CDNI Metadata Model Extensions

2.1.1. MI.CachePolicy

CachePolicy is a new GenericMetadata object that allows for 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).¶

• Property: internal¶

  • Description: Specifies the internal cache control policy to be used by the dCDN.¶
  • Type: Number in seconds encoded as string (e.g. 5 is a five second cache ) and/or a list of Enumeration [as-is|no-cache|no-store]¶
  • Mandatory-to-Specify: No. The default is to use the cache control policy specified in the response from the uCDN.¶

• Property: external¶

  • Description: Specifies the external cache control policy to be used by clients of this dCDN.¶
  • Type: Number in seconds encoded as string (e.g. 5 is a five second cache ) and/or a list of Enumeration [as-is|no-cache|no-store]¶
  • Mandatory-to-Specify: No. The default is to use the cache control policy specified in the response from the uCDN.¶

• Property: force¶

  • Description: If set to True, the metadata interface cache policy defined in the MI.CachePolicy will override any cache control policy set in the response from the uCDN. If set to False, the MI.CachePolicy is only used if there is no cache control policy provided in the response from the uCDN.¶
  • Type: Boolean¶
  • Mandatory-to-Specify: No. The default is "False", which will apply the MI.CachePolicy only if no policy is provided in the response from the uCDN.¶

Example 1: An MI.CachePolicy that sets the internal cache control policy to five seconds. The external cache policy is set to 'no-cache':¶

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

Example 2: An MI.CachePolicy that 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:¶

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

Example 3: An MI.CachePolicy in the context of the processing stages model that sets a caching policy only if the HTTP status code received from the origin is a 200. In this example, the internal cache control policy is set to five seconds. The external cache policy is set to 'no-cache'. Force is set to 'False', indicating that the MI.CachePolicy only applies if there is no cache policy in the response from the uCDN.¶

{
  "generic-metadata-type": "MI.ProcessingStages",
  "generic-metadata-value": {
    "origin-response": [
      {
        "match": {
          "expression": "resp.status == 200"
      },
      "stage-metadata": {
          "generic-metadata": [
            {
              "generic-metadata-type": "MI.CachePolicy",
              "generic-metadata-value": {
                "internal": "5",
                "external": "no-cache",
                "force": "false"
              }
            }
          ]
        }
      }
    ]
  }
}

2.1.2. MI.NegativeCachePolicy

NegativeCachePolicy is a new GenericMetadata object that allows for the specification of caching policies based on error response codes received from the origin.¶

• Property: error-codes¶

  • Description: Array of HTTP response error status codes (See Sections 6.5 and 6.6 of [RFC7231] , that if returned from the uCDN, will be cached using the cache policy defined by the cache-policy property.¶
  • Type: Array of HTTP response error status codes¶
  • Values: ["404", "503", "504"]¶
  • Mandatory-to-Specify: No. The default is to revert to [RFC8006] behavior.¶

• Property: cache-policy¶

  • Description: MI.CachePolicy to apply to the HTTP response error status codes returned by the uCDN.¶
  • Mandatory-to-Specify: Yes¶

Example: A MI.NegativeCachePolicy that applies to HTTP error codes: "404", "503", "504" and sets the internal cache control policy to five seconds and external to 'no-cache'.¶

{
  "generic-metadata-type": "MI.NegativeCachePolicy",
  "generic-metadata-value": {
    "error-codes": [ "404", "503", "504" ],
    "cache-policy": {
      "internal": "5",
      "external": "no-cache",
      "force": "true"
    }
  }
}

2.1.3. 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.¶

• Property: stale-while-revalidating¶

  • Description: Instructs the dCDN to serve a stale version of a resource while refreshing the resource with the uCDN. When set to "True", the dCDN will return a previously cached version of a resource while the resource is refreshed with the uCDN in the background.¶
  • Type: Boolean¶
  • Mandatory-to-Specify: No. The default is False, which waits for the uCDN to refresh a resource before responding to the client.¶

• Property: stale-if-error¶

  • Description: Instructs the dCDN to serve a stale version of a resource if an error was received when trying to refresh the resource with the uCDN. When set, the dCDN will return a previously cached version of a resource instead of caching the error response. Per Section 4 of [RFC5861], an error is any situation that would result in a 500, 502, 503, or 504 HTTP response status code being returned¶
  • Type: Array of HTTP response error status codes. Example: [ "503", "504"]¶
  • Mandatory-to-Specify: No. The default is to cache the error response received from the uCDN.¶

• Property: failed-refresh-ttl¶

  • Description: Instructs the dCDN to serve a stale version of a resource for the number of seconds specified in failed-refresh-ttl before trying to revalidate the resource with the uCDN. Use of failed-refresh-ttl allows the load to be reduced on the uCDN during times of system stress.¶
  • Type: Integer¶
  • Mandatory-to-Specify: No¶

Example 1: A MI.StaleContentCachePolicy where stale-while-revalidating is true, instructing 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
  }
}

Example 2: A 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 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"
  }
}

Example 3: A MI.StaleContentCachePolicy where stale-while-revalidating is true, instructing 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, 503, or 504 when trying to refresh the resource with the uCDN.¶

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", "503", "504" ],
      "failed-refresh-ttl": "5"
  }
}

2.1.4. MI.CacheBypassPolicy

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 cache when testing the uCDN fill path. Note: 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 CacheBypassPolicy) is not evicted.¶

• Property: bypass-cache¶

  • Description: A Boolean value that can activate the feature for a given client request. It is expected that this feature will be used within ProcessingStages to allow a client request to be marked to bypass cache.¶
  • Type: Boolean¶
  • Mandatory-to-Specify: No. The default is False.¶

Example 1: A MI.CacheBypassPolicy with the client HTTP header of: CDN-BYPASS: "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"
              }
            }
          ]
        }
      }
    ]
  }
}

Example 2: A MI.CacheBypassPolicy that applies to all requests where the host header is bypass.example.com:¶

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

2.1.5. MI.ComputedCacheKey

While the properties provided by the standard CDNi metadata Cache object (See Section 4.2.6 of [RFC8006]) provide some simple control over the construction of the cache key, 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 may specify a cache key as a value returned in a specific HTTP response header.¶

ComputedCacheKey is a new GenericMetadata object that allows for the specification of a cache key using the metadata expression language. Typical use cases would involve the construction of a cache key from one or more elements of the HTTP request. In cases where both the ComputedCacheKey and the Cache object are applied, the ComputedCacheKey will take precedence.¶

• Property: expression¶

  • Description: The expression that specifies how the cache key shall be constructed.¶
  • Type: String. An expression using [CDNI-MEL] to dynamically construct the cache key from elements of the HTTP request and/or response.¶
  • Mandatory-to-Specify: Yes¶

