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ALTO                                                             H. Song
Internet-Draft                                                    Huawei
Intended status: Standards Track                                M. Tomsu
Expires: December 28, 2010                      Alcatel-lucent Bell Labs
                                                               G. Garcia
                                                          Telefonica I+D
                                                                 Y. Wang
                                                         Microsoft Corp.
                                                              V. Pascual
                                                                 Tekelec
                                                           June 26, 2010


                         ALTO Service Discovery
                  draft-song-alto-server-discovery-02

Abstract

   Application-Layer Traffic Optimization (ALTO) service aims to provide
   distributed applications with information to perform better-than-
   random initial peer selection when multiple peers in the network are
   available to provide a resource or service.  In order to discover an
   Application-Layer Traffic Optimization (ALTO) Server, a set of
   mechanisms are required.  These mechanisms enable applications to
   find an information source which provides them with information
   regarding the underlying network.  This document discusses various
   scenarios of ALTO discovery and specifies the use of several
   available options such as DHCP or DNS.

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 December 28, 2010.

Copyright Notice




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   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 Simplified BSD License.







































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  History  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  ALTO Service Deployment  . . . . . . . . . . . . . . . . . . .  5
     3.1.  ISP-Centric vs. Application-Specific ALTO Service
           Deployments  . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  Cross-domain vs. Localized ALTO Server Discovery . . . . .  8
   4.  ALTO Service Discovery Scenarios . . . . . . . . . . . . . . .  8
     4.1.  Discovery Metrics  . . . . . . . . . . . . . . . . . . . .  8
       4.1.1.  Discovery Clients  . . . . . . . . . . . . . . . . . .  8
       4.1.2.  Service Location . . . . . . . . . . . . . . . . . . .  9
       4.1.3.  Layering Perspective . . . . . . . . . . . . . . . . .  9
     4.2.  Discovery Scenarios  . . . . . . . . . . . . . . . . . . .  9
       4.2.1.  Local ALTO service discovery by end terminals  . . . . 10
       4.2.2.  Local ALTO service discovery by application
               trackers . . . . . . . . . . . . . . . . . . . . . . . 10
   5.  ALTO Service Discovery Mechanisms  . . . . . . . . . . . . . . 11
     5.1.  ALTO service discovery using Domain Name System (DNS)  . . 12
       5.1.1.  DNS-based ALTO discovery . . . . . . . . . . . . . . . 12
       5.1.2.  Determine Service Name of Local ALTO servers . . . . . 13
         5.1.2.1.  Using DHCP option for access domain name . . . . . 13
         5.1.2.2.  Use IANA Database  . . . . . . . . . . . . . . . . 14
         5.1.2.3.  Reverse DNS lookup . . . . . . . . . . . . . . . . 14
     5.2.  DHCP . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     5.3.  XRD  . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     5.4.  Provisioning . . . . . . . . . . . . . . . . . . . . . . . 15
     5.5.  Manual Configuration . . . . . . . . . . . . . . . . . . . 15
     5.6.  Multicast and broadcast  . . . . . . . . . . . . . . . . . 15
     5.7.  Caching  . . . . . . . . . . . . . . . . . . . . . . . . . 16
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 17
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19












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1.  Introduction

1.1.  History

   This document represents a merge of features from two previous
   drafts:

   (1). draft-wang-alto-discovery-00

   (2). draft-song-alto-server-discovery-00

   The ALTO service architecture and protocol are currently under
   discussion and development within the IETF ALTO working group.

   Although it is identified in the charter that a discovery mechanism
   is needed, the preference is to adopt one or more existing mechanisms
   for ALTO discovery rather than designing a new one.  Note though
   certain design decisions of the final ALTO framework will affect the
   selection of discovery mechanisms.  As a result, this document makes
   minimum assumptions of the ALTO framework, and presents different
   scenarios and available options based on prior and related discovery
   mechanisms.  This document will be updated to track the progress of
   the ALTO requirements and solution.

1.2.  Overview

   The ALTO problem statement draft [I-D.ietf-alto-problem-statement]
   describes that in P2P applications or client/server applications,
   resources or services are often available through multiple replicas
   and each of those are sometimes provided by different providers.
   ALTO service gives guidance to a consumer or directory about which
   resource provider(s) to select, in order to optimize the client's
   performance or quality of experience while optimizing resource
   consumption in the underlying network infrastructure.

