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Network Working Group                                           R. Penno
Internet-Draft                                              S. Raghunath
Intended status: Experimental                                  J. Medved
Expires: April 28, 2011                                 Juniper Networks
                                                                R. Alimi
                                                                  Google
                                                                 R. Yang
                                                         Yale University
                                                              S. Previdi
                                                           Cisco Systems
                                                        October 25, 2010


                   ALTO and Content Delivery Networks
                        draft-penno-alto-cdn-02

Abstract

   Networking applications can request through the ALTO protocol
   information about the underlying network topology from the ISP or
   Content Provider (henceforth referred as Provider) point of view.  In
   other words, information about what a Provider prefers in terms of
   traffic optimization -- and a way to distribute it.  The ALTO Service
   provides information such as preferences of network resources with
   the goal of modifying network resource consumption patterns while
   maintaining or improving application performance.

   One of the main use cases of the ALTO Service is its integration with
   Content Delivery Networks (CDN).  The purpose of this draft is
   twofold: first, to describe how ALTO can be used in existing and new
   CDNs, both within an ISP and in separate organizational entities from
   the ISP; second, to collect requirements for ALTO usage in CDNs and
   to provide recommendations into the development of the ALTO protocol
   for better support of CDNs.

Requirements Language

   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 RFC 2119 [RFC2119].

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



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   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
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

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

   This Internet-Draft will expire on April 28, 2011.

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.





















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Request Routing as an Integration Point of ALTO into CDN . . .  5
     4.1.  HTTP Redirect  . . . . . . . . . . . . . . . . . . . . . .  5
     4.2.  DNS Request Routing  . . . . . . . . . . . . . . . . . . .  6
   5.  Basic Scheme of CDN/ALTO Integration . . . . . . . . . . . . .  6
     5.1.  Basic Integration Scheme . . . . . . . . . . . . . . . . .  6
       5.1.1.  ALTO for HTTP Redirect . . . . . . . . . . . . . . . .  7
       5.1.2.  ALTO for DNS Resolution  . . . . . . . . . . . . . . .  8
     5.2.  Multi-hop Redirection  . . . . . . . . . . . . . . . . . .  8
     5.3.  CDN Node Discovery and Status Notification . . . . . . . .  8
       5.3.1.  CDN Node Status Updates received by Request Router . .  9
       5.3.2.  CDN Node Status Updates received by ALTO . . . . . . . 10
   6.  Request Routing using ALTO Services  . . . . . . . . . . . . . 10
     6.1.  Request Routing using the Map Service  . . . . . . . . . . 10
     6.2.  Request Routing using the Endpoint Cost Service  . . . . . 11
   7.  Multiple Administrative Domains  . . . . . . . . . . . . . . . 12
     7.1.  CDN nodes/Request Router in a separate administrative
           domain  from that of ISP . . . . . . . . . . . . . . . . . 12
     7.2.  Managed DNS Domain with Three Administrative Domains . . . 15
       7.2.1.  Managed DNS  Redirect to Local CDN . . . . . . . . . . 15
       7.2.2.  Managed DNS with CDN-Provided Request Routing  . . . . 16
   8.  Protocol Recommendations . . . . . . . . . . . . . . . . . . . 17
     8.1.  Necessary Additions  . . . . . . . . . . . . . . . . . . . 17
       8.1.1.  NA1: PID Attributes  . . . . . . . . . . . . . . . . . 17
       8.1.2.  NA2: PID Attributes and Query  . . . . . . . . . . . . 17
     8.2.  Helpful Additions  . . . . . . . . . . . . . . . . . . . . 18
       8.2.1.  HA1: Push Mechanism  . . . . . . . . . . . . . . . . . 18
       8.2.2.  HA2: Incremental Map Updates . . . . . . . . . . . . . 18
       8.2.3.  HA3: ALTO Border Router PID attribute  . . . . . . . . 18
       8.2.4.  HA4: CDN ALTO Server Discovery . . . . . . . . . . . . 18
       8.2.5.  HA5: Extensible ALTO Cost Maps . . . . . . . . . . . . 18
       8.2.6.  NA4: Federated Deployment of ALTO Servers  . . . . . . 19
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 19
     12.2. Informative References . . . . . . . . . . . . . . . . . . 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20








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1.  Introduction

   Content Delivery Networks are becoming increasingly important in the
   Internet [ARBOR] and many CDNs today already use some form of
   proximity through geolocation.  But in many cases the content
   provider/distributor and the Internet Service Provider are disjoint
   and even if content servers are co-located into the ISP's networks,
   there is no standardized way to share server location and/or network
   topology information.  Therefore a natural step forward would be to
   use ALTO to share this information.

