CDNI Request Routing Extensions
draft-ietf-cdni-request-routing-extensions-07

Abstract

Open Caching is a use case of Content Delivery Networks Interconnetion (CDNI) in which the commercial Content Delivery Network (CDN) is the upstream CDN (uCDN) and the ISP caching layer serves as the downstream CDN (dCDN). The extensions specified in this document to the CDNI Metadata and FCI interfaces are derived from requirements raised by Open Caching but are also applicable to CDNI use cases in general.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

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 https://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 March 27, 2020.

Copyright Notice

Copyright (c) 2019 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 (https://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.

Table of Contents

• 1. Introduction
• 1.1. Terminology
• 2. Redirect Target Capability
• 2.1. Properties of Redirect Target Capability Object
• 2.2. DnsTarget
• 2.3. HttpTarget
• 2.4. Usage Example
• 3. Fallback Target Address Metadata
• 3.1. Properties of Fallback Target Address Metadata Object
• 3.2. Usage Example
• 4. IANA Considerations
• 4.1. CDNI Payload Types
• 4.1.1. CDNI FCI RedirectTarget Payload Type
• 4.1.2. CDNI MI FallbackTarget Payload Type
• 5. Security Considerations
• 5.1. Confidentiality and Privacy
• 6. Acknowledgements
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Authors' Addresses

1. Introduction

The Open Caching working group of the Streaming Video Alliance (SVA) is focused on the delegation of video delivery requests from commercial CDNs to a caching layer at the Internet Service Provider's (ISP) network. Open Caching is a specific use case of CDNI where the commercial CDN is the upstream CDN (uCDN) and the ISP caching layer is the downstream CDN (dCDN). This document defines and registers CDNI generic metadata object [RFC8006] and CDNI Footprint and Capabilities object [RFC8008] that are required for Open Caching request routing. For consistency with other CDNI documents this document follows the CDNI convention of uCDN (upstream CDN) and dCDN (downstream CDN) to represent the commerical CDN and ISP caching layer respectively.

This document also registers CDNI Payload Types [RFC7736] for the defined objects:

• Redirect Target Capability (for dCDN advertising redirect target address)
• Fallback Target Metadata (for uCDN configuring fallback target address)

1.1. Terminology

The following terms are used throughout this document:

• FQDN - Fully Qualified Domain Name
• CDN - Content Delivery Network

Additionaly, this document reuses the terminology defined in [RFC6707], [RFC7336], [RFC8006], [RFC8007], and [RFC8008]. Specifically, we use the following CDNI acronyms:

• FCI - Footprint and Capability Interface (see [RFC8008])
• MI - Metadata Interface (see [RFC8006])
• uCDN, dCDN - Upstream CDN and Downstream CDN respectively (see [RFC7336])
• RT - Redirection Target. Endpoint for redirection from uCDN to dCDN.
• RR - Request Router. An element responsible for routing user requests.

2. Redirect Target Capability

Iterative request redirection is defined in Section 1.1 of [RFC7336] and elaborated by examples in Sections 3.2 and 3.4 of [RFC7336]. A Redirection Target (RT) is defined in Section 2 of [RFC7975] for Recursive Request Redirection as:

• "The endpoint to which the User Agent is redirected. In CDNI, a RT may point to a number of different components, some examples include a surrogate in the same CDN as the request router, a request router in a dCDN, or a surrogate in a dCDN".

In this document we adopt the same defintion of the RT for the Iterative Request Redirect use case. This use case requires the provisioning of the RT address to be used by the uCDN in order to redirect to the dCDN. RT addresses can vary between different footprints, for example, between different regions, and they may also change over time, for example as a result of network problems. Given this variable and dynamic nature of the redirect target address, it may not be suitable to advertise it during bootstrap. A more dynamic and footprint oriented interface is required. Section 4.3 of [RFC7336] suggests that it could be one of the roles of the FCI [RFC8008]. Following this suggestion we have, therefore, chosen to use the CDNI Footprint and Capabilities interface for redirect target address advertisement.

