Request Interface for CDN Interconnection
draft-choi-cdni-req-intf-00

The information below is for an old version of the document
Document Type Active Internet-Draft (individual)
Authors Taesang Choi  , Yeong-il Seo  , Laboratory Kt , Jongmin Lee  , Ja-Ryeong Koo  , John Shinn  , June-koo Rhee
Last updated 2013-07-13
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Networking Working Group                                         T. Choi
Internet-Draft                                                      ETRI
Intended Status: Standards Track                                  Y. Seo
Expires: January 14, 2014                                         D. Kim
                                                                      KT
                                                                  J. Lee
                                                                     SKT
                                                                  J. Koo
                                                                    LGU+
                                                                J. Shinn
                                                             Solbox Inc.
                                                                 J. Rhee
                                                                   KAIST
                                                           July 13, 2013

               Request Interface for CDN Interconnection
                      draft-choi-cdni-req-intf-00

Abstract

   This document specifies the request interface between a CDN end-user
   and an upstream CDN or downstream CDN to request CDN request routing
   redirection or URI signing.  It specifies the CDNI Request Interface
   information elements and the actual protocol for exchanging them.

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
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   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 29, 2012.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

 

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   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
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   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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2. Request Interface Function Overview . . . . . . . . . . . . . .  3
   3. Request Interface Information Elements  . . . . . . . . . . . .  4
   4.  Request Procedures . . . . . . . . . . . . . . . . . . . . . .  5
     4.1.  Iterative Request Routing Redirection with Loop 
           Prevention Procedures  . . . . . . . . . . . . . . . . . .  6
       4.1.1.  HTTP-based Redirection . . . . . . . . . . . . . . . .  6
       4.1.2.  DNS-based Redirection  . . . . . . . . . . . . . . . . 10
     4.2. HTTP-based Redirection with Multiple Location Return  . . . 12
     4.3.  URI Signing  . . . . . . . . . . . . . . . . . . . . . . . 16
   5  Security Considerations . . . . . . . . . . . . . . . . . . . . 16
   6  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 16
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 17
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17

 

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1.  Introduction

   According to the CDNi requirements[I-D.ietf-cdni-requirements] and
   the CDNI Framework [I-D.ietf-cdni-framework], the need for
   interconnected CDNs to be able to support iterative and recursive
   CDNi request routing and implement an access control mechanism that
   enforces the CSP's distribution policy is described.  Recursive CDNi
   request routing is specified by request routing redirection interface
   [I-D.ietf-cdni-redirection].  URI signing is specified in URI signing
   [I-D.leung-cdni-uri-signing]. 

   For generic CDN services, the high availability is a major issue as
   most contents providers want more than 99% availability for their
   services. Response time is also a major issue as it might be the main
   reason why contents providers use CDN services. For CDNi services,
   both high availability and response time are required for the same
   reason. However, due to the limitation of redirection feature on
   HTTP/1.1 which allows only a single location for redirection,
   unnecessary connection cost and point of failures are expected during
   request routing redirection and content acquisition processes.  A new
   mechanism to cope with such a limitation is required to support
   availability for CDNI services.

   Meanwhile, both iterative request routing redirection and URI signing
   require interactions between an end-user and uCDN or dCDN.  This
   implies the need for an interface to support such interactions.

   To meet the needs, this document specifies an interface called
   "Request Interface" which resides between end-users and CDN
   providers.  It describes information elements and transport protocols
   for the interface.

   [Editor's Note: Request Interface is out of scope in the current
   charter.  There are some contradictions between the requirements and
   scope.  According to the requirements, both iterative request routing
   redirection and secure access control need to be supported.  Both
   operations cannot be realized without support of request interface. 
   Thus, scope discussion is necessary in the WG.]

1.1.  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 RFC 2119 [RFC2119].

2. Request Interface Function Overview

   Request interface operates between a user agent and CDN provider
 

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   (either uCDN or dCDN).  Since one end of this interface is user
   application such as web browser, it is desirable not to invent a new
   protocol for this function.  Instead, it uses well-known protocols
   used by the existing CDN applications such as HTTP and DNS.  

