CDNI Metadata Expression Language
draft-power-metadata-expression-language-01
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Will Power , Glenn Goldstein , Arnon Warshavsky | ||
| Last updated | 2024-03-04 | ||
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draft-power-metadata-expression-language-01
Content Delivery Networks Interconnection W. Power
Internet-Draft G. Goldstein
Intended status: Standards Track Lumen Technologies
Expires: 5 September 2024 A. Warshavsky
Qwilt
4 March 2024
CDNI Metadata Expression Language
draft-power-metadata-expression-language-01
Abstract
This document specifies the syntax and provides usage examples for an
expression language to be used within Streaming Video Technology
Alliance (SVTA)/Content Delivery Network Interconnection (CDNI)
Metadata Interface (MI) objects. The purpose of this expression
language is to enable metadata to be applied conditionally (based on
aspects of an HTTP request), and to enable Hypertext Transfer
Protocol (HTTP) responses to be generated or altered dynamically.
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
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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 5 September 2024.
Copyright Notice
Copyright (c) 2024 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
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extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Expression Usage In CDNI Metadata Model . . . . . . . . . . . 3
4. Expression Syntax BNF . . . . . . . . . . . . . . . . . . . . 3
5. Core Variables . . . . . . . . . . . . . . . . . . . . . . . 5
6. Operators And Keywords . . . . . . . . . . . . . . . . . . . 6
7. Built-In Functions . . . . . . . . . . . . . . . . . . . . . 8
7.1. Type Conversions . . . . . . . . . . . . . . . . . . . . 8
7.2. String Conversions . . . . . . . . . . . . . . . . . . . 9
7.3. Convenience Functions . . . . . . . . . . . . . . . . . . 9
8. User-Defined Variables . . . . . . . . . . . . . . . . . . . 11
8.1. MI.SetVariable . . . . . . . . . . . . . . . . . . . . . 12
9. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Compile-Time Errors . . . . . . . . . . . . . . . . . . . 13
9.2. Runtime Errors . . . . . . . . . . . . . . . . . . . . . 14
10. FCI Capabilities . . . . . . . . . . . . . . . . . . . . . . 14
10.1. FCI.SupportedMELFeatures . . . . . . . . . . . . . . . . 15
11. Informative Examples . . . . . . . . . . . . . . . . . . . . 16
11.1. MI.ComputedCacheKey . . . . . . . . . . . . . . . . . . 16
11.2. MI.ExpressionMatch . . . . . . . . . . . . . . . . . . . 17
11.3. MI.ResponseTransform . . . . . . . . . . . . . . . . . . 17
12. Security Considerations . . . . . . . . . . . . . . . . . . . 18
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
13.1. CDNI Payload Types . . . . . . . . . . . . . . . . . . . 18
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
15. Normative References . . . . . . . . . . . . . . . . . . . . 19
16. Informative References . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
The CDNI metadata model defined in [RFC8006] allows metadata to be
applied conditionally based on matches to an HTTP request hostname
and/or path While this is sufficient for many use cases, the need
often arises for configuration metadata (such as caching rules) to be
applied conditionally based on matching values of HTTP request
headers. Additionally, content delivery networks (CDNs) often need
to conditionally modify responses before forwarding them to a client.
These modifications typically involve adding, deleting, or modifying
HTTP response headers or synthesizing responses. A set of CDNI
metadata objects for conditionally applying metadata and modifying
responses is defined in the Processing Stages Metadata Specification
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[SVTA2032], which requires an expression language to enable the
specification of match expressions and synthetically generated
values.
This document specifies the syntax and provides usage examples for an
expression language to be used within SVTA/CDNI Metadata Interface
(MI) objects, such as those defined within CDNI Metadata Model
Extensions [SVTA2029] and its subparts.
