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ALTO                                                           S. Kiesel
Internet-Draft                                           NEC Europe Ltd.
Intended status: Informational                                  M. Tomsu
Expires: April 29, 2010                         Alcatel-Lucent Bell Labs
                                                        October 26, 2009


                   Third-party ALTO server discovery
                      draft-kiesel-alto-3pdisc-01

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   This Internet-Draft will expire on April 29, 2010.

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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.


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 . . . . . .  4
   3.  Peer-to-peer application scenario  . . . . . . . . . . . . . .  6
   4.  Classification of solution approaches  . . . . . . . . . . . .  8
     4.1.  Solutions that do not require an update of the
           application protocol . . . . . . . . . . . . . . . . . . .  8
     4.2.  Solutions that do require an update of the application
           protocol . . . . . . . . . . . . . . . . . . . . . . . . . 10
   5.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     5.1.  Approach #1  . . . . . . . . . . . . . . . . . . . . . . . 14
     5.2.  Approach #2  . . . . . . . . . . . . . . . . . . . . . . . 14
     5.3.  Approach #3  . . . . . . . . . . . . . . . . . . . . . . . 14
     5.4.  Approach #4  . . . . . . . . . . . . . . . . . . . . . . . 15
     5.5.  Approach #5  . . . . . . . . . . . . . . . . . . . . . . . 15
     5.6.  Approach #6  . . . . . . . . . . . . . . . . . . . . . . . 15
   6.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 17
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 20
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22











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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.  ALTO
   queries in the latter case 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
   [I-D.ietf-alto-problem-statement] 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 asks
   the resource directory for a list of potential 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.

   There are two possibilities where to apply the ALTO guidance, in the
   resource consumer or in the resource directory:

   In the first scenario, the resource directory returns a list of
   potential resource providers without considering ALTO.  It is then
   the duty of the resource consumer to invoke ALTO, in order to solicit
   guidance regarding this list.  The problem with this approach is,
   that while the resource directory might know thousands of peers
   taking part in a swarm, the list returned to the resource consumer is
   usually shortened for efficiency reasons.  Therefore, the "best" (in
   the sense of ALTO) potential resource providers might not be
   contained in that list anymore, even before ALTO can consider them.

   In the second scenario, the resource directory has an embedded ALTO
   client, which we will refer to as RDAC in this document.  The
   resource directory invokes the RDAC to evaluate all resource
   providers it knows.  Then it returns a, possibly shortened, list
   containing the "best" resource providers to the resource consumer.
   From an overall optimization perspective, this scenario 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.  One 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,



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   respectively.

   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.

   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.


































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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 1.
   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 1: ALTO scenario

   Figure 1 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 2).

   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 3) or redirects (see Figure 4) 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 2: 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 3: 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 4: 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 5).  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 5: 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 6: 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 7: 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.penno-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.penno-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.  Informative References

   [I-D.ietf-alto-problem-statement]
              Seedorf, J. and E. Burger, "Application-Layer Traffic
              Optimization (ALTO) Problem Statement",
              draft-ietf-alto-problem-statement-02 (work in progress),
              July 2009.

   [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-01 (work in progress),
              July 2009.

   [I-D.penno-alto-protocol]
              Penno, R. and Y. Yang, "ALTO Protocol",
              draft-penno-alto-protocol-03 (work in progress),
              July 2009.

   [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.



























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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.

   Sebastian Kiesel is partially supported by the NAPA-WINE project
   (Network-Aware P2P-TV Application over Wise Networks,
   http://www.napa-wine.org), a research project supported by the
   European Commission under its 7th Framework Program (contract no.
   214412).  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 NAPA-WINE project or the European Commission.

   Marco Tomsu is partially supported by the 4WARD project
   (http://www.4ward-project.eu), a research project supported by the
   European Commission under its 7th Framework Program (contract no.
   216041).  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 4WARD project or the European Commission.






























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Authors' Addresses

   Sebastian Kiesel
   NEC Laboratories Europe
   Kurfuerstenanlage 36
   Heidelberg  69115
   Germany

   Email: kiesel@nw.neclab.eu
   URI:   http://www.nw.neclab.eu/


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

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































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