Internet-Draft CDNI Cache Control Metadata July 2023
Power & Goldstein Expires 11 January 2024 [Page]
Workgroup:
Network Working Group
Internet-Draft:
draft-power-cdni-cache-control-metadata-01
Published:
Intended Status:
Standards Track
Expires:
Authors:
W. Power
Lumen Technologies
G. Goldstein
Lumen Technologies

CDNI Cache Control Metadata

Abstract

This specification adds to the basic cache control metadata defined in RFC8006, providing content providers and upstream CDNs (uCDNs) more fine-grained control over downstream CDN (dCDN) caching. Use cases include overriding or adjusting cache control headers from the origin, bypassing caching altogether, or altering cache keys with dynamically generated values.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on 11 January 2024.

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Table of Contents

1. Introduction

In addition to the cache control parameters currently specified by the Cache object in [RFC8006], content providers and uCDNs often need more fine-grained control over dCDN caching, including scenarios where it is desirable to override or adjust cache control headers from the origin.

The following capabilities are required for commercial CDN and Open Caching use cases:

  1. Positive Cache Control - Allows the uCDN to specify internal caching policies for the dCDN and external caching policies advertised to clients of the dCDN, overriding any cache control policy set in the response from the uCDN.
  2. Negative Cache Control - Allows the specification of caching policies based on error response codes received from the origin, allowing for fine-grained control of the downstream caching of error responses. For example, it may be desirable to cache error responses at the dCDN for a short period of time to prevent an overwhelmed origin service or uCDN from being flooded with requests.
  3. Cache Bypass Control - Allows content providers to bypass CDN caching when needed (typically for testing or performance benchmarking purposes).
  4. Stale Content Policies - Allows control over how the dCDN should process requests for stale content. For example, this policy allows the content provider to specify that stale content be served from cache for a specified time period while refreshes from the origin occur asynchronously.
  5. Dynamically Constructed Cache Keys - It is typical in advanced CDN configurations to generate cache keys that are dynamically constructed via lightweight processing of various properties of the Hypertext Transfer Protocol (HTTP) request and/or response. As an example, an origin may specify a cache key as a value returned in a specific HTTP response header. The Metadata Expression Language (MEL) is used to allow for such advanced cache key construction.

2. Requirements

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

3. MI.CachePolicy

MI.CachePolicy is a new GenericMetadata object that allows the uCDN to specify internal caching policies for the dCDN, as well as external caching policies advertised to clients of the dCDN (overriding any cache control policy set in the response from the uCDN).

In example 1, an MI.CachePolicy sets the internal cache control policy to five seconds. The external cache policy is set to 'no-cache' and both policies are forced:

{
  "generic-metadata-type": "MI.CachePolicy",
  "generic-metadata-value": {
    "internal": "5",
    "external": "no-cache",
    "force-internal": true,
    "force-external": true
  }
}

In example 2, an MI.CachePolicy sets the internal cache control policy to "as-is" (keep the policy set in the response from the uCDN). The external cache policy is set to 'no-cache' and forced:

{
  "generic-metadata-type": "MI.CachePolicy",
  "generic-metadata-value": {
    "internal": "as-is",
    "external": "no-cache",
    "force-external": true
  }
}

In example 3, cache policies are set in the context of the Processing Stages Model (see the Processing Stages Metadata Specification [SVTA2032]). If the HTTP status code received from the origin is a 200, cache expiration is set to 300 seconds if no caching directives were set from the uCDN (unforced). If the HTTP status code received from the origin is a 503 or 504, the internal CDN caching policy is forced to 5 seconds and the external downstream policy is forced to “no-cache”.