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. Expression Usage In CDNI Metadata Model
The CDNI Metadata Expression Language (MEL) provides a syntax with a
rich set of variables, operators, and built-in functions to
facilitate two key use cases within the CDNI metadata model:
* Match Expressions - Expressions that evaluate to a Boolean value
are used to dynamically determine if metadata should be applied
based on evaluation of aspects of an inbound HTTP request
(matching to a header value, for example).
* Value Expressions - Enable the dynamic construction of a value to
be used in scenarios such as constructing a cache key, setting an
HTTP response header or status code, rewriting a request Uniform
Resource Identifier (URI), or dynamically generating a response
body.
Refer to the Processing Stages Metadata Specification [SVTA2032] for
example usage of both match expressions and value expressions within
the Processing Stages Model.
Refer to the Cache Control Metadata Specification [SVTA2033] for
example usage of Value Expressions to dynamically generate cache
keys.
4. Expression Syntax BNF
<list> ::= "[" "]" | "[" <expr_list> "]"
<expr_list> ::= <expr> | <expr_list> "," <expr>
<expr> ::= <identifier> "=" <expr> | <function_call> |
<expr> <operator> <expr> |
<prefix_op> <expr> | <literal>
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<function_call> ::= <function_name> "(" ")" |
<function_name> "(" <expr_list> ")"
<operator> ::= <logic_operator> | <math_operator> |
<bit_operator> | <string_operator> |
<comparison_operator> | <regex_operator> |
<glob_operator> | <ip_operator>
<prefix_op> ::= "+" | "-" | "!" | "~"
<literal> ::= NUMBER | STRING | "true" | "false" | "nil"
<logic_operator> ::= "and" | "or"
<math_operator> ::= "+" | "-" | "*" | "/" | "%"
<bit_operator> ::= "|" | "&" | "~" | "<<" | ">>"
<string_operator> ::= "."
<comparison_operator> ::= "==" | "!=" | ">" | "<" | "<=" | ">="
<regex_operator> ::= "regexmatch" | "~=" | "regexmatchi" |
"!regexmatch" | "!regexmatchi"
<glob_operator> ::= "globmatch" | "*=" | "globmatchi" |
"%*=" | "!globmatch" | "!*=" |
"!globmatchi" | "!%*="
<ip_operator> ::= "ipmatch" | "!ipmatch"
<function_name> ::= "integer" | "real" | "string" | "boolean" |
"upper" | "lower" |
"match" | "match_replace" | "add_query" |
"add_query_multi" | "remove_query" |
"remove_query_multi" |
"path_element" | "path_elements"
<identifier> ::= <alpha_char> <identifier_word> |
"_" <identifier_word>
<identifier_word> ::= <identifier_char> |
<identifier_char> <identifier_word>
<identifier_char> ::= <alpha_numeric> | "." | "-" | "_" | "#"
<alpha_numeric> ::= <alpha_char> | <digit>
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<alpha_char> ::= "a-z" | "A-Z"
<digit> ::= "0" | "1" | "2" | "3" | "4" |
"5" | "6" | "7" | "8" | "9"
Figure 1
5. Core Variables
The expression language supports two namespaces of core variables,
each with a distinct prefix:
* req. Prefix for accessing attributes of the HTTP request object
* resp. Prefix for accessing attributes of the HTTP response object
+=======================+================================+==========+
| Variable | Meaning | Type |
+=======================+================================+==========+
| req.h.<name> | Request header <name> | String |
+-----------------------+--------------------------------+----------+
| req.uri | Request URI (includes query | String |
| | string and fragment | |
| | identifier, if any) | |
+-----------------------+--------------------------------+----------+
| req.uri.path | Request URI path | String |
+-----------------------+--------------------------------+----------+
| req.uri.pathquery | Request path and query | String |
| | string | |
+-----------------------+--------------------------------+----------+
| req.uri.query | Request query string | String |
+-----------------------+--------------------------------+----------+
| req.uri.query.<key> | Request query string value | String |
| | associated with <key>. If | |
| | the key is not present in | |
| | the uri, nil is returned. | |
| | If the key is present with | |
| | no value, as in "a=", then | |
| | an empty string is returned. | |
+-----------------------+--------------------------------+----------+
| req.uri.querykv.<key> | Request query string key and | String |
| | value associated with <key>, | |
| | returned as a single String | |
| | exactly as-is from the | |
| | request url.For example, | |
| | when used with a uri | |
| | containing a query string | |
| | "key=xxx", expression would | |
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| | return the string "key=xxx". | |
| | When used with a uri | |
| | containing a query string | |
| | "key=", expression would | |
| | return the string "key=". | |
+-----------------------+--------------------------------+----------+
