F3 Tracker Q (Addr. of ALTO Srv2)             |
       |==============================>|               |
       |               |               | F4 ALTO cp Q  |
       |               |               |-------------->|
       |               |               | F5 ALTO cp R  |
       | F6 Tracker R  |               |<--------------|
       |<==============================|               |
       |               |               |               |


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

              Figure 8: Message sequence chart for Approach 5



























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    Peer 2a        ConfigSrv        Tracker        ALTO Srv2
   ----+----       ----+----       ----+----       ----+----
       | F1 Config Q   |               |               |
       |;;;;;;;;;;;;;;>|               |               |
       |               |               |               |
       | F2 Config R   |               |               |
       | + Addr. of ALTO Srv2          |               |
       |<;;;;;;;;;;;;;;|               |               |
       |               |               |               |
       | F3 ALTO cp Q  |               |               |
       |---------------------------------------------->|
       | F4 ALTO cp R  |               |               |
       |<----------------------------------------------|
   optional: perform   |               |               |
   measurements        |               |               |
       |               |               |               |
       | F5 Tracker Q (Info from ALTO reply + meas'mt.)|
       |==============================>|               |
       | F6 Tracker R  |               |               |
       |<==============================|               |
       |               |               |               |


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

              Figure 9: Message sequence chart for Approach 6























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5.  Discussion

   This section assesses and compares the different approaches
   introduced above, regarding trust, scalability, integration into
   existing ISP infrastructure and management processes, modification of
   existing applications, and ongoing ALTO architecture specification
   works.

5.1.  Approach #1

   The existence of a mechanism according to approach #1 is assumed by
   [I-D.ietf-alto-protocol].

   This approach does not require any changes of existing (P2P)
   application protocols.  However, the RDAC needs to implement an
   additional protocol for performing third-party ALTO server discovery.

   One possible way of implementing this approach would be based on DNS,
   providing a mapping from the resource consumer's IP address to the IP
   address of the corresponding "right" ALTO server.  DNS is proven to
   be scalable and has well-understood mechanisms for delegating
   authority.  Network operators are used to DNS management.

   This approach does not support intra-domain traffic optimization for
   large domains behind a NAT.

5.2.  Approach #2

   This approach does not require any changes of existing (P2P)
   application protocols.

   Furthermore, the RDAC does not need to implement an additional
   protocol besides the ALTO client protocol.  However, this approach
   relocates the discovery problem from the RDAC to the first ALTO
   server.

   This first ALTO server, when preconfigured in the RDAC of a large
   resource directory, would raise serious concerns about scalability
   and trust/security issues.

   This approach does not support intra-domain traffic optimization for
   large domains behind a NAT.

5.3.  Approach #3

   This approach requires changes to all existing (P2P) application
   protocols that want to benefit from ALTO.




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   This approach supports intra-domain traffic optimization for large
   domains behind a NAT.

   Except for the above mentioned statements, the same results as for
   approach #1 apply.

5.4.  Approach #4

   This approach requires changes to all existing (P2P) application
   protocols that want to benefit from ALTO.

   This approach supports intra-domain traffic optimization for large
   domains behind a NAT.

   Except for the above mentioned statements, the same results as for
   approach #2 apply.

5.5.  Approach #5

   This approach requires changes to all existing (P2P) application
   protocols that want to benefit from ALTO.

   This approach does not need a mechanism for third-party ALTO server
   discovery, as the ALTO server is discovered by the resource consumer.
   However, a mechanism for this kind of discovery is needed, see, e.g.,
   [I-D.song-alto-server-discovery].

   Unlike approaches #1 .. #4 this approach supports scenarios, in which
   there is not exactly one "right" ALTO server for any given resource
   consumer.  Instead of sending the address of the ALTO server
   provisioned by the ISP, the resource consumer can also send the
   address of another ALTO server of its choice to the RDAC.

5.6.  Approach #6

   This approach requires changes to all existing (P2P) application
   protocols that want to benefit from ALTO.