   Another key aspect of ALTO in the context of CDNs deployments is that
   it is desirable that no changes to the hosts are needed (or that
   changes to hosts would be transparent to the user).  In other words,
   a traditional web browser is all there is needed to take advantage of
   ALTO information.  This is a significant difference from the P2P
   applications where a special client is typically needed and ALTO is
   normally used as a way to reduce operational expense.


2.  Scope

   This document discusses how Content Delivery Networks can benefit
   from ALTO through integration of the ALTO Service with the main
   request routing techniques.  There are two objectives:

   o  Present basic integration schemes of ALTO into CDNs.

   o  Provide protocol recommendations to ALTO: Whenever a new
      requirement on protocol functionality is identified to achieve
      integration with CDNs, it will be enumerated with 'REQ-<N>'.  Each
      requirement is documented in a section of its own in order to
      foster parallel discussions and possible adoption.


3.  Terminology

      Content-aware Proximity Request Router: The Request Router knows
      about locations and presence of content & media objects in the
      network.  Therefore the redirection to a CDN node is made based on
      both the availability of content or content-type in that CDN node
      and the proximity of the CDN node to the requesting user.

      Service-aware Proximity Request Router: The Request Router knows
      about locations of CDN nodes in the network and redirects user to
      the closest CDN node.  A redirection is made irrespective of
      content presence in the CDN node; if content is not present, the
      node will be populated with the content while the content is



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      served to the user.

      HTTP Request Router: a Content-aware or Service-aware Proximity
      Request Router for HTTP.  It embeds an HTTP Server that performs
      HTTP Redirects, an ALTO client that retrieves network mapping from
      the ALTO Server, and a Location Database which stores network
      mappings received from the ALTO Client.  The HTTP Server consults
      the Location Database when making redirection decisions.


4.  Request Routing as an Integration Point of ALTO into CDN

   Content Distribution is a rich and evolving field.  New architectures
   and approaches (e.g., a hybrid architecture using both servers and
   P2P) continue to be developed in the research community and industry
   and some are being deployed in production networks.  While we would
   like to provide a survey of each possible CDN architecture and show
   how it may be integrated with ALTO, it would be a daunting task to
   track such a rapidly-changing field.

   One scheme that is out of the scope of this document is P2P-only
   CDNs, where the application tracker takes the role of the ALTO
   Client, fetching the Network and Cost Maps from the ALTO Server and
   integrating them with its peer database.  The result is a peer
   database that takes into account both the current peer metrics, such
   as peer availability or content availability, and network metrics,
   such as topological localization.  This architecture in context of
   file sharing was extensively studied and trialed by ISPs such as
   Comcast [RFC5632] and China Telecom [I-D.lee-alto-chinatelecom-trial]
   under the ALTO/P4P [P4P] protocol.  Thus, P2P-only CDNs are not
   discussed in this document.

   Today, multiple request routing approaches can be used even in CDNs
   with purely server-based infrastructure.  Thus, we take the approach
   of developing a basic request routing scheme covering all major CDN
   types.  Specifically, the Request Routing Component of a CDN directs
   a request to a serving CDN node, and thus is the major integration
   point to utilize information available through ALTO.  There are
   multiple request routing mechanisms, including HTTP Redirect, DNS
   name resolution, and anycast.  We focus on HTTP Redirect and DNS name
   resolution.  We briefly review the two mechanisms.

4.1.  HTTP Redirect

   In this mechanism, an HTTP GET request from a host is received by an
   HTTP Request Router which sends back an HTTP responses with Status-
   Code 302 (Redirect) informing the host of the most optimal location
   to fetch the content.  The HTTP Redirection method is already



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   commonly used in production CDNs as described in RFC3568 [RFC3568].
   ALTO integration provides localization services where the device that
   performs the redirection becomes an ALTO client.

4.2.  DNS Request Routing

   In this mechanism, the DNS server handling host requests provides the
   Request Routing Component.  When the host performs a DNS query/
   lookup, the IP address contained in the response is already optimal
   for that query.

   DNS queries can be either iterative or recursive.  Iterative queries
   can be used with ALTO if the host itself queries the DNS Servers, or
   if the DNS Proxy used by the host is topologically close to the host.
   If the Host queries the DNS Servers, the authoritative DNS Server can
   see directly the host's IP address.  If the DNS Proxy's is
   topologically close to the Host, its IP address is a good
   approximation for the host's location.  In recursive queries, the
   authoritative DNS Server sees the IP address of the previous DNS
   Server in the resolution chain, and the IP address of the host is
   unknown.  DNS-based request routing does not work with recursive DNS
   queries.