Example, using a custom request header as the cache key instead of the URI path:¶

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

2.2.1. MI.SourceMetadataExtended

SourceMetadataExtended is an alternative to the CDNI standard SourceMetadata object, which adds a property to specify load balancing across multiple sources, as well as a SourceExtended sub-object with additional attributes to the CDNI standard Source object. While both SourceMetadataExtended and SourceMetadata can be provided for backward compatibility, a dCDN that advertises capability for SourceMetadataExtended will ignore SourceMetadata if both are provided for a given host or path match.¶

• Property: sources¶

  • Description: Sources from which the dCDN can acquire content, listed in order of preference.¶
  • Type: Array of SourceExtended objects¶
  • Mandatory-to-Specify: No. Default is to use static configuration, out-of-band from the CDNI metadata interface.¶

• Property: load-balance¶

  • Description: Specifies load balancing rules for the set of sources.¶
  • Type: LoadBalanceMetadata object¶
  • Mandatory-to-Specify: No¶

Example of a SourceMetadataExtended object with the associated LoadBalanceMetadata configuration object:¶

{
  "generic-metadata-type": "MI.SourceMetadataExtended",
  "generic-metadata-value": {
    "sources": [
      {
        "endpoints": [
          "a.service123.ucdn.example",
          "b.service123.ucdn.example"
        ],
        "protocol": "http/1.1"
      },
      {
        "endpoints": [
          "origin.service123.example"
        ],
        "protocol": "http/1.1"
      }
    ],
    "load-balance": {
      "balance-algorithm": "content-hash",
      "balance-path-pattern": "^/prod/(.*)/.*\\.ts$"
    }
  }
}

{
  "generic-metadata-type": "MI.SourceMetadataExtended",
  "generic-metadata-value": {
    "sources": [
      {
        "endpoints": [
          "a.service123.ucdn.example",
          "b.service123.ucdn.example:8443"
        ],
        "protocol": "https/1.1",
        "origin-host": "internal.example.com",
        "webroot": "/prod",
        "follow-redirects": false,
        "timeout-ms": 4000,
        "failover-errors": [ "502", "503", "504" ]
      },
      {
        "endpoints": [ "origin.service123.example" ],
        "protocol": "http/1.1",
        "webroot": "/prod",
        "follow-redirects": true,
        "timeout-ms": 8000
      }
    ]
  }
}

{
  "generic-metadata-type": "MI.LoadBalanceMetadata",
  "generic-metadata-value": {
    "balance-algorithm": "content-hash",
    "balance-path-pattern": "^/prod/(.*)/.*\\.ts$"
  }
}

{
  "generic-metadata-type": "MI.LoadBalanceMetadata",
  "generic-metadata-value": {
    "balance-algorithm": "random",
    "balance-weights": [ 1, 2, 1 ]
  }
}

2.2.2. MI.Auth

To meet the typical industry requirements for authenticating CDNs to external origins, two new authentication types are defined.¶

• auth-type: MI.HeaderAuth¶

  • Description: Header based authentication is used to pass an HTTP header (secret-name) and value (secret-value) to a uCDN when requesting content. The header name and value are agreed upon between parties out of band.¶
  • Note: We may want to add a way to encrypt or separately communicate the secret; this could be a general capability for CDNI.¶
  • auth-value: MI.HeaderAuth object specifying the header name and value (secret name, secret key) required for authenticated access to an origin. For more information, refer to the MI.HeaderAuth section below.¶

• auth-type: MI.AWSv4Auth¶

  • Description: Allows for the specification of a set of headers to be added to requests that are forwarded to an origin to enable Amazon Web Services (AWS) authentication, as documented by AWS (See Specifications & Standards References).¶
  • auth-value: MI.AWSv4Auth object specifying the access parameters. For more information, refer to the MI.AWSv4Auth section below.¶

{
  "generic-metadata-type": "MI.SourceMetadataExtended",
  "generic-metadata-value": {
    "sources": [
      {
        "endpoints": [ "origin.example.com" ],
        "protocol": "http/1.1",
        "acquisition-auth": {
          "generic-metadata-type": "MI.Auth",
          "generic-metadata-value": {
            "auth-type": "MI.HeaderAuth",
            "auth-value": {
              "header-name": "X-Origin-Auth",
              "header-value": "SECRETKEYJKSDHFSIFUI4UFH78HW4NF7"
            }
          }
        }
      }
    ]
  }
}

{
  "generic-metadata-type": "MI.SourceMetadataExtended",
  "generic-metadata-value": {
    "sources": [
      {
        "endpoints": [ "origin.example.com" ],
        "protocol": "http/1.1",
        "acquisition-auth": {
          "generic-metadata-type": "MI.Auth",
          "generic-metadata-value": {
            "auth-type": "MI.AWSv4Auth",
            "auth-value": {
              "access-key-id": "MYACCESSKEYID",
              "secret-access-key": "SECRETKEYJKSDHFSIUHKWRGHHF",
              "aws-region": "us-west-1"
            }
          }
        }
      }
    ]
  }
}

2.3.1. MI.CrossOriginPolicy

Delegation of traffic between a CDN over an open caching node based on HTTP redirection does change the domain name in the client requests. This represents a cross-origin request that must be managed appropriately using Cross-Origin Resource Sharing (CORS) headers in the responses.¶

The dynamic generation of CORS headers is typical in modern HTTP request processing and avoids CORS validation forwarded to the CDN origin servers, particularly with the preflight OPTIONS requests. The CDNI metadata model requires extensions to specify how a CDN or open caching node should generate and evaluate these headers.¶

Required capabilities:¶

1. Set a default value for CORS response headers independent of the origin request header value.¶
2. Match the origin request header with a list of valid values, including PatternMatch, to return or not return the CORS response headers.¶
3. Set a list of custom headers that can be exposed to the client (expose headers).¶
4. Support for preflight requests using the OPTIONS method, including custom header validation, expose headers, and methods.¶
5. Support for credentials validation within CORS.¶

Simple CORS requests are those where both HTTP method and headers in the request are included in the safe list defined by the World Wide Web Consortium [W3C]. The user agent (UA) request can include an origin header set to the URL domain of the webpage that the player runs. Depending on the metadata configuration, the logic to apply by the open caching node (OCN) is:¶

1. Validation of the origin header - Metadata can include a list of valid domains to validate the request origin header. If it does not match, the CORS header must not be included in the response.¶
2. WIldcard usage - Depending on the configuration, the resultant CORS header to include in the response will be the same as the request origin header, or a wildcard.¶
3. If no validation of request is included in the origin header, set a default value for CORS response headers independent of the origin request header value.¶