   The main function of this interface is to support the iterative
   request routing redirection and the URI signing mechanism.  For the
   iterative request routing redirection, it also supports loop
   prevention mechanism.

   For the high availability of the CDN service, it also supports
   multiple CDN connection choices by returning multiple CDN locations
   to a user agent.  When the first CDN location fails to serve the
   request due to some reason (e.g., the request routing system of that
   CDN fails for some reason), the second CDN location can be tried by a
   user agent until it succeeds.  Detailed semantics on how uCDN selects
   multiple locations, the number of locations, and selection priority,
   etc. need further work.

3. Request Interface Information Elements

   To meet the requirements of request routing redirection, we define
   the term "CDN-Provider-ID".  It uniquely identifies each CDN provider
   during the course of request routing redirection.  It consists of
   "CDN provider name" and "MaxNumRedHops".  A pair of an AS number and
   an additional qualifier is used for CDN provider name.  Since more
   than one CDN providers can belong to the same AS, an additional
   qualifier is used to guarantee the uniqueness.  MaxNumRedHops
   represents a maximum allowed redirections.  The value is decreased
   once every redirection occurs until it reaches 0.  To avoid its usage
   abuse (e.g., end user or CDN operator can set huge number like 100 or
   above), a reasonable upper bound has to be agreed among CDN
   providers.  Security aspect of it is for further study.

   A few examples of the CDN provider names are 100:0 and 200:1.  The
   former means that a CDN provider belong to AS 100 and it is the only
   CDN provider within that AS.  The latter represents the first CDN
   provider in the AS 200.  There are other CDN providers in the same
   AS.

   One example of CDN-Provider-ID is "CDN-Provider-Name=100:0 &
   MaxNumRedHops=10", which means that a CDN provider that belong to AS
   number 100 and it is the only CDN provider and a maximum allowed
   redirection is 10.  An example how a list of CDN-Provider-IDs can be
   carried in the URI query string when a certain cascaded request
   routing redirection occurs is the following.  We assume that
   redirection is cascaded three times: uCDN -> dCDN1 -> dCDN2.  dCDN1,
   then, carries the following URL, "http://cdn.csp.com?uCDN-Provider-
 

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   ID=100:0&dCDN1-Provider-ID=200:1&MaxNumRedHops=9".  Note that
   MaxNumbRedHops carries the latest number instead of adding in every
   CDN-Provider-ID to save the space in URI query string.

   It is applicable for both HTTP-based and DNS redirections.  For HTTP-
   based redirection, we define a HTTP request routing redirection
   header "CDN-Provider-ID".  For each step of redirection, it is
   attached to the beginning of the provider domain URL.  For example,
   uCDN initiates a redirection with its URL,
   http://100:0:10.cdn.csp.com.  dCDN further attaches its own CDN-
   Provider-ID in the front when another level of redirection is
   required.  For DNS-based redirection, the CDN-Provider-ID can be
   attached in the DNS CNAME.

   For URI singing, detailed format of information elements carried over
   this interface is defined in [I-D.ietf-leung-cdni-uri-signing-02] 

   For multiple CDN locations support, we define an extension tag
   called, "M-Location".  As there are still many clients that support
   HTTP/1.1 with single location return, it will take some time for them
   to upgrade to support the multi location return capability. This will
   also guarantee the backward compatibility.

   The following is an example of using location tag  with HTTP/1.1 or
   older versions.

      HTTP/1.1 302
      Location: http://www.example.com/sample.zip

   For backward compatibility, we keep the original location tag and add
   an extension tag. The example is as following.

      HTTP/2.0 302
      Location: http://www.example.com/sample.zip   
      M-Location: http://www.example.com/sample.zip
                  http://www2.example.com/sample.zip
                  http://www3.example.com/sample.zip
      M-Redirection-Count: 0

   In this way, legacy clients can parse the location tag as usual, and
   new clients which support the multiple location tag can parse
   multiple locations return.