   In an iterative DNS lookup with DNS Proxy, the host queries the
   Proxy, which in turn first queries one of the root servers to find
   the server authoritative for the top-level domain (com in our
   example).  The Proxy then queries the obtained top-level-domain DNS
   server for the address of the DNS server authoritative for the CDN
   domain.  Finally, the Proxy queries the DNS server that is
   authoritative for the cdn.com domain.  The authoritative DNS Server
   for the cdn.com will perform the request routing to the most
   appropriate CDN node, based on the source IP address of the
   requestor.  The host will then request the content directly from the
   CDN Node.


5.  Basic Scheme of CDN/ALTO Integration

   Although HTTP Redirect and DNS are quite different mechanisms to
   direct a request to a serving CDN node, as we will see, the basic
   structure of integrating ALTO with them can be quite similar.  Thus,
   we first present common structures.  We refer to the HTTP Redirect
   component or the DNS component of a CDN as a CDN Request Router.

5.1.  Basic Integration Scheme

   Figure 1 shows a general structure to embed an ALTO Client into a CDN
   Request Router.



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                                 +-----------------+
                                 |  Request Router |
         +-----------+   1       |                 |
         |           |---------> |                 |
         | Requestor |<--------  |                 |
         +-----------+   2       |                 |
                                 | +-------------+ |
                                 | | ALTO Client | |
                                 | +-------------+ |
                                 +-----------------+
                                        |   ^
                                        |   |  ALTO Protocol
                                        v   |
                                 +-----------------+
                                 |   ALTO Server   |
                                 +-----------------+

                    Figure 1: Request Router with ALTO

5.1.1.  ALTO for HTTP Redirect

   To make the basic scheme more concrete, Figure 2 shows the case that
   the Request Router uses HTTP Redirect.

                                 +---------------------+
                                 | HTTP Request Router |
         +------+      1         |   +-------------+   |
         |      |--------------> |   | HTTP Server |   |
         | Host |<-------------- |   +-------------+   |
         +------+      2         |          ^          |
                                 |          |          |
                                 |   +-------------+   |
                                 |   | ALTO Client |   |
                                 |   +-------------+   |
                                 +---------------------+
                                          |   ^
                                          |   |  ALTO Protocol
                                          v   |
                                 +---------------------+
                                 |     ALTO Server     |
                                 +---------------------+

                  Figure 2: ALTO for HTTP Request Router








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5.1.2.  ALTO for DNS Resolution

   Figure 3 shows the case that the Request Router uses DNS Resolution.

                           2      +----------------+
            +------------------> |       root     |
            | +----------------- |   Name Server  |      +----------+
            | |           3      +----------------+      | Content  |
            | |                                          | Provider |
            | |           4      +----------------+      +----------+
            | |   +------------> |       com      |
            | |   | +----------- |   Name Server  |
            | |   | |     5      +----------------+
            | |   | |
            | V   | V
           +---------+           +----------------+
           |   DNS   |---------> |     cdn.com    |
           |  Proxy  |<--------- |   Name Server  |
           +---------+    7      |                |
               ^ |               | +------------+ |
             1 | | 8             | |ALTO Client | |
               | V               | +------------+ |
           +---------+           +----------------+
           |  Host   |                  |   ^
           +---------+                  |   |  ALTO Protocol
                |                       |   |
                |                       V   |
                V                +----------------+
             CDN Node            |   ALTO Server  |
                                 +----------------+

                    Figure 3: ALTO for DNS Resolution.

5.2.  Multi-hop Redirection

   The preceding examples show the logical flow for redirection.  It is
   important to state that there maybe multiple redirection hops.

   For HTTP Redirect, the requestor may be redirected again by the first
   CDN node.  For DNS, the first DNS server may direct, using aggregated
   ALTO information (e.g., from multiple ALTO Servers of multiple ISPs),
   the DNS resolution to a second level DNS server, which then may use
   more specific ALTO information as well as CDN node status.

5.3.  CDN Node Discovery and Status Notification

   Since ALTO for HTTP Redirect and that for DNS have many common
   issues, we use the basic general scheme unless stated otherwise.



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   One common issue is how Request Router discovers the available CDN
   nodes and their locations.  The exact mechanism is outside the scope
   of this document.

   It is desirable that not only CDN node locations, but also real-time
   CDN node status (like health, load, cache utilization, CPU, etc.) is
   communicated to the CDN.