When a UA makes a request that includes a method or headers that are not included in the safe-list, the client will make a CORS preflight request using the OPTIONS method to the resource including the origin header. If CORS is enabled and the requests passes the origin validation, the OCN SHOULD respond with the set of headers that indicate what is permitted for that resource, including one or more of the following:¶

1. Allowed methods¶
2. Allowed credentials¶
3. Allowed request headers¶
4. Max age that the OPTIONS request is valid¶
5. Headers that can be exposed to the client¶

CrossoriginPolicy is a GenericMetadata object that allows for the specification of dynamically generated CORS headers.¶

• Property: allow-origin¶

  • Description: Validation of simple CORS requests.¶
  • Type: Object MI.AccessControlAllowOrigin¶
  • Values: One element for each of the following properties.¶
  • Mandatory-to-Specify: Yes¶

• Property: expose-headers¶

  • Description: A list of values the OCN will include in the Access-Control-Expose-Headers response header to a preflight request.¶
  • Type: Array of strings¶
  • Mandatory-to-Specify: No¶

• Property: allow-methods¶

  • Description: A list of values the OCN will include in the Access-Control-Allow-Methods response header to a preflight request.¶
  • Type: Array of strings¶
  • Mandatory-to-Specify: No¶

• Property: allow-headers¶

  • Description: A list of values the OCN will include in the Access-Control-Allow-Headers response header to a preflight request.¶
  • Type: Array of strings¶
  • Mandatory-to-Specify: No¶

• Property: allow-credentials¶

  • Description: The value the OCN will include in the Access-Control-Allow-Credentials response header to a preflight request.¶
  • Type: Boolean¶
  • Mandatory-to-Specify: No¶

• Property: max-age¶

  • Description: The value the OCN will include in the Access-Control-Max-Age response header to a preflight request.¶
  • Type: Integer¶
  • Mandatory-to-Specify: No¶

{
  "generic-metadata-type": "MI.CrossoriginPolicy",
  "generic-metadata-value": {
    "allow-origin": {
      "allow-list": [
        {
          "pattern": "*"
        }
      ],
      "wildcard-return": true
    }
  }
}

{
  "generic-metadata-type": "MI.CrossoriginPolicy",
  "generic-metadata-value": {
    "allow-origin": {
      "allow-list": [
        {
          "pattern": "*://sourcepage.example.com"
        },
        "wildcard-return": false
      },
      "allow-methods": [ "GET", "POST" ],
      "allow-credentials": true,
      "allow-headers": [ "X-PINGOTHER", "Content-Type" ],
      "expose-headers": [ "X-User", "Authorization" ],
      "max-age": 3600
    }
  }
}

2.3.2. MI.AllowCompress

Downstream CDNs often have the ability to compress HTTP response bodies in cases where the client has declared that it can accept compressed responses (via an Accept-Encoding header), but the source/origin has returned an uncompressed response.¶

The specific compression algorithm used by the dCDN is negotiated by the client's Accept-Encoding header according to [RFC7694] (including q= preferences) and the compression capabilities available on the dCDN.¶

In addition, HeaderTransform allows the uCDN to normalize, or modify, the Accept-Encoding header to allow for fine-grain control over the selection of the compression algorithm (e.g., gzip, compress, deflate, br, etc.).¶

AllowCompress is a new GenericMetadata object that allows the dCDN to compress content before sending to the client.¶

• Property: allow-compress¶

  • Description: If set to "True", then the dCDN will try to compress the response to the client based on the Accept-Encoding request header.¶
  • Type: Boolean¶
  • Values: True or False¶
  • Mandatory-to-Specify: No. The default is "False".¶

Example 1: An MI.AllowCompress that allows manifests (*.m3u8) to be compressed by the dCDN:¶

{
  "match": {
    "expression": "req.h.uri *= '*.m3u8'"
  },
  "stage-metadata": {
    "generic-metadata": [
      {
        "generic-metadata-type": "MI.AllowCompress",
        "generic-metadata-value": {
          "allow-compress": "true"
        }
      }
    ]
  }
}

Example 2: An MI.AllowCompress that allows manifests (*.m3u8) to be compressed by the dCDN but normalizing the client's Accept-Encoding header:¶

{
  "match": {
    "expression": "req.h.accept-encoding *= '*gzip*'"
  },
  "stage-metadata": {
    "generic-metadata": [
      {
        "generic-metadata-type": "MI.AllowCompress",
        "generic-metadata-value": {
          "allow-compress": "true"
        }
      }
    ]
  }
}

2.3.3. MI.TrafficType

Content delivery networks often apply different infrastructure, network routes, and internal metadata for different types of traffic. Delivery of large static objects (such as software downloads), may, for example, use different network routes than video stream delivery. In an HTTP adaptive bitrate video service, every video title corresponds to a set of video files and descriptors according to different video protocols, and this is independent of the type of service (Video-on-Demand, Live, Catch-up, etc.).¶

The way the video service is consumed by the user agents can vary. For instance, a segment that belongs to a Video-on-Demand (VoD) title can be requested for every moment the content is available for the user agents to consume, while a segment of live content will be only requested as long as the time-shift duration is configured for that service. Knowing those differences, a CDN or OCN provider can implement specific strategies that will maximize performance and thereby provide more available capacity to the upstream provider. It should be noted that the dCDNs handling of the traffic types is implementation-specific and not prescribed here.¶

TrafficType metadata defines a set of descriptors that characterize either the type or usage of the traffic, enabling CDNs and OCNs to apply any internal configuration rules without exposing an unnecessary number of internal details. Note that the interpretation of these traffic types and application of rules such as rate limiting or delivery pacing are implementation specific.¶

• Property: traffic-type¶

  • Description: A literal that defines the traffic type. uCDN will use the literal that is most representative of the traffic being delegated.¶
  • Type: Enumeration [vod, live, object-download] encoded as lowercase string¶
  • Mandatory-to-Specify: Yes¶

• Property: hints¶

  • Description: Other traffic characteristics that the uCDN can indicate to the dCDN as suggestions for service optimization. Accepts free-form unconstrained values.¶
  • Type: Array of strings¶
  • Mandatory-to-Specify: No¶

A TrafficType definition example for HostMetadata:¶

{
  "generic-metadata-type": "MI.TrafficType",
  "generic-metadata-value": {
    "traffic-type": "vod",
    "hints": [ "low-latency", "catch-up" ]
  }
}

2.3.4. MI.OcnSelection

Configuration metadata is required to permit several levels of OCN selection policies. For example, in a mobile network, several physical locations are possible (i.e., candidates) for hosting the OCN that will take charge in the delegation for the uCDN. This is the case when the cache is virtualized and deployed dynamically. Depending on the OCN selection policy (which may be a cost driver), the dCDN may attempt to favor certain types of caches at the edge, for example. The default OCN selection policy might be "best-effort". Another one might be linked to the network characteristics like "Edge" or ("average latency< 10ms").¶