Use cases

• Footprint: The dCDN may want to have a different target per footprint. Note that a dCDN may spread across multiple geographies. This makes it easier to route client requests to a nearby request router. Though this can be achieved using a single canonical name and Geo DNS, that approach has limitations; for example a client may be using a third party DNS resolver, making it impossible for the redirector to detect where the client is located, or Geo DNS granularity may be too rough for the requirement of the application.
• Scaling: The dCDN may choose to scale its request routing service by deploying more request routers in new locations and advertise them via an updatable interface like the FCI.

The Redirect Target capability object is used to indicate the target address the uCDN should use in order to redirect a client to the dCDN. A target may be attached to a specific uCDN host, a list of uCDN hosts, or used globally for all the hosts of the uCDN.

When a dCDN is attaching the redirect target to a specific uCDN host or a list of uCDN hosts, the dCDN MUST advertise the hosts within the Redirect Target capability object as "redirecting-hosts". In this case, the uCDN can redirect to that dCDN address, only if the User Agent request was to one of these uCDN hosts.

A redirect target for DNS redirection is an IPv4 address used as an A record response, an IPv6 address used as an AAAA record response or a FQDN used as an alias in a CNAME record response (see [RFC1034]) of the uCDN DNS router. Note that DNS routers make routing decisions based on either the DNS resolver's IP address or the client IP address when EDNS0 client-subnet is used (see [RFC7871]). The dCDN may choose to advertise redirect targets and footprints to cover both cases. A uCDN DNS router implemenation SHOULD prefer routing based on client IP address when it is available.

A redirect target for HTTP redirection is the URI to be used as the value for the Location header of a HTTP redirect 3xx response, typically a 302 (Found) (see Section 7.1.2 of [RFC7231] and section 6.4 of [RFC7231]).

If the redirect target capability object does not contain a target or the target is empty, the uCDN MUST interpret it as "no target available for these uCDN hosts for the specified footprint". In case such a target was already advertised in a previous FCI object, the uCDN MUST interperet it as an update that deletes the previous redirect target.

2.1. Properties of Redirect Target Capability Object

The Redirect Target capability object consists of the following properties:

• Property: redirecting-hosts
  • Description: One or more uCDN hosts to which this redirect target is attached. A redirecting host SHOULD be a host that was published in a HostMatch object by the uCDN as defined in Section 4.1.2 of [RFC8006].
  • Type: A list of Endpoint objects (see Section 4.3.3 of [RFC8006])
  • Mandatory-to-Specify: No. If not present, or empty, the redirect target applies to all hosts of the redirecting uCDN.
• Property: dns-target
  • Description: Target address for a DNS A record, AAAA record or CNAME record.
  • Type: DnsTarget object (see Section 2.2)
  • Mandatory-to-Specify: No. If the dns-target is not present or empty the uCDN MUST interpret it as "no dns-target available".
• Property: http-target
  • Description: Target URI for a HTTP redirect.
  • Type: HttpTarget object (see Section 2.3)
  • Mandatory-to-Specify: No. If the http-target is not present or empty the uCDN MUST interpret it as "no http-target available".

The following is an example of a Redirect Target capability object serialization that advertises a dCDN target address that is attached to a specific list of uCDN "redirecting-hosts". A uCDN host that is included in that list can redirect to the advertised dCDN redirect target. The capabilities object is serialized as a JSON object as defined in Section 5 of [RFC8008]

{
  "capabilities": [
    {
      "capability-type": "FCI.RedirectTarget",
      "capability-value": {
          "redirecting-hosts": [
             "a.service123.ucdn.example.com", 
             "b.service123.ucdn.example.com"
          ],
          "dns-target": {
             "host": "service123.ucdn.dcdn.example.com"
          },
          "http-target": {
              "host": "us-east1.dcdn.example.com",
              "path-prefix": "/cache/1/",
              "include-redirecting-host": true          
          }
      },
      "footprints": [
          <Footprint objects>
      ]
    }
  ]
}
         

2.2. DnsTarget

The DnsTarget object gives the target address for the DNS response to delegate from the uCDN to the dCDN.