4.  Request Procedures

   This section specifies two request procedures: iterative request
   routing redirection and URI singing.  More specifically, the former
   consists of iterative request routing redirection with loop
 

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   prevention and iterative request routing redirection with multiple
   location return.

4.1.  Iterative Request Routing Redirection with Loop Prevention
   Procedures

4.1.1.  HTTP-based Redirection

   In this section, we describe an iterative procedure of HTTP-based
   request routing redirection with loop prevention.

   End-User           dCDNn       dCDNn-1     dCDN1      uCDN           
   |DNS cdn.csp.com    |            |            |          |
   |------------------------------------------------------->|
   |                   |            |            |          |(1)
   |IPaddr of uCDN's Request Router |            |          |
   |<-------------------------------------------------------|
   |HTTP cdn.csp.com   |            |            }          |
   |------------------------------------------------------->|
   |                   |            |            |          |(2)
   |302 peer-uCDN.dCDN1.net/cdn.csp.com?uCDN-Provider-ID    |
   |<-------------------------------------------------------|
   |DNS peer-uCDN.dCDN1.net         |            |          |
   |-------------------------------------------->|          |
   |                   |            |            |(3)       |
   |IPaddr of dCDN1's Request Router|            |          |
   |<--------------------------------------------|          |
   |                   |            |            |          |
   |HTTP peer-uCDN.dCDN1.net/cdn.csp.com?uCDN-Provider-ID   |
   |-------------------------------------------->|          |
   |                   |            |            |(4)       |
   |302 peer-dCDN1.dCDN2.net/cdn.csp.com?dCDN1-Provider-ID  |
   |<--------------------------------------------|          |
   |                   |            |            |          |
   |                   |          .......        |(5)       |
   |                   |            |            |          |
   |302 peer-dCDNn-1.dCDNn.net/cdn.csp.com?dCDNn-1-Provider-ID
   |<-------------------------------|            |          |
   |                   |            |            |          |
   |DNS peer-dCDNn-1.dCDNn.net      |            |          |
   |------------------------------->|            |          |
   |                   |            |(6)         |          |
   |IPaddr of dCDNn's Request Router|            |          |
   |<-------------------------------|            |          |
   |                   |            |            |          |
   |HTTP peer-dCDNn-1.dCDNn.net/cdn.csp.com      |          |
   |------------------------------->|            |          |
 

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   |                   |            |(7)         |          |
   |302 node1.peer-dCDNn-1.dCDNn.net/cdn.csp.com |          |
   |<-------------------------------|            |          |
   |                   |            |            |          |
   |DNS node1.peer-dCDNn-1.dCDNn.net|            |          |
   |------------------>|            |            |          |
   |                   |            |(8)         |          |
   |IPaddr of dCDNn's Delivery Node }            |          |
   |<------------------|            |            |          |
   |                   |            |            |          |
   |HTTP node1.peer-dCDNn-1.dCDNn.net/cdn.csp.com|          |
   |------------------>|            |            |          |
   |                   |(9)         |            |          |
   |                   |DNS dCDNn-acq.dCDNn-1.net|          |
   |                   |----------->|            |          |
   |                   |            |(10)        |          |
   |                   |IPaddr of dCDNn-1's Request Router  |
   |                   |<-----------|            |          |
   |                   |HTTP dCDNn-acq.dCDNn-1.net?dCDNn-Provider-ID
   |                   |----------->|            |          |
   |                   |            |(11)        |          |
   |                   |            |   .......  |          |
   |                   |            |            |          |
   |                   |HTTP dCDN1-acq.uCDN.net?dCDN1-Provider-ID
   |                   |            |            |--------->|
   |                   |            |            |          |(12)
   |                   |            |302 node2.dCDN1.acq.uCDN.net
   |                   |            |            |<---------|
   |                   |            |DNS node2.dCDN1-acq.uCDN.net
   |                   |            |            |--------->|
   |                   |            |            |          |(13)
   |                   |            |IPaddr of uCDN's Delivery Node
   |                   |            |            |<---------|
   |                   |            |            |          |(14)
   |                   |            |            |   Data   |
   |                   |            |            |<---------|
   |                   |            |   .......  |          |
   |Data               |            |            |          |
   |<------------------|            |            |          |