   Specifically, CDN node status can be retrieved from the existing Load
   Balancer infrastructure.  Most Load Balancers today have mechanisms
   to poll caches/servers via ping, HTTP Get, traceroute, etc.  Most LBs
   have SNMP trap capabilities to let other devices know about these
   thresholds.

   [yry: move]In addition to the CDN node status, network status can
   also be retrieved from TE/RP databases.

   We see two ways that CDN node status can be communicated into the
   request routing decision process.

5.3.1.  CDN Node Status Updates received by Request Router

   In this use case the Request Router receives CDN Status updates
   directly.

   Specifically, the Request Router can implement an SNMP agent and get
   to know whatever is needed.

                                 +-----------------+
                                 |  Request Router |
         +-----------+   1       |                 |
         |           |---------> |                 |<--- Real-time CDN
         | Requestor |<--------  |                 |     status updates
         +-----------+   2       |                 |
                                 | +-------------+ |
                                 | | ALTO Client | |
                                 | +-------------+ |
                                 +-----------------+
                                        |   ^
                                        |   |  ALTO Protocol
                                        v   |
                                 +-----------------+
                                 |   ALTO Server   |
                                 +-----------------+

                Figure 4: CDN Node Status to Request Router





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5.3.2.  CDN Node Status Updates received by ALTO

   This model generally simplifies the Request Router.  It allows an
   easier distribution of the Request Router, and to keep real time CDN
   status data updates in a logically centralized ALTO Server or in an
   ALTO Server Cluster.  It allows for the Request Router and the ALTO
   Server to be in different administrative domains.  For example, the
   Request Router can be in a Content Provider's domain, the ALTO Server
   and CDN Nodes in a Network Service Provider's domain.

   Specifically, ALTO Server could provide an API (for example, a Web
   Service or XMPP-based API) that could be used by CDN nodes to
   communicate their status to the ALTO server directly.

                                 +-----------------+
                                 |  Request Router |
         +-----------+   1       |                 |
         |           |---------> |                 |
         | Requestor |<--------  |                 |
         +-----------+   2       |                 |
                                 | +-------------+ |
                                 | | ALTO Client | |
                                 | +-------------+ |
                                 +-----------------+
                                        |   ^
                                        |   |  ALTO Protocol
                                        v   |
                                 +-----------------+
                                 |   ALTO Server   |<--- Real-time CDN
                                 +-----------------+     status updates

                     Figure 5: CDN Node Status to ALTO


6.  Request Routing using ALTO Services

   Either the Map Service or the Endpoint Cost Service of ALTO can be
   used by the Request Router.

6.1.  Request Routing using the Map Service

   The ALTO client embedded in the Request Router fetches the Network
   and Cost Maps from the ALTO Server and provides that information to
   the Request Router.

   As an illustrative example, we consider the case of HTTP Redirect.  A
   simple Request Router may be given (from an external source) the list
   of available CDN nodes.  The Request Router precomputes a redirection



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   table indexed by source PID with values being the closest CDN nodes.
   This redirection table can be built based on information from Network
   and Cost Maps.  Then when the Request Router receives an HTTP GET
   request, it looks up the PID of the source IP address on the request,
   indexes the redirection table using the request PID to select a CDN
   node, and finally returns a response that is an HTTP redirect with
   the URL of the selected CDN node.  The URL in 302 Redirect may
   contain the IP address of the selected CDN node or a domain name
   instead of IP address due to virtual hosting.  Therefore the IP
   addresses contained in the cost maps may need to be correlated to
   domain names a priori.  In practice, the redirection table may be
   indexed by both source and content to provide better redirection.

   The illustrative example can also be extended to DNS.

   The Network Maps generated by the ALTO Server will contain both Host
   PIDs and CDN Node PIDs, i.e., Host PIDs contain host subnets; CDN
   PIDs contain IP addresses of available CDN nodes.  Cost Maps may
   contain only cost from each host PID to each CDN PID and not the full
   matrix across all PIDs.  The reason is that the Request Router may
   redirect a host only to a CDN node, not to another host as in the P2P
   case.  Moreover, there is no generic way to disambiguate PIDs
   containing only hosts from PIDs containing CDN nodes.

   It is possible that a Request Router may be designated as being
   responsible only for a fixed set of Host PIDs.  This information can
   be made available to the Request Router before it receives requests
   from hosts.  If the set of Host PIDs is not known ahead of time, the
   latency for serving requests will be impacted by the capabilities of
   the ALTO server.