   In order to query the ALTO server, clients must first know one or
   more ALTO servers that might provide useful information.  The purpose
   of the ALTO discovery mechanism is to find those servers in different
   network and application scenarios.

   Section 3 and Section 4 discuss various scenarios of ALTO service
   deployment and discovery.  Section 5 provides a description of
   available discovery mechanisms and its application to the ALTO
   service discovery use case addressing potential issues and
   consideration for each.






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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   The document uses terms defined in [I-D.ietf-alto-problem-statement].

   In addition to the generic ALTO descriptions, the following terms are
   used to describe the discovery mechanisms in this document:

   o ALTO Discovery Client: The logical entity discovering the ALTO
   Service.  Depending on the scenario, this could be a Peer or a Super-
   peer/Tracker.

   o ALTO Discovery Server: The logical entity providing information to
   locate the ALTO Service.  Depending on the discovery mechanism, this
   could be another Peer or a dedicated entity in the network.

   o ALTO Discovery Domain: The scope of the network handled by a
   particular ALTO Discovery Server.


3.  ALTO Service Deployment

   This section explores the various dimensions of the ALTO service
   deployment and access scenarios, and briefly discusses their
   implications to the discovery mechanisms.  Figure 1 below shows a
   generic ALTO framework diagram with discovery. .






















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                                                +------+
                                              +-----+  |  Peers
               +-----+      +------+    +=====|     |--+-oo
               |     |......|      |====+   oo+--*--+     o
               +-----+      +------+    |   o    *  ooooooo
             Source of       ALTO       |   o    *
             Topological     Service    |   o +--*--+
             information                +===o=|     | Tracker
                                            o +-----+ /Super-peer
                             ALTO Discovery o    o
                                 Service    o    o
                                +------+    o    o
                                |      |oooooooooo
                                +------+

              Legend:

              ===   ALTO query protocol
              ooo   ALTO service discovery protocol
              ***   Application protocol (out of scope)
              ...   Provisioning or initialization (out of scope)

    Figure 1  ALTO Discovery Diagram

3.1.  ISP-Centric vs. Application-Specific ALTO Service Deployments

   An ALTO Server is the logical entity that provides query interfaces
   for ALTO Clients.  ALTO servers can be deployed in an ISP-centric
   deployment or on the application level by application providers or
   other trusted third parties:

   1.  A network operator which wants to optimize its traffic, e.g. to
   reduce its transit traffic volume across the network boundaries; a
   third party on behalf of one or even several ISPs.

















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      +-----------+           +-----------+   +---------+
      |ALTO Server|           |ALTO Server|   |ALTO     |
      |    A      |           |    B      |   |Service  |
      +-----------+           +-----------+   |Discovery|
           ^                          ^       +---------+
       +---+--+                   +---+--+    o    o
      +|------|-------------------|------|-+  o    o
      ||      | P2P Appication A  |      | |ooo    o
      ++------+-------------------+------+-+       o
       |      |                   |      |         o
      ++------+-------------------+------+-+       o
      ||      |  P2P Applcation B |      | |oooooooo
      ++------+-------------------+------+-+
       |ISP A |  ..............   | ISP B|
       +------+                   +------+

    Figure 2: ISP-centric ALTO service deployment example

   2.  The application provider itself, a trusted third party or a user
   community: acting on the application level and spanning different
   networks.  They run distributed algorithms to measure the overall
   network through some mechanisms and provide an ALTO service to
   numerous application clients.  However in this case, the application
   level service may not know the policy of network operators, so with
   high probability it will also cause suboptimal result for the network
   operator while benefitting the applications.  (Haibin: not the main
   focus of this WG?  Need to update or remove from the spec)

                                            +---------+
                +-----------+               |ALTO     |
                |ALTO Server|               |Service  |
                +-----------+               |Discovery|
                 ^         ^                +---------+
       +------+  |         |      +------+    o    o
      +|------|--+---------|------|------|-+  o    o
      ||      | P2P Appication A  |      | |ooo    o
      ++------+------------+------+------+-+       o
       |      |            |      |      |         o
      ++------+------------|------+------+-+       o
      ||      |  P2P Applcation B |      | |oooooooo
      ++------+-------------------+------+-+
       |ISP A |  ..............   | ISP B|
       +------+                   +------+

    Figure 3: Application-centric ALTO service deployment example






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3.2.  Cross-domain vs. Localized ALTO Server Discovery

   For cross domain scenarios, the ALTO service includes the ALTO
   servers provided by p2p application community as well as the ALTO
   servers provided by a third party on behalf of multiple cooperating
   network operators.