   Figure 1: HTTP-based request routing redirection iterative procedure 

   The steps illustrated in the figure are as follows:

   1.   A DNS resolver for uCDN provider processes the DNS request for
        its customer based on CDN-domain cdn.csp.com.  It returns the IP
        address of a request router in uCDN provider.

 

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   2.   A Request Router for uCDN provider processes the HTTP request
        and recognizes that the end-user is best served by dCDN1. So it
        returns a 302 redirect message for a new URL constructed by
        "stacking" dCDN1's distinguished CDN-domain (peer-
        uCDN.dCDN1.net) on the front of the original URL.  It also adds
        uCDN's CDN-Provider-ID in the URI query string of the HTTP
        request message. (e.g., uCDN-Provider-ID=100:0 &
        MaxNumRedHops=10).  This information is not processed by the
        customer but conveyed in the HTTP message without any
        modification of the step 4.  The details on how it is used for
        loop prevention is described in the step 4.

   3.   The end-user does a DNS lookup using dCDN1's distinguished
        CDN-domain (peer-uCDN.dCDN1.net).  dCDN1's DNS resolver returns
        the IP address of a request router for dCDN1.

   4.   The request router for dCDN1 processes the HTTP request.  There
        are two options: redirect further to another dCDN (i.e.,
        cascading the request) or process it by itself.  In either
        cases, it performs loop prevention step first.  It checks a list
        of CDN-provider-IDs in the URI query string: it contains a list
        of CDN providers which requested redirections so far.  If either
        it contains own CDN provider name or MaxNumRedHops becomes 0, it
        means that the redirection loop has occurred or the number of
        redirection hops has reached the maximum.  Once loop is
        detected, details on the next steps is described in the section
        3.  If it is loop free, it either redirects further or processes
        based on the local policy.  For the former, it selects another
        dCDN provider and and sends an HTTP redirect message with its
        own CDN-Provider-ID included in its URI query string (e.g.,
        uCDN-Provider-ID=100:0 & dCDN1-Provider-ID=200:1 &
        MaxNumRedHops=9) attached.   For the latter, it selects a
        suitable delivery node to serve the end-user request, and
        returns a 302 redirect message for a new URL constructed by
        replacing the hostname by a subdomain of the dCDN1's
        distinguished CDN-domain that points to the selected delivery
        node.  Then it goes to the step 6.

   5.   If further redirection is decided, it repeats steps 2 - 4 until
        it either selects dCDN provider to serve the request or
        MaxNumRedHops expires.  If the former occurs, it resumes the
        step 6.  If the latter occurs, it follows the processes
        described in the section 3.

   6.   Assuming that dCDNn is selected as a serving dCDN provider, the
        end-user does a DNS lookup using dCDNn's distinguished
        CDN-domain (peer-dCDNn-1.dCDNn.net).  dCDNn-1's DNS resolver
        returns the IP address of a request router for dCDNn.
 

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   7.   The request router for dCDN1 processes the HTTP request and
        selects a suitable delivery node to serve the end-user request,
        and returns a 302 redirect message for a new URL constructed by
        replacing the hostname by a subdomain of the dCDNn's
        distinguished CDN-domain that points to the selected delivery
        node.

   8.   The end-user does a DNS lookup using dCDNn's delivery node
        subdomain (node1.peer-dCDNn-1.dCDNn.net).  dCDNn's DNS resolver
        returns the IP address of the delivery node.