   With such information ahead of time, a Request Router that uses the
   Network Maps Service may pre-download the Network Map for the
   interesting Host PIDs and the CDN PIDs.  It can also start
   periodically pulling Cost Map for relevant PID 2-tuples.

   The Request Router can rely on the ALTO Server generated Cache-
   Control headers to decide how often to fetch CDN PID network map and
   Host PID network maps.

   For Alto protocol requirements related to request routing with the
   Map Service see Section 8.1.1 and Section 8.1.2.

6.2.  Request Routing using the Endpoint Cost Service

   Alternatively, the Request Router may request the Endpoint service
   from the ALTO client.




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   Specifically, the Request Router requests the Endpoint Cost Service
   in order to rank/rate the content locations (i.e., IP addresses of
   CDN nodes) based on their distance/cost (by default the Endpoint Cost
   Service operates based on Routing Distance) from/to the user address.

   Once the Request Router obtained from the ALTO Server the ranked list
   of locations (for the specific user) it can incorporate this
   information into its selection mechanisms in order to point the user
   to the most appropriate location.

   A Request Router that uses the Endpoint Cost Service may query the
   ALTO Server for rankings of CDN Node IP addresses for each
   interesting Host and cache the results for later usage.


7.  Multiple Administrative Domains

   The preceding discussion works well in a single administrative domain
   setting: the CDN nodes are in the administrative domain of the ISP.
   However, the CDN nodes, the ISP, and the Request Router can be in
   different administrative domains.  In this section, we consider a few
   such deployment cases.  We use DNS as an example.

7.1.  CDN nodes/Request Router in a separate administrative domain  from
      that of ISP

   In many situations, the CDN nodes and the Request Router are in a
   separate network managed by an entity that is distinct from the ISP.
   Consequently, the CDN nodes belong to a network with its own ALTO
   server that is distinct from the ALTO server of the ISP where the
   subscriber belongs.




















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                                       .................................
                                       :      +-----------------+      :
                                       :      |     cdn.com     |      :
                                       :      |   Name Server   |      :
            +----------+               :      |                 |      :
            | Content  |               :      | +-------------+ |      :
            | Provider |               :      | | ALTO Client | |      :
            +----------+               :      | +-------------+ |      :
                                       :      +-----------------+      :
                                       :              ^                :
                                       :              |                :
                                       :      +-----------------+      :
   .................................   :      |   ALTO Server   |      :
   :                               :   :      |                 |      :
   :    +----------------+         :   :      | +-------------+ |      :
   :    |   ALTO Server  |--------------------->| ALTO Client | |      :
   :    +----------------+         :   :      | +-------------+ |      :
   :                               :   :      +-----------------+      :
   :                               :   :                               :
   : +------+ C(1-4)    +--------+ :   : +--------+    C(6-8) +------+ :
   : | Host |<--------->| Border |: c6  :| Border |<--------->| CDN  | :
   : | PID1 |       +-->| Router |-------| Router |<--+       | PID8 | :
   : +------+       |+->|  PID4  | :    :|  PID6  |<-+|       +------+ :
   :                ||  +--------+ :   : +--------+  ||                :
   :                ||             :   :             ||                :
   : +------+ C(2-4)||             :   :             ||C(6-9) +------+ :
   : | Host |<------+|             :   :             |+------>| CDN  | :
   : | PID2 |        |             :   :             |        | PID9 | :
   : +------+        |             :   :             |        +------+ :
   :                 |             :   :             |                 :
   :                 |             :   :             |                 :
   : +------+ C(3-4) |  +--------+ :   : +--------+  | C(6-10)+------+ :
   : | Host |<-------+  | Border |: c7  :| Border |  +------->|  CDN | :
   : | PID3 |           | Router |-------| Router |           | PID10| :
   : +------+           |  PID5  | :   : |  PID7  |           +------+ :
   :                    +--------+ :   : +--------+                    :
   :                               :   :                               :
   :  ISP Administrative Domain    :   :   CDN Administrative Domain   :
   :...............................:   :...............................:

           Figure 6: Map advertising between ISP and CDN domains

   The ALTO server in the CDN provider network is assumed to be
   initialized with information about the ISP networks it serves.  For
   every such ISP network, it consults the routing plane to find the set
   of Border routers.  The CDN network ALTO server computes the cost of
   reaching each Border router from every CDN node (say, C_cdn).