   So there may be several ALTO servers distributed in different
   operator's networks.

   Each operator may provide the ALTO service using their own ALTO
   servers.  Each network operator may have its own traffic optimization
   policy based on his network topology, however it may not know other
   network operator's policies, nor be clear of other network operator's
   topologies (e.g. topology hiding).  Each of the ALTO servers may have
   a FQDN.

   The ALTO client (e.g. the Tracker) must be able to discover and
   choose the ALTO server that has the information that is specific to
   those clients located within that network.

   In localized discovery deployments one or several ALTO servers
   provide the service only to clients in their own network or
   autonomous system.


4.  ALTO Service Discovery Scenarios

4.1.  Discovery Metrics

4.1.1.  Discovery Clients

   The ALTO Client can be the Peer in the end-user host or an external
   entity like a Super-peer or Resource Directory (aka Tracker) on
   behalf of the Peer [I-D.ietf-alto-problem-statement].  If a Super-
   Peer or Tracker acts as an ALTO Client it needs to know and select
   the suitable ALTO Service for the Peer being served.  Possible
   mechanisms for third party ALTO discovery have been proposed
   in[I-D.kiesel-alto-3pdisc]

   In a hybrid model the location of the ALTO Server could be
   communicated from the Peer to the Super-Peer using the application
   protocol.  It could also be discovered by the Super-Peer from other
   Peer information received implicitly (like the Peer public IP
   address) or received explicitly.

   There could be scenarios where only the Peer (and not the Super-Peer/
   Tracker) is able to access the ALTO Service, for example if the ALTO



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   Server is located in a private network.

4.1.2.  Service Location

   The ALTO service could be provided in a distributed and cooperative
   fashion by the Peers in an overlay, or it can be provided by a
   centralized entity (the ALTO Server) for a given scope.  In the
   former case, a DHT-style key-based routing algorithm could be used to
   locate the peers with the target network information in this type of
   distributed environment.  For the latter case, where a centralized
   ALTO Server is implicitly or explicitly assigned to a specific
   network scope, an out-of-band discovery mechanism is often required.

   All current ALTO solution proposals, ([Infoexp],
   [I-D.wang-alto-p4p-specification]), fall into the second category.

   The ALTO Server for a Peer could be in the same Local Area Network
   (LAN), within the same ISP Network but not on the same LAN, or in the
   Global Internet outside the ISP Network.  Different network scopes
   place different constraints on the discovery mechanisms.  Multicast
   discovery generally works within a single LAN only, whereas DNS-based
   or DHCP-based discovery can span multiple subnets within a single ISP
   or a single network administrative domain.  Internet scope discovery
   usually requires cross-domain indexing or directory services.  Note
   that peers participating in a single P2P application may reside on
   the same or different ISP networks.  Scenarios like this may require
   hybrid discovery solutions that can adapt to multiple network scopes
   at the same time.  The discovery mechanisms listed in this document
   should take into account possible limitations of the ALTO service
   deployment in those network scopes.

4.1.3.  Layering Perspective

   The discovery process takes place before the first access to the ALTO
   server.  This discovery process could be done at host network
   initialization time, at application initialization time or just
   before the first ALTO query is sent.

4.2.  Discovery Scenarios

   The ALTO service discovery scenarios are classified into two types:
   one is the ALTO server providing service to the overall network, and
   the other is where the ALTO server is providing service to the local
   network.







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4.2.1.  Local ALTO service discovery by end terminals

   In p2p applications without a tracker like DHTs and other
   conventional client/server applications, an end device needs to
   discover the local ALTO server by itself.

   After the discovery of an ALTO server, the p2p client can get
   guidance from the ALTO server directly or through its application
   tracker.