| req.method | Request HTTP method (GET, | String |
| | POST, etc.) | |
+-----------------------+--------------------------------+----------+
| req.scheme | Request scheme (http or | String |
| | https) | |
+-----------------------+--------------------------------+----------+
| req.clientip | IP address of the client | String |
| | that made the request. | |
| | Note: IPv6 addresses MUST | |
| | NOT be enclosed in square | |
| | brackets []. | |
+-----------------------+--------------------------------+----------+
| req.clientport | Request port number | Unsigned |
+-----------------------+--------------------------------+----------+
| resp.h.<name> | Response header <name> | String |
+-----------------------+--------------------------------+----------+
| resp.status | Response status code | Unsigned |
+-----------------------+--------------------------------+----------+
Table 1
6. Operators And Keywords
+==========+==========+==========+================================+
| Operator | Type | Result | Meaning |
| | | Type | |
+==========+==========+==========+================================+
| == | infix | Boolean | Equality test |
+----------+----------+----------+--------------------------------+
| != | infix | Boolean | Inequality test |
+----------+----------+----------+--------------------------------+
| ! | infix | Boolean | Logical NOT operator |
+----------+----------+----------+--------------------------------+
| > | infix | Boolean | Greater than test |
+----------+----------+----------+--------------------------------+
| < | infix | Boolean | Less than test |
+----------+----------+----------+--------------------------------+
| >= | infix | Boolean | Greater than or equal test |
+----------+----------+----------+--------------------------------+
| <= | infix | Boolean | Less than or equal |
+----------+----------+----------+--------------------------------+
| *= | infix | Boolean | Glob style match |
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+----------+----------+----------+--------------------------------+
| ~= | infix | Boolean | Regular expression match |
| | | | (see [PCRE] for details on |
| | | | PCRE RegEx matching) |
+----------+----------+----------+--------------------------------+
| ipmatch | infix | Boolean | Match against IP address or |
| | | | CIDR (IPv4 and IPv6). For an |
| | | | example IP address of |
| | | | '10.2.3.4', matching against |
| | | | '10.2.3.0/24' would return |
| | | | true, while matching against |
| | | | '10.2.3.5' would return false. |
+----------+----------+----------+--------------------------------+
| + | infix | Numeric | Addition |
+----------+----------+----------+--------------------------------+
| - | infix | Numeric | Subtraction |
+----------+----------+----------+--------------------------------+
| * | infix | Numeric | Multiplication |
+----------+----------+----------+--------------------------------+
| / | infix | Numeric | Division |
+----------+----------+----------+--------------------------------+
| % | infix | Unsigned | Modulus |
| | | or | |
| | | Integer | |
+----------+----------+----------+--------------------------------+
| . | infix | String | Concatenation. Note: There |
| | | | MUST be at least 1 space |
| | | | before/after the dot operator. |
+----------+----------+----------+--------------------------------+
| ? : | ternary | * | Conditional operator: <e> ? |
| | | | <v1> : <v2>Evaluates <v1> if |
| | | | <e> is true, <v2> otherwise. |
+----------+----------+----------+--------------------------------+
| ( ) | grouping | | Used to override precedence |
| | | | and used for function calls. |
+----------+----------+----------+--------------------------------+
Table 2
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+=========+=========================================+
| Keyword | Meaning |
+=========+=========================================+
| and | Logical AND |
+---------+-----------------------------------------+
| or | Logical OR |
+---------+-----------------------------------------+
| not | Logical NOT (see also the "!" operator) |
+---------+-----------------------------------------+
| nil | No value (distinct from empty value) |
+---------+-----------------------------------------+
| true | Boolean constant: true |
+---------+-----------------------------------------+
| false | Boolean constant: false |
+---------+-----------------------------------------+
Table 3
7. Built-In Functions
To enable the types of expressions typically used in content delivery
scenarios to evaluate and generate or modify HTTP headers, the
following set of built-in functions are defined to facilitate format
conversions, matching, and query string management. Any dCDN that
advertises support for a metadata property that leverages MEL MUST
provide implementations of these built-in functions.