   9.   The end-user requests the content from dCDNn's delivery node. 
        In the case of a cache hit, steps 10 ~ 14 below do not happen,
        and the content data is directly returned by the delivery node
        to the end-user.  In the case of a cache miss, the content needs
        to be acquired by dCDN from either parent dCDN or uCDN (not the
        CSP). The distinguished CDN-domain peer-dCDNn-1.dCDNn.net
        indicates that this content is to be acquired from dCDNn-1;
        stripping the CDN-domain reveals the original CDN-domain
        cdn.csp.com and dCDNn may verify that this CDN-domain belongs to
        a known peer (so as to avoid being tricked into serving as an
        open proxy).  It then does a DNS request for an inter-CDN
        acquisition CDN-domain as agreed above (in this case, dCDNn-
        acq.dCDNn-1.net).  This process repeats recursively until it
        finds a CDN provider that can serve the requested content.

   10.   dCDNn-1's DNS resolver processes the DNS request and returns
        the IP address of a request router in dCDNn-1.

   11.   The request router for dCDNn-1 processes the HTTP request from
        dCDNn's delivery node.  dCDNn-1 request router recognizes that
        the request is from a peer CDN rather than an end-user because
        of the dedicated inter-CDN acquisition domain
        (dCDNn-acq.dCDNn-1.net).  It also performs loop prevention
        process as described in step 4 based on the provided CDN-
        Provider-ID (e.g., uCDN-Provider-ID=100:0 & dCDN1-Provider-
        ID=200:1 & ... & dCDNn-Provider-ID=1000:0 & MaxNumRedHops=1). 
        Depending on the number of levels of redirection and
        availability of contents, the same process repeats until either
        content serving CDN provider is found or MaxNumRedHps expires.  

   12.  Assuming that all intermediate dCDNs also have a cache miss, 
        The request router for uCDN selects a suitable delivery node to
        serve the inter-CDN acquisition request and returns a 302
        redirect message for a new URL constructed by replacing the
        hostname by a subdomain of the uCDN's distinguished inter-CDN
        acquisition domain that points to the selected delivery node.

 

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   13.   uCDN DNS resolver processes the DNS request and returns the IP
        address of the delivery node in uCDN.

   14.  uCDN serves content for the requested CDN-domain to dCDN and
        finally to end-user. Although not shown, it is at this point
        that uCDN processes the rest of the URL: it extracts information
        identifying the origin server, validates that this server has
        been registered, and determines the content provider that owns
        the origin server.  It may also perform its own content
        acquisition steps if needed before returning the content to
        dCDN.

4.1.2.  DNS-based Redirection

   In this section, we describe an iterative procedure of DNS-based
   request routing redirection with loop prevention.

 

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   End-User           dCDNn       dCDNn-1     dCDN1      uCDN           
   |DNS cdn.csp.com    |            |            |          |
   |------------------------------------------------------->|
   |                   |            |            |          |
   |CNAME uCDNProviderID.dCDN1.cdn.csp.com       |          |(1)
   |NS records for dCDN1.cdn.csp.com|            |          |
   |<-------------------------------------------------------|
   |DNS dCDN1.cdn.csp.com           |            |          |
   |-------------------------------------------->|          |
   |                   |            |            |          |
   |                   |          .......        |(2)       |
   |                   |            |            |          |
   |CNAME dCDN1.cdn.csp.com         |            |          |
   |NS records for dCDN1.cdn.csp.com|            |          |
   |<-------------------------------|            |          |  
   |DNS dCDNn.cdn.csp.com           |            |          |
   |------------------>|            |            |          |
   |                   |            |(3)         |          |
   |IPaddr of dCDNn's Delivery Node }            |          |
   |<------------------|            |            |          |
   |                   |            |            |          |
   |HTTP cdn.csp.com   |            |            |          |
   |------------------>|            |            |          |
   |                   |(4)         |            |          |
   |                   |DNS dCDNn-acq.dCDNn-1.net|          |
   |                   |----------->|            |          |
   |                   |            |            |          |
   |                   |            |   .......  |(5)       |
   |                   |            |            |          |
   |                   |DNS dCDN1 ProviderID.dCDN1-acq.uCDN.net
   |                   |            |            |--------->|
   |                   |            |            |          |(6)
   |                   |            |IPaddr of uCDN's Delivery Node
   |                   |            |            |<---------|
   |                   |            |            |          |(7)
   |                   |            |            |   Data   |
   |                   |            |            |<---------|
   |                   |            |   .......  |          |
   |Data               |            |            |          |
   |<------------------|            |            |          |