OcnSelection is a new GenericMetadata object that allows the uCDN to indicate to the dCDN a preference in terms of OCN selection.¶

• Property: ocn-delivery¶

  • Description: Instructs the dCDN to perform delegation operating a particular medium and/or a transport arrangement.¶
  • Type: Object MI.OcnDelivery¶
  • Mandatory-to-Specify: No. At least one of the two properties, ocn-type or ocn-delivery, must be present.¶

• Property: ocn-type¶

  • Description: Instructs the dCDN to perform delegation operating the type of open caching nodes.¶
  • Type: A string corresponding to one of the open caching node types announced by the dCDN through the FCI interface.¶
  • Mandatory-to-Specify: No. At least one of the two properties, ocn-type or ocn-delivery, must be present.¶

• Property: ocn-selection¶

  • Description: This property enforces the selection of OCNs, considering the ocn-type and/or the ocn-delivery properties. "False" means best-effort.¶
  • Type: string. "attempt-or-failed" and "attempt-or-besteffort" mean that the delegation must be attempted considering the ocn-type and/or the ocn-delivery properties. If not possible, it is considered as an error and either fails (configuration failure) or the dCDN continues with a best-effort procedure. Last, "best effort" means the dCDN tries its best to fulfil the requested ocn-selection policy.¶
  • Mandatory-to-Specify: No. Best-effort is the default OCN selection policy.¶

2.4.1. MI.ProcessingStages

A ProcessingStages object is a new GenericMetadata which describes the matching rules, metadata, and transformations to be applied at specific stages in the request processing pipeline. The processing rules and transformations are defined as a child data model referenced within a ProcessingStages object, as defined below.¶

                        +----------------+
                        |ProcessingStages|
                        +----------------+
                               (*)
                                |
       +----------------+-------+---------+----------------+
       |                |                 |                |
       v                v                 v                v
+-------------+ +-------------+ +--------------+ +--------------+
|ClientRequest| |OriginRequest| |OriginResponse| |ClientResponse|
+-------------+ +-------------+ +--------------+ +--------------+
      (*)             (*)              (*)              (*)
       |               |                |                |
       +---------------+----------------+----------------+
       |
       |                    +---------------+
       |          +-------->|ExpressionMatch|
       |          |         +---------------+
       |          |
       |          |                                 *****************
       |         (*)                           +--->*GenericMetadata*
       |    +----------+    +-------------+    |    *     Objects   *
       +--->+StageRules+--->|StageMetadata+(*)-+    *****************
            +----------+    +-------------+
                             (*)      (*)
                              |        |
                      +-------+        +-----------+
                      |                            |
                      v                            v
            +----------------+              +-----------------+
            |RequestTransform|(*)-+    +-(*)|ResponseTransform|
            +----------------+    |    |    +-----------------+
                    (*)           |    |
                     |            |    |
                     |            v    v
                     |       +---------------+
                     |       |HeaderTransform|(*)-+
                     |       +---------------+    |
                     |                            |
                     v                            |
             +-----------------+                  |
             |SyntheticResponse|(*)--+            v
             +-----------------+     |      +----------+
                                     +----->|HTTPHeader|
                                            +----------+

Each of the four processing stages is represented by an array of StageRules objects, with each StageRules object defining criteria along with metadata that MUST be applied if the match applies to "True". It should be noted that the StageRules objects in the array are evaluated and processed in order. A possible future extension to this processing model could allow for an if-else structure, where processing for a stage is halted upon the first match of a StageRules expression.¶

• Property: client-request¶

  • Description: Allows for the specification of conditional metadata to be applied at the client request processing stages, as defined in the Rule Processing Stages section. The StageRules in the array are evaluated in order.¶
  • Type: Array of StageRules objects¶
  • Mandatory-to-Specify: No¶

• Property: origin-request¶

  • Description: Allows for the specification of conditional metadata to be applied at origin request processing stages, as defined in the Rule Processing Stages section. The StageRules in the array are evaluated in order.¶
  • Type: Array of StageRules objects¶
  • Mandatory-to-Specify: No¶

• Property: origin-response¶

  • Description: Allows for the specification of conditional metadata to be applied at origin response processing stages, as defined in the Rule Processing Stages section. The StageRules in the array are evaluated in order.¶
  • Type: Array of StageRules objects¶
  • Mandatory-to-Specify: No¶

• Property: client-response¶

  • Description: Allows for the specification of conditional metadata to be applied at client response processing stages, as defined in the Rule Processing Stages section. The StageRules in the array are evaluated in order.¶
  • Type: Array of StageRules objects¶
  • Mandatory-to-Specify: No¶

Example specifying all four processing stages. In this example, the client-request stage has two StageRules, appling one set of metadata if "ExpressionMatch1" evaluates to "True" and applying another set of metadata if "ExpressionMatch2" evaluates to "True".¶

{
  "generic-metadata-type": "MI.ProcessingStages",
  "generic-metadata-value": {
    "client-request" : [
      {
        "match" : < ExpressionMatch1 for conditional metadata >
        "stage-metadata" : <StageMetadata1 for clientRequest stage>,
      },
      {
        "match" : <Additional ExpressionMatch2 for conditional metadata >
        "stage-metadata" : <StageMetadata2 for clientRequest stage>,
      }
    ],
    "origin-request" : [{
      "match" : <Optional ExpressionMatch for conditional metadata >
      "stage-metadata" : <StageMetadata for originRequest stage>,
    }],
    "origin-response" : [{
      "match" : <Optional ExpressionMatch for conditional metadata >
      "stage-metadata" : <StageMetadata for originResponse stage>,
    }],
    "client-response" : [{
      "match" : <Optional ExpressionMatch for conditional metadata >
      "stage-metadata" : <StageMetadata for clientResponse stage>,
    }]
  }
}

2.4.2. MI.StageRules

A StageRules object is used within the context of ProcessingStages to define elements in a list of match rules and stage-specific metadata and transformations that MUST be applied conditionally on a rich expression match.¶

• Property: match¶

  • Description: An ExpressionMatch object encapsulating a rich expression using the CDNI Metadata Expression Language [CDNI-MEL] to evaluate aspects of the HTTP request and/or response. The stage-metadata rules are only applied if the match evaluates to "True" or if no match expression is provided¶
  • Type: ExpressionMatch object¶
  • Mandatory-to-Specify: No. The stage-metadata rules are always applied if no match expression is provided. This would be the case when stage-metadata should be applied unconditionally within the context of the higher-level host and path matches.¶