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   Next, the CDN ALTO server contacts the ISP network's ALTO server and
   downloads the network map.  In order to help the CDN ALTO server
   compute the cost from a CDN node to a subscriber's PID, we break it
   down into two parts - the cost from the CDN node to the Border Router
   (C_cdn) and the cost from the Border Router to the subscriber's PID
   (say, C_isp).  Note that for any chosen exit point, C_cdn may be
   computed locally by the CDN ALTO Server.  However, the fundamental
   issue is that C_isp depends on the exit point (Border outer) chosen
   by the CDN.  There are multiple ways for the CDN ALTO Server to
   compute C_isp given the Network Map and Cost Map from the ISP's ALTO
   Server.

   One possibility is for the ISP ALTO Server to define a special Border
   Router PID (denoted by a PID attribute) which also indicates the
   corresponding Border Router PID in the CDN.  The attributes and
   values may be agreed-upon by the ISP and CDN when the ALTO Services
   are configured.  For example, in the example shown in Figure 5, the
   ISP ALTO Server indicates that its PID4 and PID5 are Border PIDs,
   with corresponding PIDs in the CDN as PID6, and PID7, respectively.
   Then, CDN ALTO Server can locally compute C_isp = cost(ISP Border
   Router PID, Subscriber PID).

   A second possibility for computing C_isp is to make use of Border
   Router IP addresses.  The CDN's Border Router can locally determine
   the IP address of the connected border router in the ISP.  In this
   approach, neither the CDN ALTO Server nor the ISP ALTO Server define
   PID attributes.  The ISP ALTO Server is not required to define
   special PIDs for Border Routers - it only needs to ensure that Border
   Router IP addresses are aggregated appropriately in its Network Map.

   Specifically, we identify two scenarios for the CDN ALTO Server to
   compute C_isp and C_cdn.

   In the first scenario, the CDN does not conduct CDN-level multi-path
   routing from the CDN nodes to the subscriber hosts.  Thus, the
   routing path from a CDN IP address to a subscriber host IP address is
   typically uniquely (if no ECMP) determined by the network routing
   system.  In this scenario, for a given CDN node IP address to a
   subscriber host IP address, the CDN ALTO Server uses the routing
   system to compute the Border Egress router inside the CDN, and the
   corresponding Border Ingress router inside the ISP.  Then the CDN
   ALTO Server has C_cdn(CDN node IP, Border Egress router IP inside the
   CDN), and C_isp(Border Ingress router IP inside the ISP, Subscriber
   IP).  The computation of C_cdn and C_isp can be done using ALTO in
   the traditional way through either the Network Map and Cost Map or
   the Endpoint Cost Service.

   In the second scenario, the CDN may support CDN-level multi-path



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   routing from the CDN nodes to the subscriber hosts.  In particular,
   from each CDN node, the CDN has a capability (e.g., through
   tunneling) to send to a subscriber host IP through multiple Border
   Egress routers (e.g., through any Egress router that receives an
   announcement from the ISP of the subscriber host IP).  In this case,
   the cost of reaching a host PID from a given CDN node is then
   determined as the minimum cost among all possible intermediate Border
   Routers.

   If the network is homogeneous, then a good approximation of the cost
   between each host PID and a given CDN node can be given as: C_cdn(CDN
   Node, Border router) + C_isp(Border router, Subscriber PID).  In this
   computation, the Border Router is the one that is on the best path
   from the CDN node to the Subscriber PID.

   The CDN ALTO server now has a cost map that provides the cost from
   each CDN node to all known Subscriber PIDs.  The ALTO client in the
   CDN DNS server downloads this cost map in preparation for subscriber
   DNS requests.

   When a subscriber DNS request arrives at the CDN provider's DNS
   server, it looks up the network map and maps the source IP address to
   a Subscriber PID.  It then uses the cost map to pick the best CDN
   node for this Subscriber PID.

7.2.  Managed DNS Domain with Three Administrative Domains

   Many organizations / content providers outsource DNS management to
   the external vendors for various reasons like reliability,
   performance improvement, DNS security etc.  Managed DNS service could
   be used either with caches owned by the organization itself (section
   6.3.1) OR with external CDNs (section 6.3.2)

7.2.1.  Managed DNS  Redirect to Local CDN

   One of the common functions offered by managed DNS service vendor is
   DNS traffic management where DNS resolver can load balance traffic
   dynamically across CDN servers.

   Typically managed DNS service provider has DNS resolvers spread
   across geographical locations to improve performance.  This also
   makes easier for DNS resolver to redirect host to the nearest cache.
   Such a DNS resolver would be an ideal candidate to implement ALTO
   client where it can fetch network map and cost map from ALTO servers
   located in the same geographical area only.  Load balancing
   implemented with the knowledge of network and cost map would be more
   efficient than other mechanisms like round robin.