• Property: host
  • Description: The host property is a hostname or an IP address, without a port number.
  • Type: Endpoint object as defined in Section 4.3.3 of [RFC8006] with the limitation that it SHOULD NOT include a port number and, in case a port number is present, the uCDN MUST ignore it.
  • Mandatory-to-Specify: Yes.

The following is an example of DnsTarget object:

 {
    "host": "service123.ucdn.dcdn.example.com"
 }
         

The following is an example of a DNS query for uCDN address "a.service123.ucdn.example.com" and the corresponding CNAME redirection response:

 Query:
 a.service123.ucdn.example.com: 
 type A, class IN 
 
 Response:
 a.service123.ucdn.example.com: 
 type CNAME, class IN, cname service123.ucdn.dcdn.example.com
         

2.3. HttpTarget

The HttpTarget object gives the necessary information to construct the target Location URI for HTTP redirection.

• Property: host
  • Description: Hostname or IP address and an optional port, i.e., the host and port of the authority component of the URI as described in Section 3.2 of [RFC3986].
  • Type: Endpoint object as defined in Section 4.3.3 of [RFC8006].
  • Mandatory-to-Specify: Yes.
• Property: path-prefix
  • Description: A path prefix for the HTTP redirect Location header. The original path is appended after this prefix.
  • Type: A prefix of a path-absolute as defined in Section 3.3 of [RFC3986]. The prefix MUST end with a trailing slash, to indicate the end of the last path segment in the prefix.
  • Mandatory-to-Specify: No. If this property is absent or empty, the uCDN MUST NOT prepend a path prefix to the original content path, i.e., the original path MUST appear in the location URI right after the authority component.
• Property: include-redirecting-host
  • Description: A flag indicating whether or not to include the redirecting host as the first path segment after the path-prefix. If set to true and a "path-prefix" is used, the uCDN redirecting host MUST be added as a separate path segment after the path-prefix and before the original URL path. If set to true and there is no path-prefix, the uCDN redirecting host MUST be prepended as the first path segment in the redirect URL.
  • Type: Boolean.
  • Mandatory-to-Specify: No. Default value is False.

Example of HttpTarget object with a path-prefix and include-redirecting-host:

{
   "host": "us-east1.dcdn.example.com",
   "path-prefix": "/cache/1/",
   "include-redirecting-host": true
}
         

Example of a HTTP request for content at uCDN host "a.service123.ucdn.example.com" and the corresponding HTTP response with Location header used for redirecting the client to the dCDN using the the http-target in the above example:

 Request:
 GET /vod/1/movie.mp4 HTTP/1.1
 Host: a.service123.ucdn.example.com
 
 Response:
 HTTP/1.1 302 Found
 Location: http://us-east1.dcdn.example.com/cache/1/
 a.service123.ucdn.example.com/vod/1/movie.mp4
         

2.4. Usage Example

Before requests can be routed from the uCDN to the dCDN the CDNs must exchange service configurations between them. Using the MI the uCDN advertises out-of-band its hosts to the dCDN, each host is designated by a host name and has its own specific metadata (see Section 4.1.2 of [RFC8006]. The dCDN, using the FCI, advertises, also out-of-band, the redirect target address object defined in Section 2.1 for the relevant uCDN hosts. The following is a generalized example of the message flow between an upstream CDN and a downstream dCDN. For simplicity, we focus on the sequence of messages between the uCDN and dCDN and not on how they are passed.

  dCDN                                                    uCDN
    +                                                       +
    |                                                       |
(1) | MI:  host: s123.ucdn.example.com                      |
    |      host-metadata: < metadata >                      |
    <-------------------------------------------------------+
    |                                                       |
(2) | FCI:  capability-type: FCI.RedirectTarget             |
    |       redirecting-hosts: us-east1.dcdn.example.com    |
    |       target host: s123.ucdn.example.com              |
    +------------------------------------------------------->
    |                                                       |
    |                                                       |
    +                                                       +

    Figure 1: Redirect target address advertisement
         

1. The uCDN advertises a host (s123.ucdn.example.com) with the host metadata.
2. The dCDN adveritses its FCI objects to the uCDN including a FCI.RedirectTarget object that contains the redirect target address (us-east1.dcdn.example.com) specified for that uCDN host.