   This approach does not need a mechanism for third-party ALTO server
   discovery, as the ALTO server is discovered by the resource consumer.
   However, a mechanism for this kind of discovery is needed, see, e.g.,
   [I-D.song-alto-server-discovery].

   Unlike approaches #1 .. #4 this approach supports scenarios, in which
   there is not exactly one "right" ALTO server for any given resource
   consumer.  Instead of querying the ALTO server provisioned by the ISP
   and forwarding that information to the resource directory, the
   resource consumer can also query any other ALTO server of its choice.



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   This approach allows the resource consumer to augment the ALTO
   server's reply with local preferences (e.g., from measurements).  It
   is also possible not to query an ALTO server at all.
















































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6.  Conclusion

   This document describes why a third-party ALTO server discovery
   mechanism is required for an important class of applications, namely
   tracker-based P2P applications.  Several solution approaches are
   classified and evaluated.  Assuming that ALTO should work together
   with already deployed application protocols, "Approach #1" seems to
   be most promising.  In this approach, the resource directory invokes
   a discovery mechanism external to the ALTO client protocol, in order
   to map from the resource consumer's IP address to the "right" ALTO
   server.

   The existence of such a mechanism according to "Approach #1" is
   assumed by [I-D.ietf-alto-protocol].

   Further action is required to standardize a protocol and procedures
   according to "Approach #1".


































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7.  IANA Considerations

   This document does not mandate any immediate IANA actions.  However,
   such IANA considerations may arise from future ALTO discovery
   specification documents which try to meet the requirements given
   here.













































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8.  Security Considerations

   This early version of this memo does not yet have any security
   considerations, but they will be added in future revision.















































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9.  References

   [I-D.ietf-alto-protocol]
              Alimi, R., Penno, R., and Y. Yang, "ALTO Protocol",
              draft-ietf-alto-protocol-02 (work in progress),
              March 2010.

   [I-D.ietf-alto-reqs]
              Kiesel, S., Popkin, L., Previdi, S., Woundy, R., and Y.
              Yang, "Application-Layer Traffic Optimization (ALTO)
              Requirements", draft-ietf-alto-reqs-03 (work in progress),
              February 2010.

   [I-D.kiesel-alto-h12]
              Kiesel, S. and M. Stiemerling, "ALTO H12",
              draft-kiesel-alto-h12-01 (work in progress), March 2010.

   [I-D.song-alto-server-discovery]
              Song, H., Tomsu, M., Garcia, G., Wang, Y., and V. Pascual,
              "ALTO Service Discovery",
              draft-song-alto-server-discovery-01 (work in progress),
              July 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

   The authors would like to thank Haibin Song, Richard Alimi, and Roni
   Even for fruitful discussions during the 75th IETF meeting.

   Marco Tomsu and Nico Schwan are partially supported by the ENVISION
   project (http://www.envision-project.org), a research project
   supported by the European Commission under its 7th Framework Program
   (contract no. 248565).  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 ENVISION project or the European Commission.

   Michael Scharf is supported by the German-Lab project
   (http://www.german-lab.de) funded by the German Federal Ministry of
   Education and Research (BMBF).



































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Authors' Addresses

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

   Email: ietf-alto@skiesel.de
   URI:   http://www.rus.uni-stuttgart.de/nks/


   Marco Tomsu
   Alcatel-Lucent Bell Labs
   Lorenzstrasse 10
   Stuttgart  70435
   Germany

   Email: marco.tomsu@alcatel-lucent.com
   URI:   www.alcatel-lucent.com/bell-labs


   Nico Schwan
   Alcatel-Lucent Bell Labs
   Lorenzstrasse 10
   Stuttgart  70435
   Germany

   Email: nico.schwan@alcatel-lucent.com
   URI:   www.alcatel-lucent.com/bell-labs


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

   Email: michael.scharf@alcatel-lucent.com
   URI:   www.alcatel-lucent.com/bell-labs











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