• Property: stage-metadata¶

  • Description: Specifies the set of StageMetadata to be applied at the processing stage if the match expression evaluates to "True" or is not present.¶
  • Type: Array of StageMetadata objects, applied in order.¶
  • Mandatory-to-Specify: Yes¶

An example of StageRules that are applied just after responses are received from the origin. In this example, receipt of a response status code of 304 from the origin indicates that CachePolicy metadata SHOULD be applied (as specified via an external HREF), and that response headers SHOULD be modified (X-custom-response-header added and ETag deleted).¶

{
  "match": {
    "expression": "resp.status == 304"
  },
  "stage-metadata": {
    "generic-metadata": [
      {
        "type": "MI.CachePolicy",
        "href": "https://metadata.ucdn.example/origin_response_cache"
      }
    ],
    "response-transform": {
      "headers": {
        "add": [
          {
            "name": "X-custom-response-header",
            "value": "header-value"
          }
        ],
        "delete": [ "ETag" ]
      }
    }
  }
}

2.4.3. MI.ExpressionMatch

The ExpressionMatch object contains the rich expression that must evaluate to "True" for the StageMetadata to be applied for the specific StageRules. Defining expressions as stand-alone objects allows for sets of reusable match expressions to be reused via metadata reference linking.¶

• Property: expression¶

  • Description: A rich expression using CDNI-MEL to evaluate aspects of the HTTP request and/or response. See documentation on the Metadata Expression Language for details on the expression of matching variables and syntax.¶
  • Type: String, using CDNI-MEL syntax. See the METADATA EXPRESSION LANGUAGE (CDNI-MEL) section.¶
  • Mandatory-to-Specify: Yes¶

Example of ExpressionMatch on the referrer and user agent request headers:¶

{
  "expression" : "req.h.user-agent *= '*Safari*' and req.h.referrer == 'www.myhost.com'"
}

2.4.4. MI.StageMetadata

The StageMetadata object contains GenericMetadata and HTTP request/response transformations that MUST be applied for a StageRules match. The following table defines the processing stages where request and response transformations are possible:¶

Note that for the stages where both request and response transformations are allowed, it is possible to specify both. This may be the case if, for example, the request URI needs alteration for cache-key generation and the response headers need to be manipulated.¶

• Property: generic-metadata¶

  • Description: Specifies the set of GenericMetadata to be applied for a StageRules match. A typical use case would be the application of a CachePolicy or TimeWindowACL conditionally on matching HTTP headers. Support for this capability is optional and can be advertised via feature-flags in the FCI interface.¶
  • Type: Array of GenericMetadata, applied in order. Note that not all GenericMetadata object types may be applicable at all processing stages.¶
  • Mandatory-to-Specify: No. The generic-metadata property would not be needed when StageMetadata is used to only specify request or response transformations, such as modifications of HTTP headers.¶

• Property: request-transform¶

  • Description: Specifies a transformation to be applied to the HTTP request for a StageRules match. The transformation can be the modification of any request header and/or the modification of the URI. Modifications are applied such that downstream processing stages receive the modified HTTP request as their input. Support for this capability is optional and can be advertised via feature-flags in the FCI interface.¶
  • Type: RequestTransform object¶
  • Mandatory-to-Specify: No¶

• Property: response-transform¶

  • Description: Specifies a transformation to be applied to the HTTP response for a StageRules match. The transformation can be the modification of any response header, HTTP response status code, or the generation of a synthetic response. Modifications are applied such that downstream processing stages receive the modified HTTP response as their input. Support for this capability is optional and can be advertised via feature-flags in the FCI interface.¶
  • Type: ResponseTransform object¶
  • Mandatory-to-Specify: No¶

Example of a StageMetadata object:¶

{
  "generic-metadata" :  [{
    < Optional list of generic metadata to apply at this stage >
  }],
  "request-transform" : {
    "headers" : { <list of request headers to add/replace/delete> },
    "uri" : < URI rewrite, either static or dynamically constructed>
  }
  "response-transform" : {
    "headers" : { <list of response headers to add/replace/delete> },
    "response-status" : <Status either static or dynamically constructed >
  }
}

2.4.5. MI.RequestTransform

The RequestTransform object contains metadata for transforming the HTTP request for a specific StageRules object. The transformation can be the modification of any request header and/or the modification of the URI. Modifications are applied such that downstream processing stages receive the modified HTTP request as their input.¶

• Property: headers¶

  • Description: A HeaderTransform object specifying HTTP request headers to add, replace, or delete.¶
  • Type: HeaderTransform object¶
  • Mandatory-to-Specify: No¶

• Property: uri¶

  • Description: Replacement value for the HTTP request.¶
  • Type: String. Either a literal (static string) or an expression using CDNI-MEL to dynamically construct a URI value from elements of the HTTP request and/or response.¶
  • Mandatory-to-Specify: No¶

• Property: uri-is-expression¶

  • Description: Flag to signal whether the URI is a static string literal or a CDNI-MEL expression that needs to be dynamically evaluated.¶
  • Type: Boolean¶
  • Mandatory-to-Specify: No. The default is "False", indicating that the URI is a string literal and does not need to be evaluated.¶

Example of a RequestTransform object illustrating a dynamically constructed URI rewrite:¶

{
  "request-transform" : {
    "headers" : { <Optional list of request headers to add/replace/delete> },
    "uri" :    "req.uri.path",
    "uri-is-expression" : true
 }

2.4.6. MI.ResponseTransform

The ResponseTransform object contains metadata for transforming the HTTP response for a StageRules match. The transformation can be the modification of any response header, HTTP response status code, or the generation of a synthetic response. Modifications are applied such that downstream processing stages receive the modified HTTP response as their input.¶

• Property: headers¶

  • Description: A HeaderTransform object specifying HTTP response headers to add, replace, or delete.¶
  • Type: HeaderTransform object¶
  • Mandatory-to-Specify: No¶

• Property: response-status¶

  • Description: Replacement value for the HTTP response status code.¶
  • Type: Integer. Either a static integer or an expression using CDNI-MEL that evaluates to an integer to dynamically generate an HTTP status code based on elements of the HTTP request and/or response. Expressions that do not evaluate to an integer shall be considered invalid and result in no override of origin-provided response status.¶
  • Mandatory-to-Specify: No¶

• Property: status-is-expression¶

  • Description: Flag to signal whether the response-status is a static integer or a CDNI-MEL expression that needs to be dynamically evaluated to generate an HTTP response status code.¶
  • Type: Boolean¶
  • Mandatory-to-Specify: No. The default is "False", indicating that the response-status is a static integer and does not need to be evaluated.¶