Once the redirect target has been set, the uCDN can start redirecting user requests to the dCDN. The following is a generic sequence of redirection using the host and redirect target that were advertised in Figure 1 above.

End User                  dCDN                   uCDN RR
    +                       +                       +
    |                       |                       |
(1) | Request sent s123.ucdn.example.com            |
    +-----------------------+----------------------->
    |                       |                       |
(2) | Redirect to us-east1.dcdn.example.com         |
    <-----------------------+-----------------------+
    |                       |                       |
(3) | Request us-east1.dcdn.example.com             |
    +----------------------->                       |
    |                       |                       |
(4) | Response              |                       |
    <-----------------------+                       |
    |                       |                       |
    +                       +                       +

    Figure 2: Generic requests redirection sequence
         

1. The End User sends a request (DNS or HTTP) to the uCDN Request Router (RR).
2. Using the previously advertised Redirect Target, the uCDN redirects the request to the dCDN.
3. The End User sends a request to the dCDN.
4. The dCDN either sends a response or reroutes it, for example, to a dCDN surrogate.

3. Fallback Target Address Metadata

Open Caching requires that the uCDN provides a fallback target server to the dCDN, to be used in cases where the dCDN cannot properly handle the request. To avoid redirect loops, the fallback target server's address at the uCDN MUST be different from the original uCDN address from which the client was redirected to the dCDN. The uCDN MUST avoid further redirection when receiving the client request at the fallback target. The fallback target is defined as a generic metadata object (see Section 3.2 of [RFC8006])

Use cases

• Failover: A dCDN request router receives a request but has no caches to which it can route the request. This can happen in the case of failures or temporary network overload.
• No coverage: A dCDN request router receives a request from a client located in an area inside the footprint but not covered by the dCDN caches or outside the dCDN footprint coverage. In such cases, the router may choose to redirect the request back to the uCDN fallback address.
• Error: A cache may receive a request that it cannot properly serve, for example, some of the metadata objects for that service were not properly acquired. In this case, the cache may resolve to redirect back to uCDN.

The Fallback target metadata object is used to indicate the target address the dCDN should use in order to redirect a client back to the uCDN. Fallback target is represented as endpoint objects as defined in section 4.3.3 of [RFC8006].

The uCDN fallback target address may be used as a DNS A record, AAAA record or CNAME record in case of DNS redirection or a hostname for HTTP redirect.

When using HTTP redirect to route a client request back to the uCDN, it is the dCDN's responsibility to use the original URL path as the client would have used for the original uCDN request, stripping, if needed, the dCDN path-prefix and/or the uCDN hostname from the redirect URL that may have been used to request the content from the dCDN.

3.1. Properties of Fallback Target Address Metadata Object

The MI.FallbackTarget Metadata object consists of the following single property:

• Property: host
  • Description: Target address to which the dCDN can redirect the client.
  • Type: Endpoint object as defined in Section 4.3.3 of [RFC8006] with the limitation that in case of DNS delegation it SHOULD NOT include a port number and, in case a port number is present, the dCDN MUST ignore it.
  • Mandatory-to-Specify: Yes.

Example of a MI.FallbackTarget Metadata object that designates the host address the dCDN should use as fallback address to redirect back to the uCDN.

{
    "generic-metadata-type": "MI.FallbackTarget",
    "generic-metadata-value":
    {
        "host": "fallback-a.service123.ucdn.example"
    }
}
         

3.2. Usage Example

The uCDN advertises out-of-band the fallback target address to the dCDN, so that the dCDN may redirect a request back to the uCDN in case the dCDN cannot serve it. Using the MI the uCDN advertises its hosts to the dCDN, along with their specific host metadata (see Section 4.1.2 of [RFC8006]. The Fallback Target generic metadata object is encapsulated within the "host-metadata" property of each host. The following is an example of a message flow between an upstream CDN and a downstream dCDN. For simplicity, we focus on the sequence of messages between the uCDN and dCDN, not on how they are passed.