{
  "generic-metadata-type": "MI.OriginResponseStage",
  "generic-metadata-value": {
    "match-groups": [
      {
        "if-rule": {
          "match": {
            "expression": "resp.status == 200"
          },
          "stage-metadata": {
            "generic-metadata": [
              {
                "generic-metadata-type": "MI.CachePolicy",
                "generic-metadata-value": {
                  "internal": "300",
                  "external": "300"
                }
              }
            ]
          }
        },
        "else-if-rules": [
          {
           "match": {
             "expression": "resp.status == 503 or resp.status == 504"
           },
           "stage-metadata": {
             "generic-metadata": [
                {
                  "generic-metadata-type": "MI.CachePolicy",
                  "generic-metadata-value": {
                    "internal": "5",
                    "external": "no-cache",
                    "force-internal": true,
                    "force-external": true
                  }
                }
              ]
            }
          }
        ]
      }
    ]
  }
}

4. MI.NegativeCachePolicy

MI.NegativeCachePolicy is a new GenericMetadata object that allows the specification of caching policies based on response codes received from the origin. MI.NegativeCachePolicy is a simple alternative to using the origin response processing stage [SVTA2032] with a match criteria on specific HTTP response codes, useful when there a single caching policy needs to be specified for a list of one or more response codes from the origin.

In the following example, a MI.NegativeCachePolicy object applies a no-cache policy whenever error codes 503 or 504 are seen from the origin..

{
  "generic-metadata-type": "MI.NegativeCachePolicy",
  "generic-metadata-value": {
    "error-codes": [ "503", "504" ],
    "cache-policy": {
      "internal": "5",
      "external": "no-cache",
      "force-internal": true,
      "force-external": true
    }
  }
}

5. MI.StaleContentCachePolicy

MI.StaleContentCachePolicy is a new GenericMetadata object that allows the uCDN to specify the policy to use by a dCDN when responding with stale content. For example, this policy allows the content provider to specify that stale content be served from cache for a specified time period while refreshes from the origin occur asynchronously.

In example 1, an MI.StaleContentCachePolicy where stale-while-revalidating is true instructs the dCDN to respond with a stale cached version of the resource while it refreshes the resource with the uCDN in the background:

{
  "generic-metadata-type": "MI.StaleContentCachePolicy",
  "generic-metadata-value": {
    "stale-while-revalidating": true
  }
}

In example 2, an MI.StaleContentCachePolicy where stale-if-error instructs the dCDN to use the stale cached resource if it receives an error of type 503 or 504 when trying to refresh the resource with the uCDN.

failed-refresh-ttl instructs the dCDN to use a five-second cache time-to-live (TTL) on the resource that receives an error when refreshing from the uCDN. That is, after five seconds, the dCDN will attempt to refresh the resource with the uCDN.

{
  "generic-metadata-type": "MI.StaleContentCachePolicy",
  "generic-metadata-value": {
    "stale-if-error": [ "503", "504" ],
    "failed-refresh-ttl": 5
  }
}

In example 3, an MI.StaleContentCachePolicy where stale-while-revalidating is true instructs the dCDN to respond with a stale cached version of the resource while it refreshes the resource with the uCDN in the background.

stale-if-error instructs the dCDN to use the stale cached resource if it receives an error of type 404 or any 5xx status when trying to refresh the resource with the uCDN.

  1. stale-if-error instructs the dCDN to use the stale cached resource if it receives an error of type 404 or any 5xx status when trying to refresh the resource with the uCDN.
  2. failed-refresh-ttl instructs the dCDN to use a five-second cache TTL on the resource that receives an error when refreshing from the uCDN. That is, after five seconds, the dCDN will attempt to refresh the resource with the uCDN. 
{
  "generic-metadata-type": "MI.StaleContentCachePolicy",
  "generic-metadata-value": {
    "stale-while-revalidating": true,
      "stale-if-error": [ "404", "5xx" ],
      "failed-refresh-ttl": 5
  }
}

6. MI.CacheBypassPolicy

MI.CacheBypassPolicy is a new GenericMetadata object that allows a client request to be set as non-cacheable. It is expected that this feature will be used to allow clients to bypass caching when testing the uCDN fill path. Note: MI.CacheBypassPolicy is typically used in conjunction with a path match or match expression on a header value or query parameter. Any content previously cached (by client requests that do not set MI.CacheBypassPolicy) MUST NOT be evicted.