              +---------------+
              |  ALTO Server  |
              +---------------+
                   ^        ^         +-----------+
                   |        |         | ALTO      |
                   |    +---+---+     | Service   |
                   |    |tracker|     | Discovery |
                   |    +-------+     +---------+-+
                   |        |           o       o
      +------------+--+     |           o       o
      |P2P Application|ooooo|oooooooooooo       o
      |   Client A    |     |                   o
      +---------------+     |                   o
                            |                   o
                         +--+-------------+     o
                         | P2P Application|oooooo
                         |   Client B     |
                         +----------------+

    Figure 4: Local ALTO service discovery by end terminals (Example)

4.2.2.  Local ALTO service discovery by application trackers

   Some p2p applications have trackers, and these applications don't
   need to have their clients looking for the ALTO server guidance.
   Trackers query the ALTO servers for guidance, and then return the
   final ranked result to the application clients.  However, application
   clients are distributed among different network operators and
   autonomous systems.  Trackers must find different ALTO servers for
   the clients located in different network operators or autonomous
   systems.

   Figure 5 shows an example for a tracker's ALTO server discovery.  For
   client 1, the tracker has not cached yet the mapping between client
   1's network operator and its ALTO server address, so it queries the
   DNS server for the ALTO server address in that operator's domain.
   And then the tracker interacts with the ALTO server on behalf of
   client 1, finally, the ranked list is sent back to client 1.  For



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   client 2, the tracker has cached the mapping between client 2's
   network operator and its ALTO server address, so it does not need to
   query the DNS for the address of ALTO server 2.

            +-------------+    +-------------+
            | ALTO Server1|    | ALTO Server2|
            +-------------+    +-------------+
               ^     |            ^   |
              3|    4|           b|   |c
               |     |            |   |
                     v /----------+-\ v
   +---+         //////              \\\\\
   |   |      |||                         |||
   |   |<--->|                               |
   |DNS|  2  |     Application Tracker       |
   |   |      |||                         |||
   |   |         \\\\\\              /////
   +---+     ^    |    \------------/  |
             |    |5               ^   |d
            1|    |               a|   |
             |    v                |   v
           +-+---------+       +---+--------+
           |Application|       |Application |
           |client1    |       |client2     |
           +-----------+       +------------+

 Figure 5:  Local ALTO service discovery by application trackers (Example)


5.  ALTO Service Discovery Mechanisms

   One ALTO client should use one or several of the introduced discovery
   mechanisms according to its application scenario until it finally
   finds an appropriate ALTO server.

   The following issues should be considered when designing the ALTO
   service discovery mechanism.

      Load Balance: When more than one ALTO server provide identical
      service for the same area, we must find a mechanism to balance the
      processing load between the ALTO servers;

      Well known port: If ALTO server provides service through a well
      known port, then the discovery mechanism only needs to discover
      the IP address of an ALTO server that can provide service for a
      client, otherwise, the discovery mechanism must discover both IP
      address and port number through which the ALTO server provides the
      service.



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         Note:It will depend on the ALTO protocol whether a well know
         port is used for the ALTO server.  If there is no well known
         port for the ALTO server, we need to discover the port
         information with the discovery process.

      IP address change: The IP address of the ALTO server may change in
      some circumstances.  The ALTO service discovery mechanism must be
      well adaptable to this case when necessary.

      Mobile Scenarios: When the end terminals are mobile equipments,
      the data traffic may route via a roaming client's home agent's
      router to the client, or route to the client directly.  Which ALTO
      server to choose should depend on the routing optimization mode
      adopted for mobility.  If the data traffic routes via the client's
      home agent, it should choose the ALTO server that serves its home
      area network, otherwise, it should choose the ALTO server that
      serves its current network.

5.1.  ALTO service discovery using Domain Name System (DNS)

   DNS is widely used on the Internet to discover the server address for
   applications.  ALTO service is a conventional client/server mode
   service, which can use DNS lookup for its service discovery.

   NAPTR [RFC2915] and SRV [RFC2782] DNS resource records are
   appropriate to provide service discovery mechanisms.  The concrete
   application of these resource records depends on the final ALTO
   requests/response protocol.  The use of NAPTR or SRV records is a
   trade-off between flexibility and simplicity.  S-NAPTR [RFC3958] and
   U-NAPTR [RFC4848] mechanisms provide a Dynamic Delegation Discovery
   System (DDDS) Application to map domain name, service name and
   protocol name to a target host and port or to a target URI.  SRV
   records provide a mechanism to map domain name, service name and
   transport protocol name to a target host and port.  The use of a
   NAPTR or SRV solution is open to discussion and depends on the
   requirements of the ALTO protocol.  Next section will asume the use
   use of SRV records in the next sections are based on SRV.