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                         2        +----------------+
           +--------------------> |       root     |
           | +------------------- |   Name Server  |
           | |           3        +----------------+
           | |
           | |           4        +----------------+
           | |   +--------------> |       com      |
           | |   | +------------- |   Name Server  |
           | |   | |     5        +----------------+
           | |   | |
          _|-|---|-|--------------------.
      ,-'' | V   | V                     `--.
     '   +---------+    6    +---------------`+.
     |   |   DNS   |-------->|     xyz.com    | `
     |   |  Proxy  |<--------|   DNS Resolver | |
     |   +---------+    7    |                | |
     |     1^  | 8           | +------------+ | |
     |      |  |             | |ALTO Client | | |
     |   +-----V---+         | +------------+ | |
     |   |  Host   |         +----------------.-'
     |   +---------+                |   ^  .-'
     |        |       DOMAIN 1      |   |-'   ALTO Protocol
     |        V                     |.-'|     (Map Service)
      `--. CDN Node          __.--:-|   |
          `----.        _.--'       |   |
                `---.-''      ,---------+-------.
                            ,'+----------------+ \
                           /  |   ALTO Server  |  :
                          (   +----------------+  |
                           \                      ;
                            \    DOMAIN 2       ,'
                             `-----------------'

   In the figure above, there exists 2 possibilities:

   Case 1: Domain 1 and Domain 2 are connected to the same service
   provider network.  This case is similar to section 6.1

   Case 2: Domain 1 and Domain 2 are connected to different service
   provider network.  This case is similar to section 6.2

7.2.2.  Managed DNS with CDN-Provided Request Routing

   It is also possible to utilize a Managed DNS service and still rely
   on a CDN's request routing.  For example, this could be done if a
   network provider wishes to utilize a Managed DNS provider, but also
   wishes to integrate its own CDN using ALTO with DNS-based request
   routing.



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   To support this, the network provider may submit any necessary
   configuration files (e.g., indicating necessary CNAME records) to
   redirect CDN requests to the CDN's DNS request routing mechanism.
   Requests for the CDN (e.g., 'cdn.isp.com') will then be directed by
   DNS request routing, while requests for other hosts are handled by
   the Managed DNS solution.


8.  Protocol Recommendations

   In the previous sections, this document has taken the approach of
   providing information on existing CDN approaches and possible
   benefits of utilizing ALTO.  However, in developing the taxonomy, use
   cases, and deployment scenarios, we have identified cases where the
   ALTO Protocol [I-D.ietf-alto-protocol] and Server Discovery
   [I-D.kiesel-alto-3pdisc] [I-D.song-alto-server-discovery]
   [I-D.stiemerling-alto-dns-discovery] may be lacking capabilities that
   may be helpful and/or necessary for usage with CDNs.  We now focus on
   detailing these gaps with the goal of providing feedback and
   recommendations.  Note that some protocol changes may be necessary in
   the core protocol, while others may be implemented as extensions.

   This section will be updated to track changes in the ALTO Protocol,
   ALTO Server Discovery, and accompanying protocols.

8.1.  Necessary Additions

   This section details changes to the ALTO protocols that would be
   necessary to make use of ALTO within CDN infrastructures.  We
   classify a change as "necessary" if there is a core feature of a CDN/
   ALTO integration that is not possible to implement with the existing
   protocols.

8.1.1.  NA1: PID Attributes

   In order to disambiguate between PIDs that contain endpoints of a
   specific class, a PID property is needed.  A PID can be classified as
   containing "CDN nodes", "Mobile Hosts", "Wireline Hosts", etc.  This
   mechanism can be used to provide an ALTO Client a list of nodes of a
   particular type, along with the ALTO Costs to each node.

8.1.2.  NA2: PID Attributes and Query

   PID attributes can be used by the ALTO Client to select a appropriate
   host and also passed as a constraint in the map filtering service.






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8.2.  Helpful Additions

   This section details changes to the ALTO Protocol that would be
   helpful to make use of ALTO within CDN infrastructures.  We classify
   a change as "helpful" if there is a compelling extension to existing
   CDNs that would be possible with additional functionality within
   ALTO, or if there is a component of CDN/ALTO integration that could
   be made more efficient or otherwised improved with additional ALTO
   functionality.

8.2.1.  HA1: Push Mechanism

   It is important for the ALTO Service through the ALTO protocol or a
   companion protocol to provide a push mechanism from server to client.
   The push mechanism can be a notification that new data is available
   or the data itself.