7.1. Type Conversions
+============+=====================+=============+=========+
| Function | Action | Argument(s) | Returns |
+============+=====================+=============+=========+
| integer(e) | Converts expression | 1 | integer |
| | to integer. | | |
+------------+---------------------+-------------+---------+
| real(e) | Converts expression | 1 | real |
| | to real. | | |
+------------+---------------------+-------------+---------+
| string(e) | Converts expression | 1 | string |
| | to string. | | |
+------------+---------------------+-------------+---------+
| boolean(e) | Converts expression | 1 | Boolean |
| | to Boolean. | | |
+------------+---------------------+-------------+---------+
Table 4
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The following table summarizes the return values of type conversion
functions when presented with edge-case input values:
+============+===============+=======+==============+============+
| Function | non-zero | nil | non-numeric | numeric |
| | numeric input | input | string input | zero input |
+============+===============+=======+==============+============+
| integer(e) | integer | 0 | 0 | 0 |
+------------+---------------+-------+--------------+------------+
| real(e) | real | 0 | 0 | 0 |
+------------+---------------+-------+--------------+------------+
| string(e) | string | 'nil' | string | '0' |
+------------+---------------+-------+--------------+------------+
| boolean(e) | true | false | true | false |
+------------+---------------+-------+--------------+------------+
Table 5
7.2. String Conversions
+==========+==============================+=============+=========+
| Function | Action | Argument(s) | Returns |
+==========+==============================+=============+=========+
| upper(e) | Converts a string to | 1 | string |
| | uppercase. Useful for case- | | |
| | insensitive comparisons. | | |
+----------+------------------------------+-------------+---------+
| lower(e) | Converts a string to | 1 | string |
| | lowercase. Useful for case- | | |
| | insensitive comparisons. | | |
+----------+------------------------------+-------------+---------+
Table 6
7.3. Convenience Functions
+===========================+=====================+======+=========+
| Function | Action | Args | Returns |
+===========================+=====================+======+=========+
| match(string Input, | Regular expression | 2 | string |
| string Match) | 'Match' is applied | | |
| | to Input and the | | |
| | matching element | | |
| | (if any) is | | |
| | returned. An empty | | |
| | string is returned | | |
| | if there is no | | |
| | match.See | | |
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| | [PCRE] for details | | |
| | on PCRE RegEx | | |
| | matching. | | |
+---------------------------+---------------------+------+---------+
| match_replace(string | Regular expression | 3 | string |
| Input, string Match, | 'Match' is applied | | |
| string Replace) | to the Input | | |
| | argument and | | |
| | replaced with the | | |
| | Replace | | |
| | argument upon | | |
| | successful match. | | |
| | It returns the | | |
| | updated (replaced) | | |
| | version of Input. | | |
+---------------------------+---------------------+------+---------+
| add_query(string Input, | Add query string | 2 | string |
| string q, string v) | element q with | | |
| | value v to the | | |
| | Input string. If v | | |
| | is nil, just add | | |
| | the query string | | |
| | element q. The | | |
| | query element q and | | |
| | value v MUST | | |
| | conform to the | | |
| | format defined in | | |
| | [RFC3986]. | | |
+---------------------------+---------------------+------+---------+