5.1.1.  DNS-based ALTO discovery

   Figure 6. shows a general DNS ALTO server discovery mechanism.  A
   server must register its SRV resource record with a well known
   service name (e.g. _ALTO._TCP.example.com) in the DNS system.  The
   service name in this document refers to the name used for DNS SRV
   query, which includes the service label, protocol name (TCP or UDP)
   and domain name.  Any ALTO client that wants to get the IP address
   and port of the ALTO server sends a DNS SRV query to the DNS server
   in that domain .



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        +-------------------------------------+
        |                DNS                  |
        +-------------------------------------+
             ^                          ^
             |                          |
             |                          |
             |DNS configuration         |DNS queries
             |or dynamic update         |and responses
             |                          |
             v                          v
       +-------------+            +-------------+
       |             |            |     ALTO    |
       | ALTO Server |            |   Discovery |
       |             |            |    Client   |
       +-------------+            +-------------+

    Figure 6: DNS query for well known ALTO servers

5.1.2.  Determine Service Name of Local ALTO servers

   An ALTO discovery client must know its ALTO service name for it
   before sending a query to the DNS system.  Some ALTO servers may
   provide service to the overall network, they may have well-known
   service name.  But in most cases, one ALTO server will only provide
   service to its own local access network or autonomous system.  There
   will be multiple ALTO servers in the overall network.  An ALTO
   discovery client needs to find the service name of its local ALTO
   server.

5.1.2.1.  Using DHCP option for access domain name

   There are DHCP options (OPTION_V4_ACCESS_DOMAIN and
   OPTION_V6_ACCESS_DOMAIN) proposed in [I-D.ietf-geopriv-lis-discovery]
   to discover the local access domain names.  (Haibin: need to read the
   new version of this draft, and update this section) The retrieved
   access domain name can be used to form a SRV name by prefixing the
   ALTO service label to the access domain name.  If it failed with the
   SRV lookup with this service name, then it will remove one tag from
   the left hand of the access domain name and prefix the ALTO service
   label to form a new SRV name.  It will iterate the process until it
   succeeds in getting an ATLO server information or failed.

   It should be noticed that there are many residential gateways (RG)
   which can act as DHCP servers themselves.  RG becomes a hindrance
   between the end terminals and the ALTO service provider's DHCP server
   if we use DHCP.  It should not depend on the update of all these RGs
   to support this new DHCP Option for ALTO server discovery.  A DHCP
   Container Option [I-D.ietf-dhc-container-opt] for server



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   configuration should be used here.  With the Container Option, the
   DHCP server for the consumer domain (e.g.  RGs) can just pass the
   server configuration to the end terminals without explicit knowledge
   of the DHCP options contained in the Container.  The DHCP Option for
   the access domain name could be contained in the Container Option.

5.1.2.2.  Use IANA Database

   The service name of a client's local ALTO server could be formed by
   adding the service and protocol label before its domain information.
   IANA and its subsidiary organizations (e.g.  APNIC) database can be
   used to lookup the physical domain of a client through its public IP
   address, i.e. which network operator and/or autonomous system the
   client belongs to.  The WHOIS service [WWW.WHOIS] on the Internet is
   also available for this purpose.  This mechanism requires ISPs assign
   the domain names to their ALTO servers according to the AS and ISP
   information (e.g. they have a rule to format the domain name,
   AS.ISP.COM), then you can rebuid the domain name with the information
   retrieved from WHOIS.  Otherwise, you can't.

   However, the mapping information may be changed due to the business
   deals and network adjustment.  For example, an ISP could sell some
   part of its network (include all equipments, IP addresses, AS number,
   and so on) to another ISP, and the ISP does not have the
   responsibility to notify the IANA, and then the information in the
   IANA database is wrong.

5.1.2.3.  Reverse DNS lookup

   BEP 22 [BEP-22] framework uses reverse DNS lookup to determine the
   domain name of a client through its public address.  And then use
   service label and the domain name to lookup the local server in DNS.
   The following limitations should be considered when use this
   mechanism.

   (1) This method assumes that the access network provider also
   provides the reverse DNS record and they control the domain that is
   indicated in the "PTR" record.  (In most cases it is true, but not
   always)

   (2) Furthermore, this method might not apply where a host is given a
   domain name that is different from the domain name of the access
   network.