   Figure 2: DNS-based request routing redirection iterative procedure 

   The steps illustrated in the figure are as follows:

   1.  Request Router for uCDN provider processes the DNS request for
       CDN- domain cdn.csp.com and recognizes that the end-user is best
 

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       served by another CDN.  (This may depend on the IP address of the
       user's local DNS resolver, or other information discussed below.)
       The Request Router returns a DNS CNAME response by "stacking" the
       distinguished identifier for dCDN1 and uCDN's CDN-Provider-ID
       (e.g., 100:0.10) onto the original CDN-domain (e.g.,
       dCDN1.cdn.csp.com), plus an NS record that maps dCDN1.cdn.csp.com
       to dCDN1's Request Router. 

   2.  The end-user does a DNS lookup using the modified CDN-domain
       (i.e., dCDN1.cdn.csp.com).  dCDN1 Request Router processes the
       request and decides to serve the request or redirect further to
       another CDN provider.  It also checks redirection loop.  This
       process iterates until either serving dCDN is selected or
       MaxNumRedHops expires.  In this case, dCDNn is selected as a
       serving dCDN.  If the former occurs, it proceeds to step 3.  If
       the latter occurs, it follows the processes described in the
       section 3.

   3.  The end-user does a DNS lookup using the modified CDN-domain
       (i.e., dCDN1.cdn.csp.com).  This causes dCDNn's request router
       returns an IP address of a suitable delivery node.

   4.   The end-user requests the content from dCDNn's delivery node. 
       In the case of a cache hit, steps 5 ~ 7 do not happen, and the
       content data is directly returned by the delivery node to the
       end-user.  In the case of a cache miss, the content needs to be
       acquired by dCDNn from either parent dCDN or uCDN (not the CSP).
       It also performs loop prevention process as described in the step
       2 based on the provided CDN-Provider-ID (e.g.,
       100:0.200:1.....900:0.1)  

   5. Depending on the number of levels of redirection and availability
       of contents, the same process repeats until either content
       serving CDN provider is found or MaxNumRedHps expires.  

   6.  Assuming that all intermediate dCDNs also miss cache, uCDN is
       selected as a content delivery CDN provider.  Thus, the request
       router for uCDN selects a suitable delivery node to serve the
       inter-CDN acquisition request and returns IP address of the
       suitable uCDN delivery node.

   7.  uCDN serves content to dCDN1 and further down to end-user. 
       Although not shown, it is at this point that uCDN processes the
       rest of the URL: it extracts information identifying the origin
       server, validates that this server has been registered, and
       determines the content provider that owns the origin server.

4.2. HTTP-based Redirection with Multiple Location Return
 

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   The following figure describes the situation that if HTTP supports
   the return of multiple locations.

 