| add_query_multi(string | Add all the query | 2 | string |
| input, string qvs) | value elements from | | |
| | qvs to the input | | |
| | string. For | | |
| | example, if qvs = | | |
| | "k1=v1, k2=v2, | | |
| | k3=v3"), parameters | | |
| | k1, k2, k3 and | | |
| | associated values | | |
| | would be added to | | |
| | input.If a | | |
| | qvs element only | | |
| | has a key but no | | |
| | value, the existing | | |
| | value of that key | | |
| | will be kept. | | |
+---------------------------+---------------------+------+---------+
| remove_query(string | Remove all | 2 | string |
| Input, string q) | occurrences of | | |
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| | query string | | |
| | element q from the | | |
| | Input string. | | |
+---------------------------+---------------------+------+---------+
| remove_query_multi(string | Remove all | 2 | string |
| input, string qs) | occurrences of the | | |
| | query string | | |
| | elements referenced | | |
| | in parameter qs | | |
| | from the input | | |
| | string. For | | |
| | example, if qs= | | |
| | "k1, k2, k3", all | | |
| | occurrences of k1, | | |
| | k2, k3 would be | | |
| | removed from input. | | |
+---------------------------+---------------------+------+---------+
| keep_query_multi(string | Remove all query | 2 | string |
| input, string qs) | string elements | | |
| | from input except | | |
| | for the elements | | |
| | referenced in | | |
| | parameter qs. For | | |
| | example, if qs = | | |
| | "k1, k2, k3", all | | |
| | query string | | |
| | elements would be | | |
| | removed from input | | |
| | except for k1, k2, | | |
| | k3. | | |
+---------------------------+---------------------+------+---------+
| path_element(string | Return the path | 2 | string |
| Input, integer n) | element n from | | |
| | Input. -1 returns | | |
| | the last element. | | |
+---------------------------+---------------------+------+---------+
| path_elements(string | Return the path | 3 | string |
| Input, integer n, integer | elements from | | |
| m) | position n to m. | | |
+---------------------------+---------------------+------+---------+
Table 7
8. User-Defined Variables
In addition to the core variable namespaces that address HTTP request
and response objects (req. and resp.), MEL supports user-defined
variables that can be referenced via the following prefix:
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* var. Prefix for accessing user-defined variables set via
MI.SetVariable
A user defined variable is available for access in MEL expressions
from the moment it is assigned, throughout the duration of the
transaction. If a user variable is accessed before it has been set,
it returns the value of nil.
8.1. MI.SetVariable
MI.SetVariable is a GenericMetadata object that allows one to set
user-defined variables that can be accessed from MEL. Variables set
via this mechanism can be accessed in matching and value construction
expressions using a var. prefix before the variable name. Note:
Variable names MUST be string literals.
Property: variable-name
* Description: The name of the variable.
* Type: String. Alphanumeric with both uppercase and lowercase
characters; the property MUST start with a letter.
* Mandatory-to-Specify: Yes
Property: variable-value
* Description: The string representation of the value to set for the
user-defined variable.
* Type: String, either the static value or a MEL expression,
determined by the value-is-expression property.
* Mandatory-to-Specify: Yes
Property: value-is-expression
* Description: A flag to signal whether the value is a static string
literal or a 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.