   (3) In case of NAT and a public ALTO server, it requires the ALTO
   client to know its public IP address.

   The advantage is that it doesn't require any update/configuration/



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   change in the DHCP servers of any residential gateway.

5.2.  DHCP

   There are other ways using DHCP to locate an ALTO server.  One
   suggestion is to use DHCP to obtain the ALTO server IP address and
   port information directly.  New DHCP options are needed for this
   purpose.  The residential gateways consideration for DHCP option must
   be considered as described in . (Section 5.1.2.1)

   With this mechanism, the DHCP server needs to support load balance if
   there are more than one ALTO servers for this access domain.  The
   maintenance is costly when the address of ALTO server changes.

5.3.  XRD

   XRD is described in [XEP-1.0].  In order to begin the XRD discovery
   you need the URL (or XRI) of the resource you want to discover links/
   services related to.  In other XRD use cases like OpenId or
   OpenSocial, it is clear that you know that URL (the OpenId url of the
   user, or the url of the OS container).  But in case of ALTO Server
   Discovery, the obtainment of this initial URL also needs to use some
   discovery framework.

5.4.  Provisioning

   A network operator can simply provide a configuration file that
   contains the ALTO server address for its clients, provided that there
   are only one or a few ALTO servers which provide identical service
   for its network.  An application can also provide such a
   configuration file containing the ALTO server address if an existing
   ALTO server provides identical service to the overall network.

5.5.  Manual Configuration

   Manual configuration of the ALTO service location(s) could work in a
   single ISP network scope, but is not scalable when multiple ISPs or
   cross-domain ALTO services are required.  P2P applications often
   connect peers from ISPs that they may not have contacted before, and
   manual configuration will not work without any prior knowledge of the
   ALTO servers.

5.6.  Multicast and broadcast

   Multicast or broadcast MAY be used in some scenarios for ALTO
   discovery.

   IP-multicast-based discovery generally works in two ways:



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   1.  Clients send out multicast discovery requests and listen for
   responses (usually unicast) from available servers or service
   providers.

   2.  Servers or service providers send out multicast announcements
   when they become available or periodically, and clients waits for the
   next available multicast announcement to identify the servers or
   service providers.

   The on-demand requests and periodic announcements are not mutually
   exclusive.  An implementation can choose to utilize both
   simultaneously.  The configuration effort of multicast discovery is
   fairly straightforward, only the multicast address and port are
   needed.  Service types and additional information are often encoded
   in the requests or announcements messages, enabling the same
   multicast channel to support discovery of different resources or
   services.  There are two main constraints of multicast-based
   discovery - scopes and flooding messages.  Routers disable multicast
   forwarding by default, making it practically a single-subnet
   solution.  Some forms of discovery proxies are needed to extend the
   scope of multicast discovery to multiple subnets.  The second issue
   is the flooding of multicast messages to all hosts on the same
   subnet.  The total bandwidth consumed by multicast depends on the
   arrival rate the client application requests, and/or the frequency of
   the service announcements.  Older generations of 802.11-based
   wireless access points often slow down the transmission of multicast
   messages or generally have a higher packet loss rate for those,
   causing some multicast discovery implementation to automatically re-
   send multicast requests or announcements by default.  This mitigation
   further increases the amount of flooding messages on the LAN.
   Examples of multicast-based discovery include [I-D.cheshire-dnsext-
   multicastdns], [I-D.cai-ssdp-v1], [WSD], SLP [RFC2165], and LLMNR
   [RFC4795].

5.7.  Caching

   Once a client has located an ALTO server for the first time, it can
   cache it for use as future ALTO server.  There are implications in
   case of mobility of devices.


6.  Security Considerations

   As this document mainly proposes to use DNS and DHCP for ALTO service
   discovery, it will depend on the DHCP security and DNS security for
   this service discovery.





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7.  IANA Considerations

   The service label for the ALTO service will depend on the final
   protocol name for application-layer traffic optimization(TBD).


8.  Acknowledgements

   The authors would like to give special thanks to Roni Even for his
   continuous contribution to this document.

   We would also like to thank the following experts for their review
   and valuable comments.