• Property: synthetic¶

  • Description: Specification of a complete replacement of any HTTP response that may have been generated in an earlier processing stage with a synthetic response. Use of this property to specify a synthetic response would override any response transformations or status codes specified by other properties.¶
  • Type: SyntheticResponse object¶
  • Mandatory-to-Specify: No¶

Example of a ResponseTransform object, illustrating a dynamically constructed header value that uses the expression language to concatenate the user agent and host header, and forces a 403 HTTP response status code:¶

{
  "response-transform": {
    "headers": {
      "add": [
        {
          "name": "X-custom-response-header",
          "value": "req.h.user-agent . ‘-‘ . req.h.host",
          "value-is-expressions": true
        }
      ]
    },
    "response-status": "403"
  }
}

2.4.7. MI.SyntheticResponse

The SyntheticResponse object allows for the specification of a synthetic response to be generated in response to the HTTP request being processed. The synthetic response can contain a set of response headers, a status code, and a response body, and is a complete replacement for any HTTP response elements generated in an earlier processing stage.¶

A dynamically generated Content-Length HTTP response header is generated based on the length of the generated response body.¶

• Property: headers¶

  • Description: An array of HTTP header objects that specify the full set of headers to be applied to the synthetic response.¶
  • Type: Array of HTTP header objects¶
  • Mandatory-to-Specify: No, although it would be unusual to not specify minimal standard response headers, such as Content-Type.¶

• Property: response-status¶

  • Description: HTTP response status code.¶
  • Type: Integer. Either a static integer or an expression using CDNI-MEL that evaluates to an integer to dynamically generate an HTTP status code based on elements of the upstream HTTP request and/or response. Expressions that do not evaluate to an integer shall be considered invalid and result in a 500 status for the synthetic response.¶
  • Mandatory-to-Specify: Yes¶

• Property: status-is-expression¶

  • Description: Flag to signal whether the response-status is a static integer or a CDNI-MEL expression that needs to be dynamically evaluated to generate an HTTP response status code.¶
  • Type: Boolean¶
  • Mandatory-to-Specify: No. The default is "False", indicating that the response-status is a static integer and does not need to be evaluated.¶

• Property: body¶

  • Description: Body for the synthetic HTTP response. The response body can either be static or dynamically constructed from a rich expression.¶
  • Type: String. Either a literal (static string) or an expression using CDNI-MEL to dynamically construct a response body from elements of the HTTP request and/or response.¶
  • Mandatory-to-Specify: No. If absent, an empty HTTP response with a zero-value Content-Length header is generated.¶

• Property: body-is-expression¶

  • Description: Flag to signal whether the synthetic response body is a static string literal or a CDNI-MEL expression that needs to be dynamically evaluated.¶
  • Type: Boolean¶
  • Mandatory-to-Specify: No. The default is "False", indicating that the body is a string literal and does not need to be evaluated.¶

Example of a SyntheticResponse object illustrating a dynamically constructed response body that uses the expression language to combine the request URI with static text and forces a 405 HTTP response status code:¶

{
  "headers": [
    {
      "name": "content-type",
      "value": "text/plain"
    },
    {
      "name": "X-custom-response-header",
      "value": "some static value"
    }
  ],
  "response-status": "405",
  "response-body": "'Sorry, Access to resource '.req.uri.' not allowed'",
  "body-is-expression": false
}

2.4.8. MI.HeaderTransform

The HeaderTransform object specifies how HTTP headers MUST be added, replaced, or deleted from HTTP requests and responses.¶

• Property: add¶

  • Description: List of HTTP headers (name/value pairs) that MUST be added to the HTTP request or response. Note that any existing headers in the request or response with the same names of those added are not affected, resulting in multiple headers with the same name.¶
  • Type: Array of HTTPHeader objects containing header name/value pairs¶
  • Mandatory-to-Specify: No¶

• Property: replace¶

  • Description: List of HTTP headers (name/value pairs) that MUST be added to the HTTP request or response, replacing any previous headers with the same name.¶
  • Type: Array of HTTPHeader objects containing header name/value pairs¶
  • Mandatory-to-Specify: No¶

• Property: delete¶

  • Description: List of names of HTTP headers that MUST be deleted from the¶
  • HTTP request or response. If a named header appears multiple times, all occurrences are deleted.¶
  • Type: Array of strings, with each string naming an HTTP header to delete¶
  • Mandatory-to-Specify: No¶

Example of a HeaderTransform object illustrating the addition of two customer headers, the replacement of any previously provided Accept-Encoding header, and the removal of any previously provided Authorization or Accept-Language headers:¶

{
  "add": [
    {
      "name": "X-custom-header1",
      "value": "header-value 1"
    },
    {
      "name": "X-custom-header2",
      "value": "header-value 2"
    }
  ],
  "replace": [
    {
      "name": "Accept-Encoding",
      "value": "gzip,deflate,br"
    }
  ],
  "delete": [
    "Authorization",
    "Accept-Language"
  ]
}

2.4.9. MI.HTTPHeader

The HTTPHeader object contains a name/value pair for an HTTP header to add or replace in a request or response. The CDNI-MEL expression language can be used to dynamically generate response values.¶

• Property: name¶

  • Description: Name of the HTTP header.¶
  • Type: String¶
  • Mandatory-to-Specify: Yes¶

• Property: value¶

  • Description: New value of the named HTTP header.¶
  • Type: String. Either a static string or an expression using [CDNI-MEL] to dynamically construct a header value from elements of the HTTP request and/or response.¶
  • Mandatory-to-Specify: Yes¶

• Property: value-is-expression¶

  • Description: Flag to signal whether the value is a static string literal or a [CDNI-MEL] expression that needs to be dynamically evaluated.¶
  • Type: Boolean¶
  • Mandatory-to-Specify: No. The default is "False", indicating that the value is a string literal and does not need to be evaluated.¶

Example of an HTTPHeader illustrating a dynamically constructed header value that equals the session parameter from the query string:¶

{
  "name": "X-custom-response-header",
  "value": "req.uri.query.session",
  "value-is-expression": true
}

2.5.1. MI.ServiceIDs

CDN configurations typically have multiple tiers of identifiers that group configurations by customer account to facilitate logging, billing, and support operations. This structure supports two tiers of identifiers (a serviceID which typically identifies a high level customer's service, and a propertyID which typically represents a logical grouping of a set of hosts within a customers' service. It should be noted, however, that the interpretation of ServiceID and PropertyID are implementation-specific, and may not be used by all CDNs and Open Caching systems.¶