8.2.2.  HA2: Incremental Map Updates

   A natural evolution to the protocol if maps are large and change
   often is to allow for incremental map updates.  In this sense the map
   contained in the reply would be considered the delta from the
   previous version.

8.2.3.  HA3: ALTO Border Router PID attribute

   In order for administrative domains to collate costs across domain
   boundaries, the border routers may be placed in their own PIDs.  Such
   PIDs may be identified by a Border Router attribute.

8.2.4.  HA4: CDN ALTO Server Discovery

   In certain deployment scenarios, it may be beneficial for an ALTO
   client to directly query a CDN's ALTO Server (instead of the CDN's
   ALTO Server only being consulted as a backend process).  For example,
   this can provide more accurate guidance than DNS request routing
   since the client's IP address may be directly used by the CDN in
   order to select a cache node.  This would require an ALTO Client
   (e.g., an ISP subscriber) to be able to discover an ALTO Server owned
   and/or managed by a CDN.  This could be done by an extension to the
   discovery protocol, or it could be done by allowing an ISP's ALTO
   Server to redirect certain queries to a CDN ALTO Server.

8.2.5.  HA5: Extensible ALTO Cost Maps

   Certain deployment scenarios may benefit from additional information
   being carried within ALTO information.  For example, a trusted
   neighboring ISP B may be able to help ISP A optimize multihoming



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   costs.  To provide an extensible way to communicate additional data,
   the ALTO Protocol could be extended to include opaque data strings
   (in addition to numeric and ordinal values) in an ALTO Cost Map.

8.2.6.  NA4: Federated Deployment of ALTO Servers

   There is a need to define how ALTO servers may communicate with each
   other in a federated model.


9.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an
   RFC.


10.  Security Considerations

   When the ALTO Server and Client are operated by different entities
   the issue of trust and security comes forward.  The exchange of
   information could be done using the encryption methods already
   present in HTTP but preventing unauthorized redistribution comes into
   play.  A further issue is if the ALTO information information is
   transitive, which modifications are allowed.


11.  Acknowledgements

   We would like to thank Mayuresh Bakshi for valuable input and
   contributions to this draft.  We would also like to thank Nabil
   Bitar, Manish Bhardwaj, Michael Korolyov, Steven Luong and Ferry
   Sutanto for their comments.


12.  References

12.1.  Normative References

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

12.2.  Informative References

   [ARBOR]    Labovitz, "Internet Traffic and Content Consolidation",
              2009, <http://www.ietf.org/proceedings/10mar/slides/
              plenaryt-4.pdf>.



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   [I-D.ietf-alto-protocol]
              Alimi, R., Penno, R., and Y. Yang, "ALTO Protocol",
              draft-ietf-alto-protocol-05 (work in progress), July 2010.

   [I-D.kiesel-alto-3pdisc]
              Kiesel, S., Tomsu, M., Schwan, N., Scharf, M., and M.
              Stiemerling, "Third-party ALTO server discovery",
              draft-kiesel-alto-3pdisc-03 (work in progress), July 2010.

   [I-D.lee-alto-chinatelecom-trial]
              Li, K., Wang, A., and K. Zhou, "ALTO and DECADE service
              trial within China Telecom",
              draft-lee-alto-chinatelecom-trial-00 (work in progress),
              July 2010.

   [I-D.song-alto-server-discovery]
              Yongchao, S., Tomsu, M., Garcia, G., Wang, Y., and V.
              Avila, "ALTO Service Discovery",
              draft-song-alto-server-discovery-03 (work in progress),
              July 2010.

   [I-D.stiemerling-alto-dns-discovery]
              Stiemerling, M. and H. Tschofenig, "A DNS-based ALTO
              Server Discovery Procedure",
              draft-stiemerling-alto-dns-discovery-00 (work in
              progress), July 2010.

   [P4P]      Xie, H., Yang, YR., Krishnamurthy, A., Liu, Y., and A.
              Silberschatz, "P4P: Provider Portal for (P2P)
              Applications", March 2009.

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

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


Authors' Addresses

   Reinaldo Penno
   Juniper Networks

   Email: rpenno@juniper.net




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   Satish Raghunath
   Juniper Networks

   Email: satishr@juniper.net


   Jan Medved
   Juniper Networks

   Email: jmedved@juniper.net


   Richard Alimi
   Google

   Email: ralimi@google.com


   Richard Yang
   Yale University

   Email: yry@yale.edu


   Stefano Previdi
   Cisco Systems

   Email: sprevidi@cisco.com























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