CDNI Edge Control Metadata

Introduction CDNs typically require a set of configuration metadata to provide directives for the processing of responses downstream (at the edge and in the user agent). This document specifies GenericMetadata objects to meet those requirements, defining edge processing rules such as CORS handling, response compressions, and client connection failures.

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 .

MI.CrossoriginPolicy Delegation of traffic between CDNs over an Open Caching node (OCN) based on Hypertext Transfer Protocol (HTTP) redirection changes the domain name in the client requests. This represents a cross-origin request that must be managed appropriately using (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 CDN Interconnection (CDNI) metadata model requires extensions to specify how a CDN or Open Caching node should generate and evaluate these headers. Required capabilities: Set a default value for CORS response headers independent of the origin request header value.

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

Simple CORS requests are those where both HTTP method and headers in the request don´t require a preflight request. 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 is: 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.

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.
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.
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 require a CORS preflight request, the UA will use the OPTIONS method to the resource, including the origin header. If CORS is enabled and the request 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: Allowed methods

Allowed methods
Allowed credentials
Allowed request headers
Maximum age that the OPTIONS request is valid
Headers that can be exposed to the client

When an upstream CDN (uCDN) configures any of those advanced parameters, it is requesting the dCDN generate synthetic responses to OPTIONS requests. Therefore, no conditional request is performed to the uCDN origin. The uCDN SHOULD configure these values taking that into account. If some of the advanced parameters are empty, the dCDN would not send the corresponding header into the UA OPTIONS request. In cases where the uCDN only configures the MI.AccessControlAllowOrigin subobject, the dCDN will not generate synthetic responses to OPTIONS requests. Instead, the dCDN will validate with the uCDN every OPTIONS request to obtain the response. MI.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
- 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
Property: no-origin-response-headers
- Description: In the case of a request that has no Origin field, return this set of headers with the response.
- Type: Array of MI.HTTPHeader
- Mandatory-to-Specify: No
Property: apply-to-all-methods
- Description: By default, the CORS configuration refers to OPTIONS requests. Setting this flag to "True" applies the entire CORS configuration to the other methods as well.
- Type: Boolean
- Mandatory-to-Specify: No. The default is "False".

MI.AccessControlAllowOrigin The MI.AccessControlAllowOrigin object has the following properties:

Property: allow-list
- Description: List of valid URLs that will be used to match the request origin header. The origin header is an HTTP extension. Its value is a version of the Referer header in some specific requests, and used for cross-origin requests. Permitted values are schema://hostname[:port]
- Type: Array of PatternMatch objects, defined in section 4.1.5
- Mandatory-to-Specify: Yes
Property: wildcard-return
- Description: If "True", the OCN will include a wildcard (*) in the Access-Control-Allow-Origin response header. If "False", the OCN will reflect the request origin header in the Access-Control-Allow-Origin response header.
- Type: Boolean
- Mandatory-to-Specify: Yes

The examples below demonstrate how to configure response headers dynamically for CORS validation. The following is an example of a simple CORS validation configuration: { "generic-metadata-type": "MI.CrossoriginPolicy", "generic-metadata-value": { "allow-origin": { "allow-list": [ { "pattern": "*" } ], "wildcard-return": true } } } The following is an example of validation of a preflight request when some of the headers included in the subsequent object request are not included in the CORS specification safelist: { "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 } } }

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 (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.). MI.AllowCompress is a new GenericMetadata object that allows the dCDN to compress content before sending it to the client.

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

The following examples illustrate the use of MI.AllowCompress in the context of the Processing Stages Model that allowed for metadata to be applied conditionally based on evaluation of HTTP request headers. See the Processing Stages Metadata Specification and Metadata Model Expression Language (MEL) Specification . The following is an example of 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 } } ] } } The following is an example of an MI.AllowCompress that allows manifests (*.m3u8) to be compressed by the dCDN but normalizes 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 } } ] } }

MI.ClientConnectionControl Configuration metadata is required to define how connections against a client are maintained by a dCDN. Since the clients are typically owned/operated by a uCDN, giving this control to a uCDN allows it to accommodate device-specific constraints and performance optimization. A dCDN can also benefit from this configuration metadata to meet its security and resource consumption requirements. MI.ClientConnectionControl is a new GenericMetadata object that specifies how a dCDN manages its connections to clients/players.

Property: connection-keep-alive-time-ms
- Description: Specifies the time, in milliseconds, to keep an idle connection open.
- Type: Integer
- Mandatory-to-Specify: No. When not specified, a default value selected by the dCDN will be used.

The following example shows how a connection setup and a keep alive timeout can be set for client connections to a dCDN: { "generic-metadata-type": "MI.ClientConnectionControl", "generic-metadata-value": { "connection-keep-alive-time-ms": 3 } }

Conclusion This specification has defined configuration metadata objects related to controlling edge access to resources via CDNs and Open Caching systems. It has added to the set of basic CDNI configuration metadata objects defined in , and can be extended in the future with additional Edge Control Metadata object definitions.

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

IANA Considerations

CDNI Payload Types TBD.

Acknowledgements The authors would like to express their gratitude to the members of the Streaming Video Technology Alliance 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:

Guillaume Bichot - Broadpeak
Christoph Neumann - Broadpeak
Pankaj Chaudhari - Disney Streaming Services
Rajeev RK - picoNETS
Yoav Gressel - Qwilt
Arnon Warshavsky - QWilt
Shmuel Asafi . Qwilt
Nir Sopher - Qwilt
Arnon Warshavsky - Qwilt
Ben Rosenblum - Vecima