This metadata model extension allows for the association service identifier metadata to a host or path match and to allow for these IDs to be dynamically generated via an expression language. For example, it may be necessary to extract a portion of the Request URI path to derive a service identifier (e.g.: /news/* maps to one propertyID and /movies/* maps to a different propertyID). When processing the MI.ServiceIDs metadata for a request, implementations SHOULD override any previously assigned service identifiers with those specified by this metadata.¶

MI.ServiceIDs is a new GenericMetadata object that allows for the specification of the two tiers of CDN-specific service identifiers and service names.¶

• Property: service-id¶

  • Description: A provider-specific identifier for the service (typically a customer account identifier).¶
  • Type: String. Either a literal (static string) or an expression using CDNI-MEL to dynamically construct the ID from elements of the HTTP request and/or response.¶
  • Mandatory-to-Specify: No¶

• Property: service-id-is-expression¶

  • Description: Flag to signal whether the service-id is a static string literal or a CDNI-MEL expression that needs to be dynamically evaluated.¶
  • Type: Boolean¶
  • Mandatory-to-Specify: No. The default is "False", indicating that the service-id is a string literal and does not need to be evaluated.¶

• Property: service-name¶

  • Description: Human-readable name for the service-id.¶
  • Type: String¶
  • Mandatory-to-Specify: No¶

• Property: property-id¶

  • Description: A provider-specific identifier for the property (typically identifies a child configuration within the parent service-id).¶
  • Type: String. Either a literal (static string) or an expression using CDNI-MEL to dynamically construct the ID from elements of the HTTP request and/or response.¶
  • Mandatory-to-Specify: No¶

• Property: property-id-is-expression¶

  • Description: Flag to signal whether the property-id is a static string literal or a CDNI-MEL expression that needs to be dynamically evaluated.¶
  • Type: Boolean¶
  • Mandatory-to-Specify: No. The default is "False", indicating that the property-id is a string literal and does not need to be evaluated.¶

• Property: property-name¶

  • Description: Human-readable name for the property-id.¶
  • Type: String¶
  • Mandatory-to-Specify: No¶

Example illustrating the assignment of a literal service-id along with a dynamically computed property-id that is extracted from the root element of the request URI path.¶

{
  "generic-metadata-type": "MI.ServiceIDs",
  "generic-metadata-value": {
    "service-id": "12345",
    "service-name": "My Streaming Service",
    "property-id": "path_element(req.uri, 1)",
    "property-id-is-expression": true
  }
}

2.5.2. MI.PrivateFeatureList

The dCDN may gather a certain number of private features (i.e., not [yet] adopted by SVA or considered marginal) that it may want to expose to the content provider and/or the uCDN. Although private, the announcement, selection, and configuration of this private feature could be done through the OC API.¶

One example could be the support in OCNs of a new protocol that allows the ability to get additional insight about the user agent status (e.g., CTA Wave CMCD).¶

As another example, Broadpeak has developed a feature called S4Streaming, and would like to give the opportunity to control that feature to the uCDN.¶

PrivateFeatureListis a GenericMetadata configuration object as a base generic object that permits the control of private features.¶

• Property: features¶

  • Description: The list of feature configuration objects.¶
  • Type: List (array) of MI.PrivateFeature objects .¶
  • Mandatory-to-Specify: Yes¶

{
  "generic-metadata-type": "MI.PrivateFeatureList",
  "generic-metadata-value": {
    "feature": {
      "feature-oid": "Broadpeak",
      "feature-type": "S4Streaming",
      "feature-value": {
        "footprint": {
          "footprint-type": "ipv4cidr",
          "footprint-value": [
            "192.0.2.0/24",
            "198.51.100.0/24"
          ]
        },
        "activation": "ON",
        "mode": "transparent",
        "policy": "bandwidth-max"
      }
    }
  }
}

2.5.3. MI.RequestRouting

The uCDN requires the ability to indicate whether HTTP redirect, DNS redirect, and manifest rewrite are allowed, and indicate which is preferable. This is required in cases where the uCDN would like to delegate the traffic relying on the iterative method but knows the client will not support HTTP redirect. In that case, the uCDN needs a means to force the dCDN to perform request routing based on DNS redirect (or manifest rewrite).¶

This configuration possibility is useful only if the dCDN can advertise the mode of redirection it supports. There is an ongoing discussion in the IETF CDNI group to understand the semantics behind the redirection modes currently in Footprint & Capabilities Advertising Interface (I-DNS and I-HTTP). It is not clear whether this indicates that the dCDN supports one or both delegation modes (the request routing performed by the uCDN can only be based on DNS redirect or HTTP redirect or both), or whether it indicates that the dCDN supports, as its own request routing mode, DNS redirect and/or HTTP redirect. The latter is required for this new configuration object to be valid.¶

MI.RequestRouting is a new GenericMetadata object that allows the uCDN to force the dCDN request routing mode(s) to be applied when working in iterative redirection mode. The list of redirection modes supported by the dCDN is advertised through the FCI.RedirectionMode object. The list of request routing modes supported by the dCDN is advertised through the FCI.RequestRoutingMode object.¶

• Property: request-routing-modes¶

  • Description: Instructs the dCDN to perform request routing according to one or more preferred modes among those supported and advertised by the dCDN through the FCI.RequestRouting object. One must understand that forcing (instead of letting the dCDN request router select) one particular request routing mode may trigger some inefficiency in the request routing process.¶
  • Type: List (array) of iterative request routing modes¶
  • Values: "DNS", "HTTP", "MANIFEST_REWRITE"¶
  • Mandatory-to-Specify: No. By default, all request routing modes supported by the dCDN can be used by the dCDN as part of its request routing process.¶

Example, illustrating the uCDN forcing the dCDN to use DNS or HTTP as the method for request routing in case the uCDN performs an iterative delegation (i.e., iterative redirection mode):¶

{
  "generic-metadata-type": "MI.RequestRouting",
  "generic-metadata-value": {
    "request-routing-modes": [ "DNS", "HTTP" ]
  }
}

3.3.1. Basic Functions: Type Conversions

3.3.2. Basic Functions: String Conversions

3.3.3. Convenience Functions

3.4.1. Compile Time Errors

To ensure reliable service, all CDNI Metadata configurations MUST be validated for syntax errors before they are ingested into a dCDN. That is, existing configurations should be kept as the live running configuration until the new configuration has passed validation. If errors are detected in a new configuration, the configuration MUST be rejected. A HTTP 500 Internal Server Error should be returned with a message that indicates the source of the error (line number, and configuration element that caused the error).¶

Examples of compile-time errors:¶

1. Configuration does not parse relative to the CDNI Metadata JSON schema¶
2. Unknown CDNI Metadata object referenced in the configuration¶

3. CDNI Metadata object parse error¶

   1. Missing mandatory CDNI Metadata property¶
   2. Unknown CDNI Metadata property¶
   3. Incorrect type for a CDNI Metadata property value¶