  dCDN                                                    uCDN
    +                                                       +
    |                                                       |
(1) | MI:  host: s123.ucdn.example.com                      |
    |      host-metadata:                                   |
    |          < metadata objects >                         |
    |          < MI.FallbackTarget                          |
    |            host: fallback-a.service123.ucdn.example > |
    |          < metadata objects >                         |
    <-------------------------------------------------------+
    |                                                       |
(2) | FCI:  capability-type: FCI.RedirectTarget             |
    |       redirecting-hosts: us-east1.dcdn.example.com    |
    |       target host: s123.ucdn.example.com              |
    +------------------------------------------------------->
    |                                                       |
    |                                                       |
    +                                                       +

    Figure 3: Advertisement of host metadata with Fallback Target 
         

1. The uCDN advertises a host (s123.ucdn.example.com) with the host metadata. The host-metadata property contains a MI.FallbackTarget object.
2. The dCDN adveritses its FCI objects to the uCDN including a FCI.RedirectTarget object that contains the redirect target address (us-east1.dcdn.example.com) specified for that uCDN host.

The following is a generic sequence of redirection using the configurations that were advertised in Figure 3 above. In this case the dCDN redirects back to the uCDN fallback target address.

End User              dCDN            uCDN fallback          uCDN RR
    +                   +                   +                   +
    |                   |                   |                   |
(1) | Request sent s123.ucdn.example.com    |                   |
    +-------------------+-------------------+------------------->
    |                   |                   |                   |
(2) | Redirect to us-east1.dcdn.example.com |                   |
    <-------------------+-------------------+-------------------+
    |                   |                   |                   |
(3) | Request us-east1.dcdn.example.com     |                   |
    +------------------->                   |                   |
    |                   |                   |                   |
(4) | Redirect back to fallback-a.service123.ucdn.example       |
    <-------------------+                   |                   |
    |                   |                   |                   |
(5) | Request fallback-a.service123.ucdn.example                |
    +--------------------------------------->                   |
    |                   |                   |                   |
(6) | Response          |                   |                   | 
    <-------------------+-------------------+                   |
    |                   |                   |                   |
    +                   +                   +                   +

    Figure 4: Redirection to Fallback Target
         

1. The End User sends a request (DNS or HTTP) to the uCDN Request Router (RR).
2. Using the previously advertised Redirect Target, the uCDN redirects the request to the dCDN.
3. The End User sends a request to the dCDN.
4. The dCDN cannot handled the request and, therefore, redirects it back to the uCDN fallback target address.
5. The End User sends the request to the uCDN fallback target address.
6. The uCDN either sends a response or reroutes it, for example, to a uCDN surrogate.

4. IANA Considerations

4.1. CDNI Payload Types

This document requests the registration of the following CDNI Payload Types under the IANA "CDNI Payload Types" registry defined in [RFC7736]:

[RFC Editor: Please replace RFCthis with the published RFC number for this document.]

4.1.1. CDNI FCI RedirectTarget Payload Type

Purpose: The purpose of this payload type is to distinguish RedirectTarget FCI objects

Interface: FCI

Encoding: see Section 2.1

4.1.2. CDNI MI FallbackTarget Payload Type

Purpose: The purpose of this payload type is to distinguish FallbackTarget MI objects (and any associated capability advertisement)

Interface: MI/FCI

Encoding: see Section 3.1

5. Security Considerations

This specification is in accordance with the CDNI Metadata Interface and the CDNI Request Routing: Footprint and Capabilities Semantics. As such, it is subject to the security and privacy considerations as defined in Section 8 of [RFC8006] and in Section 7 of [RFC8008] respectively.

5.1. Confidentiality and Privacy

The redirect Target FCI object potentially exposes information about the internal strcture of the dCDN network. A third party could intercept the FCI transactions and use the information to attack the dCDN. An implemenation of the FCI MUST therefore use strong authentication and encryption and strictly follow the directions for securing the interface as defined for the Metadata Interface in Section 8.3 of [RFC8006].

6. Acknowledgements

The authors thank Nir B. Sopher for reality checks against production use cases, his contribution is significant to this document. The authors also thank Ben Niven-Jenkins for his review and feedback and Kevin J. Ma for his guidance throughout the development of this document including his regular reviews.

7. References

7.1. Normative References

7.2. Informative References

Authors' Addresses