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      End-User           dCDNn       dCDNn-1     dCDN1      uCDN           
      |DNS cdn.csp.com    |            |            |          |
      |------------------------------------------------------->|
      |                   |            |            |          |(1)
      |IPaddr of uCDN's Request Router |            |          |
      |<-------------------------------------------------------|
      |HTTP cdn.csp.com   |            |            |          |
      |------------------------------------------------------->|
      |                   |            |            |          |(2)
      |302 peer-uCDN.dCDN1.net/cdn.csp.com?uCDN-Provider-ID,   |
      |    peer-uCDN.dCDN2.net/cdn.csp.com?uCDN-Provider-ID,   |
      |    peer-uCDN.dCDNN.net/cdn.csp.com?uCDN-Provider-ID,   |
      |    peer-uCDN.uCDN.net/cdn.csp.com?uCDN-Provider-ID     |
      |<-------------------------------------------------------|
      |DNS peer-uCDN.dCDN1.net         |            |          |
      |-------------------------------------------->|          |
      |                   |            |            |(3)       |
      |IPaddr of dCDN1's Request Router|            |          |
      |<--------------------------------------------|          |
      |                   |            |            |          |
      |HTTP peer-uCDN.dCDN1.net/cdn.csp.com?uCDN-Provider-ID   |
      |-------------------------------------------->|          |
      |  Connection fail  |            |            |(4)       |
      |<--------------------------------------------|          |
      |DNS peer-uCDN.dCDN2.net         |            |          |
      |------------------------------->|            |          |
      |                   |            |            |(3)       |
      |IPaddr of dCDN1's Request Router|            |          |
      |<--------------------------------------------|          |
      |                   |            |            |          |
      |HTTP peer-uCDN.dCDN2.net/cdn.csp.com?uCDN-Provider-ID   |
      |-------------------------------------------->|          |
      |  Connection fail  |            |            |(4)       |
      |<--------------------------------------------|          |
      |                   |            |            |          |
      |                   |          .......        |(5)       |
      |<-------------------------------|            |          |
      |                   |            |            |          |
      |DNS peer-dCDNn-1.dCDNn.net      |            |          |
      |------------------------------->|            |          |
      |                   |            |(6)         |          |
      |IPaddr of dCDNn's Request Router|            |          |
      |<-------------------------------|            |          |
      |                   |            |            |          |
      |HTTP peer-dCDNn-1.dCDNn.net/cdn.csp.com      |          |
      |------------------------------->|            |          |
      |                   |            |(7)         |          |
      |302 node1.peer-dCDNn-1.dCDNn.net/cdn.csp.com |          |
 

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      |<-------------------------------|            |          |
      |                   |            |            |          |
      |DNS node1.peer-dCDNn-1.dCDNn.net|            |          |
      |------------------>|            |            |          |
      |                   |            |(8)         |          |
      |IPaddr of dCDNn's Delivery Node }            |          |
      |<------------------|            |            |          |
      |                   |            |            |          |
      |HTTP node1.peer-dCDNn-1.dCDNn.net/cdn.csp.com|          |
      |------------------>|            |            |          |
      |                   |(9)         |            |          |
      |                   |DNS dCDNn-acq.dCDNn-1.net|          |
      |                   |----------->|            |          |
      |                   |            |(10)        |          |
      |                   |IPaddr of dCDNn-1's Request Router  |
      |                   |<-----------|            |          |
      |                   |HTTP dCDNn-acq.dCDNn-1.net?dCDNn-Provider-ID
      |                   |----------->|            |          |
      |                   |            |(11)        |          |
      |                   |            |   .......  |          |
      |                   |            |            |          |
      |                   |HTTP dCDN1-acq.uCDN.net?dCDN1-Provider-ID
      |                   |            |            |--------->|
      |                   |            |            |          |(12)
      |                   |            |302 node2.dCDN1.acq.uCDN.net,
      |                   |            |    node3.dCDN1.acq.uCDN.net,
      |                   |            |    origin.csp.com     |
      |                   |            |            |<---------|
      |                   |            |DNS node2.dCDN1-acq.uCDN.net
      |                   |            |            |--------->|
      |                   |            |            |          |(13)
      |                   |            |IPaddr of uCDN's Delivery Node2
      |                   |            |            |<---------|
      |                   |            |            |          |(14)
      |                   |            |            |   Connection fail
      |                   |            |            |<---------|
      |                   |            |DNS node3.dCDN1-acq.uCDN.net
      |                   |            |            |--------->|
      |                   |            |            |          |(13)
      |                   |            |IPaddr of uCDN's Delivery Node3
      |                   |            |            |<---------|
      |                   |            |            |          |(14)
      |                   |            |            |   Connection fail
      |                   |            |            |<---------|
      |                   |            |DNS origin.csp.com
      |                   |            |            |--------->|
      |                   |            |            |          |(13)
      |                   |            |IPaddr of csp's origin server
 

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      |                   |            |            |<---------|
      |                   |            |            |          |(14)
      |                   |            |            |   Data
      |                   |            |            |<---------|
      |                   |            |   .......  |          |
      |Data               |            |            |          |
      |<------------------|            |            |          |

   Figure 3: HTTP-based request routing redirection iterative procedure
   with return of multiple locations

   The major difference of this figure from Figure 1 is that multiple
   locations are returned when a request routing redirect occurs via
   HTTP.