      Y. Richard Yang

      Xingfeng Jiang

      Jay Gu

      Ning Zong

      (To be added)


9.  References

9.1.  Normative References

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

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC3958]  Daigle, L. and A. Newton, "Domain-Based Application
              Service Location Using SRV RRs and the Dynamic Delegation
              Discovery Service (DDDS)", RFC 3958, January 2005.

   [RFC4848]  Daigle, L., "Domain-Based Application Service Location
              Using URIs and the Dynamic Delegation Discovery Service
              (DDDS)", RFC 4848, April 2007.

   [RFC2915]  Mealling, M. and R. Daniel, "The Naming Authority Pointer
              (NAPTR) DNS Resource Record", RFC 2915, September 2000.

   [I-D.ietf-alto-problem-statement]



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              Seedorf, J. and E. Burger, "Application-Layer Traffic
              Optimization (ALTO) Problem Statement",
              draft-ietf-alto-problem-statement-04 (work in progress),
              September 2009.

   [I-D.ietf-geopriv-lis-discovery]
              Thomson, M. and J. Winterbottom, "Discovering the Local
              Location Information Server (LIS)",
              draft-ietf-geopriv-lis-discovery-15 (work in progress),
              March 2010.

   [I-D.ietf-dhc-container-opt]
              Droms, R., "Container Option for Server Configuration",
              draft-ietf-dhc-container-opt-05 (work in progress),
              March 2009.

9.2.  Informative References

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              June 1999.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              July 2003.

   [RFC2165]  Veizades, J., Guttman, E., Perkins, C., and S. Kaplan,
              "Service Location Protocol", RFC 2165, June 1997.

   [RFC4795]  Aboba, B., Thaler, D., and L. Esibov, "Link-local
              Multicast Name Resolution (LLMNR)", RFC 4795,
              January 2007.

   [I-D.kiesel-alto-3pdisc]
              Kiesel, S., Tomsu, M., Schwan, N., and M. Scharf, "Third-
              party ALTO server discovery", draft-kiesel-alto-3pdisc-02
              (work in progress), March 2010.

   [I-D.cheshire-dnsext-multicastdns]
              Cheshire, S. and M. Krochmal, "Multicast DNS",
              draft-cheshire-dnsext-multicastdns-11 (work in progress),
              March 2010.

   [I-D.wang-alto-p4p-specification]
              Wang, Y., Alimi, R., Pasko, D., Popkin, L., and Y. Yang,
              "P4P Protocol Specification",
              draft-wang-alto-p4p-specification-00 (work in progress),
              March 2009.




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   [I-D.narten-iana-considerations-rfc2434bis]
              Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs",
              draft-narten-iana-considerations-rfc2434bis-09 (work in
              progress), March 2008.

   [I-D.cai-ssdp-v1]
              Goland, Y., Cai, T., Leach, P., Gu, Y., and S. Albright,
              "Simple Service Discovery Protocol/1.0 Operating without
              an Arbiter", October 1999, <draft-cai-ssdp-v1-03>.

   [WWW.WHOIS]
              "http://www.whois.net".

   [BEP-22]   Harrison, D., Shalunov, S., and G. Hazel, "BitTorrent
              Local Tracker Discovery Protocol", October 2008,
              <http://bittorrent.org/beps/bep_0022.html>.

   [XEP-1.0]  Hammer-Lahav, E., "Extensible Resource Descriptor (XRD)
              Version 1.0", May 2009, <http://www.oasis-open.org/
              committees/download.php/32686/xrd-1.0-wd01.html>.

   [WSD]      Beatty, J., "Web Services Dynamic Discovery (WS-
              Discovery)", April 2005, <http://specs.xmlsoap.org/ws/
              2005/04/discovery/ws-discovery.pdf>.


Authors' Addresses

   Song Haibin
   Huawei

   Email: melodysong@huawei.com


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

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








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   Gustavo Garcia
   Telefonica I+D
   Emilio Vargas
   Madrid, Madrid
   Spain

   Phone: +34 913129826
   Email: ggb@tid.es


   Yu-Shun Wang
   Microsoft Corp.
   One Microsoft Way
   Redmond, WA  98052
   USA

   Email: yu-shun.wang@microsoft.com


   Victor Pascual
   Tekelec
   Am Borsigturm 11
   Berlin  D-13507
   Germany

   Phone: +49 30 32 51 32 12
   Email: victor@iptel.org
























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