In the following example, an MI.CacheBypassPolicy is invoked when the clientrequest HTTP header of “cdn-bypass” is true:

{
  "generic-metadata-type": "MI.ProcessingStages",
  "generic-metadata-value": {
    "client-request": [
      {
        "match": {
          "expression": "req.h.cdn-bypass == 'true'"
        },
        "stage-metadata": {
          "generic-metadata": [
            {
              "generic-metadata-type": "MI.CacheBypassPolicy",
              "generic-metadata-value": {
                "bypass-cache": true
              }
            }
          ]
        }
      }
    ]
  }
}

7. MI.ComputedCacheKey

It is typical in advanced CDN configurations to generate cache keys that are dynamically constructed via lightweight processing of various properties of the HTTP request and/or response. As an example, an origin might specify a cache key as a value returned in a specific HTTP response header.

MI.ComputedCacheKey is a new GenericMetadata object that allows the specification of a cache key using the Metadata Expression Language (MEL) defined in [SVTA2031]. Typical use cases would involve constructing a cache key from one or more elements of the HTTP request. In cases where both the MI.ComputedCacheKey and the Cache object are applied, the MI.ComputedCacheKey MUST take precedence.

MI.ComputedCacheKey is, by default, allowed within any of the processing stages defined in [SVTA2032]. It should be noted, however, that a dCDN MAY only allow cache keys to be altered at certain processing stages (such as the clientRequestStage) but not at other stages (such as the originResponse or clientResponseStage). A dCDN MAY use FCI.MetadataExtended [SVTA2041] to advertise such restricted usage contexts.

In the following example, a custom request header is used as the cache key instead of the Uniform Resource Identifier (URI) path:

{
  "generic-metadata-type": "MI.ComputedCacheKey",
  "generic-metadata-value": {
    "expression": "req.h.X-Cache-Key"
  }
}

8. Conclusion

This specification defines a new set of cache control metadata objects that meet the needs of content providers, CDNs, and Open Caching Systems. As the standard matures and gains wider adoption, it is expected that additions to this set of cache control policies will be required.

9. Security Considerations

The FCI and MI objects defined in the present document are transferred via the interfaces defined in CDNI [RFC8006]. [RFC8006] describes how to secure these interfaces, protecting the integrity, confidentiality and ensuring the authenticity of the dCDN and uCDN. The security provide by [RFC8006] should therefore address the above security concerns.

10. IANA Considerations

11. Acknowledgements

The authors would like to express their gratitude to the members of the Streaming Video Technology Alliance [SVTA] Open Caching Working Group for their guidance / contribution / reviews ...)

Particulary the following people contribute in one or other way to the content of this draft:

12. References

12.1. Normative References

[RFC9110]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, , <https://www.rfc-editor.org/info/rfc9110>.
[RFC8006]
Niven-Jenkins, B., Murray, R., Caulfield, M., and K. Ma, "Content Delivery Network Interconnection (CDNI) Metadata", RFC 8006, DOI 10.17487/RFC8006, , <https://www.rfc-editor.org/info/rfc8006>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[STD98]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Caching", STD 98, RFC 9111, .

12.2. Informative References

[SVTA]
"Streaming Video Technology Alliance Home Page", <https://www.svta.org>.
[SVTA2032]
Goldstein, G., Ed., Chaudhari, P., Power, W., Gressel, Y., and A. Warshavsky, "Processing Stages Metadata Specification", , <https://svta.org/documents/SVTA2032>.
[SVTA2031]
Goldstein, G., Ed., Chaudhari, P., Power, W., Gressel, Y., and A. Warshavsky, "Metadata Model Expression Language (MEL) Specification", , <https://svta.org/documents/SVTA2031>.
[SVTA2041]
Goldstein, G., Ed., Chaudhari, P., Power, W., Gressel, Y., and A. Warshavsky, "Metadata Capabilities", <https://svta.org/documents/SVTA2041>.

Authors' Addresses

Will Power
Lumen Technologies
United States of America
Glenn Goldstein
Lumen Technologies
United States of America