4. CDNI-MEL¶

   1. Unknown CDNI-MEL variable name referenced in an expression¶
   2. Unknown CDNI-MEL operator, key-word, or functions referenced in an expression¶
   3. Incorrect number of arguments used in a CDNI-MEL expression operator or function¶
   4. Incorrect type of argument used in a CDNI-MEL expression operator or function¶

3.4.2. Runtime Errors

If a runtime error is detected when processing a request, the request should be terminated, and a HTTP 500 'Internal Server Error' returned to the caller. To avoid security leaks, sensitive information MUST be removed from the error message before it is returned to an external client. In addition to returning the HTTP 500 error, the dCDN SHOULD log additional diagnostics information to assist in troubleshooting.¶

Examples of runtime errors:¶

1. Failure to allocate memory (or other server resources) when evaluating a CDNI-MEL expression¶
2. Incorrect runtime argument type in a CDNI-MEL expression. E.g., trying to convert a non-numeric string to a number¶

4.2.1. FCI.AuthTypes

The AuthTypes object is used to indicate the support of authentication methods to be used for content acquisition (while interacting with an origin server) and authorization methods to be used for content delivery.¶

This specification document defines two new authentication methods (see MI.Auth) while there is one authorization method currently under specification in CDNI called [URI.signing]¶

• Property: stage-metadata¶

  • Description: Specifies the set of StageMetadata to be applied at the processing stage if the match expression evaluates to "True" or is not present.¶
  • Type: Array of StageMetadata objects, applied in order.¶
  • Mandatory-to-Specify: Yes¶

• Property: authe-types¶

  • Description: List of supported authentication methods (possibly required for content acquisition)¶
  • Type: Array of strings¶
  • Values: "AWSv4Auth", "HeaderAuth"¶
  • Mandatory-to-Specify: No. No authentication method is supported in this case.¶

• Property: autho-types¶

  • Description: List of supported authorization methods (possibly required for content delivery)¶
  • Type: Array of strings¶
  • Values: "UriSigning"¶
  • Mandatory-to-Specify: No. No authorization method is supported in this case.¶

FCI.AuthTypes example:¶

{
  "capabilities": [
    {
      "capability-type": "FCI.AuthTypes",
      "capability-value": {
        "authe-types": [
          "AWSv4Auth",
          "HeaderAuth"
        ],
        "autho-types": [
          "UriSigning"
        ]
      }
    }
  ]
}

4.2.2. FCI.ProcessingStages

This object is used to signal the set of features that are supported in relation to the ProcessingStages configuration object (see MI.ProcessingStages). Those optional features depend on the CDNI-MEL language support.¶

• Property: features¶

  • Description: List of supported optional processing stages features. Note that these features all have some dependencies on support of the CDNI MEL expression language.¶
  • Type: Array of strings¶
  • Values: "ExpressionMatch", "RequestTransform", "ResponseTransform"¶
  • Mandatory-to-Specify: No. None of these optional features are supported in this case.¶

Example:¶

{
  "capabilities": [
    {
       "capability-type": "FCI.ProcessingStages",
       "capability-value": {
         "features": [
           "ExpressionMatch",
           "RequestTransform",
           "ResponseTransform"
         ]
       }
     }
  ]
}

4.2.3. FCI.SourceMetadataExtended

This object is used to signal the supported features related to the SourceMetadataExtended configuration object.¶

• Property: load-balance¶

  • Description: List of supported load balancing algorithms in relation to the SourceMetadataExtended configuration object (see MI.SourceMetadataExtended)¶
  • Type: Array of strings¶
  • Values: "random", "content-hash", "ip-hash¶
  • Mandatory-to-Specify: No. load balancing is not supported among sources.¶

If the FCI.SourceMetadtaExtended object is not exposed/advertised or if the "load-balance" array is empty, the dCDN does not support the usage of the load-balance property attached to the SourceMetadataExtended configuration object (see MI.SourceMetadataExtended).¶

Example:¶

{
  "capabilities": [
    {
      "capability-type": "FCI.SourceMetadataExtended",
      "capability-value": {
        "load-balance": [
          "random",
          "content-hash",
          "ip-hash"
        ]
      }
    }
  ]
}

4.2.4. FCI.RequestRouting

This object is used by the dCDN to signal/announce the supported request routing modes. This can be optionally used by the uCDN to further select a subset of those modes when operating one of the iterative delegation modes. See the section on the GenericMetadata RequestRouting object..¶

• Property: request-routing-modes¶

  • Description: List of supported request routing modes by the dCDN. This information is useful when the uCDN decides to perform a delegation in iterative mode.¶
  • Type: Array of strings¶
  • Values: "DNS", "HTTP-R", "MANIFEST_REWRITE"¶
  • Mandatory-to-Specify: No. If the dCDN does not advertise the supported request routing modes, they are all supported by default.¶

Example:¶

{
  "capabilities": [
    {
      "capability-type": "FCI.RequestRouting",
      "capability-value": {
        "request-routing-modes": [
          "DNS",
          "HTTP",
          "MANIFEST_REWRITE"
        ]
      }
    }
  ]
}

4.2.5. FCI.PrivateFeatures

This object is used by the dCDN to signal/announce the list of supported private features. See the section on the GenericMetadata PrivateFeatureList object.¶

• Property: features¶

  • Description: The list of supported private feature¶
  • Type: List nested objects of FCI.PrivateFeature¶

Example:¶

{
  "capabilities": [
    {
      "capability-type": "FCI.PrivateFeatures",
      "capability-value": {
        "features": [
          {
            "feature-oid": "Broadpeak",
            "feature-type": "S4Streaming"
         }
        ]
      }
    }
  ]
}

4.2.6. FCI.OcnSelection

This object is used by the dCDN to signal/announce the supported OCN types and/or their transport arrangement and/or medium supported by OCNs.¶

Property ocn-delivery-list¶

1. Description: List of supported medium and/or transport arrangements.¶
2. Type: Array of nested objects, each containing the following properties:¶

• o Property: ocn-medium¶

  • Description: This property lists the supported mediums.¶
  • Type: Array of strings. The following values are specified: "SATELLITE"¶
  • Mandatory-to-Specify: No¶

• o Property: ocn-transport¶

  • Description: Instructs the dCDN to perform delegation operating a particular transport arrangement. The following values are specified: "MABR".¶
  • Type: Array of strings¶
  • Mandatory-to-Specify: No¶
• .....Property: ocn-type-list¶

o Description: List of supported OCN types. Examples include: "HOME" or "EDGE".¶

o Type: Array of strings¶

o Mandatory-to-Specify: No¶