   On step 2, uCDN provides multiple URLs of dCDN's request routers, so
   that End-User could try to connect one after another. uCDN might
   check the availability of each dCDN occasionally to eliminate
   unavailable dCDN from the multiple location list. However, any of
   dCDN could become unavailable during or right after the redirection
   response. In that case, End-User still can try another dCDN from the
   list.

   If all dCDNs become unavailable, uCDN may want to take over the
   dCDN's role. For this, uCDN's URL at the end of the list as seen on
   step 2 is inserted.  In this way, uCDN can manage the service
   availability actively. 

   It can also be applicable to the recursive request routing
   redirection and redirection during contents acquisition as shown in
   step 12.  However, it is out of scope of this document.

4.3.  URI Signing

   The detailed URI singing procedure is defined in [I-D.ietf-leung-
   cdni-uri-signing-02].  Request interface is responsible for all the
   steps related with the interactions between a user agent and uCDN or
   dCDN.

5  Security Considerations

   TBD.

6  IANA Considerations

   TBD.

 

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7.  References

7.1.  Normative References

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

   [I-D.ietf-cdni-framework] Peterson, L. and B. Davie, "Framework for
              CDN Interconnection", February 2013.

   [I-D.ietf-cdni-requirements] Leung, K. and Y. Lee, "Content
              Distribution Network Interconnection (CDNI) Requirements",
               July 2013.

   [I-D.ietf-cdni-redirection] Wang, H. and Niven-Jenkins, B., "Request
              Routing Redirection Interface for Content Distribution
              Network Interconnection",  April 2013.

   [I-D.leung-cdni-uri-signing] Leung, K. and Le Faucheur, F., "URI
              Signing for Content Distribution Network Interconnection",
               May 2013.

7.2.  Informative References

   TBD.

Authors' Addresses

   Taesang Choi
   ETRI
   161 Gajong-Dong
   Yusong-Gu, Daejeon
   Republic of Korea

   Phone: +82-42-860-5628
   Email: choits@etri.re.kr

   Young-IL Seo
   KT Network R&D Laboratory
   463-1, Jeonmoin-dong,
   Yuseong-gu, Daejeon
   Republic of Korea

   Phone: 82-10-3235-0135
   Email: yohan.seo@kt.com
 

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   Dong-Ju Kim
   KT Network R&D Laboratory
   463-1, Jeonmoin-dong,
   Yuseong-gu, Daejeon
   Republic of Korea

   Phone: 82-10-2686-9605
   Email: dj.kim@kt.com

   Jongmin Lee
   SK Telecom
   11, Euljiro-2ga
   Jung-gu, Seoul
   Republic of Korea

   Phone: 82-10-9429-6260
   Email: jminlee@sk.com

   Ja-Ryeong Koo
   LG U plus Corporation
   Namdaemunro 5-ga
   Jung-gu, Seoul
   Republic of Korea

   Phone: 82-10-8080-6115
   Email: wjbkoo@lguplus.co.kr

   John Dongho Shinn
   Solbox Inc.
   7F, Haesung Bldg. 747-2 Yeoksam-Dong
   Kangnam-Gu, Seoul
   Republic of Korea

   Phone: +82-10-3005-4785
   Email: eastsky@solbox.com

   June-koo Rhee
   KAIST
   335 Gahak-Ro
   Yusong-Gu, Daejeon
   Republic of Korea

   Phone: +82-10-6788-2814
   Email: rhee.jk@kaist.ac.kr

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