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The following example illustrates the setting of a user-defined
variable whose value is a MEL expression suffixed by a static literal
string:
{
"generic-metadata-type": "MI.SetVariable",
"generic-metadata-value": {
"variable-name": "myvar",
"variable-value": "req.h.host . 'is my host from player'"
"value-is-expression": true
}
}
Figure 2
The following example illustrates the setting of a user-defined
variable whose value is another user-defined variable suffixed by a
static literal string:
{
"generic-metadata-type": "MI.SetVariable",
"generic-metadata-value": {
"variable-name": "myvar1",
"variable-value": "req.host . 'is my host from player'"
"value-is-expression": true
}
}
{
"generic-metadata-type": "MI.SetVariable",
"generic-metadata-value": {
"variable-name": "myvar2",
"variable-value": "var.myvar1 . 'and forwarded to Origin'"
"value-is-expression": true
}
}
Figure 3
9. Error Handling
9.1. 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. An HTTP 500 'Internal Server Error' SHOULD be returned
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with a message that indicates the source of the error (line number
and configuration element that caused the error).
Examples of MEL compile-time errors:
* Unknown MEL variable name referenced in an expression
* Unknown MEL operator, keyword, or functions referenced in an
expression
* Incorrect number of arguments used in a MEL expression operator or
function
* Incorrect type of argument used in a MEL expression operator or
function
9.2. Runtime Errors
If a runtime error is detected when processing a request, the request
SHOULD be terminated, and an 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 diagnostic information to
assist in troubleshooting.
Examples of runtime errors:
* Failure to allocate memory (or other server resources) when
evaluating a MEL expression
* Incorrect runtime argument type in a MEL expression, such as,
trying to convert a non-numeric string to a number
10. FCI Capabilities
Since implementing the full MEL specification may be complex and
onerous, a mechanism is needed for a dCDN to advertise what portions
of the MEL standard it supports (if any). This section introduces
the FCI.SupportedMELFeatures object, which can be contained within
the Capabilities Advertisement object defined in [RFC8008] section 5,
or embedded in an FCI.MetadataExtended object as defined in
[SVTA2041], the SVTA Configuration Interface Metadata Capabilities
Specification.
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If FCI.SupportedMELFeatures is provided within an Capabilities
Advertisement object and a FCI.MetadataExtended object for a given
footprint, the advertisement within FCI.MetadataExtended MUST take
precedence.
10.1. FCI.SupportedMELFeatures
The FCI.SupportedMELFeatures object is contained in FCI Capabilities
advertisements and allows a dCDN to advertise the portions of the MEL
specification that it supports. If a capabilities advertisement does
not contain a FCI.SupportedMELFeatures or a FCI.MetadataExtended
object with an embedded FCI.SupportedMELFeatures object, the upstream
content delivery network (uCDN) MUST assume that the dCDN has support
for the full MEL specification.
Property: keywords
* Description: A list of supported keywords.
* Type: Array
* Mandatory-to-Specify: No. If not specified, it is assumed to be
empty.
* Values: ["and", "or", "not", "nil", "true", "false"]
Property: operators
* Description: A list of supported operators.
* Type: Array
* Mandatory-to-Specify: No. If not specified, it is assumed to be
empty.
* Values: ["==", "!=", "!", ">", "<", ">=", "<=", "*=", "~=", "+",
"-", "*", "/", "%", " . ", "()", "?:", "ipmatch"]
Property: variables
* Description: A list of supported variables.
* Type: Array
* Mandatory-to-Specify: No. If not specified, it is assumed to be
empty.
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* Values: ["req.h.<name>", "req.uri", "req.uri.path",
"req.uri.pathquery", "req.uri.query", "req.uri.query.<key>",
"req.uri.querykv.<key>", "req.method", "req.scheme",
"resp.h.<name>", "resp.status", "req.clientip", "req.clientport",
"var.<user-variable>" ]
Property: built-in-functions
* Description: A list of supported built-in function names.
* Type: Array
* Mandatory-to-Specify: No. If not specified, it is assumed to be
empty
* Values: ["integer", real", "string", "boolean", upper", "lower",
"match", "match_replace", "add_query", "remove_query",
"path_element", "path_elements", "add_query_multi",
"remove_query_multi", "keep_query_multi"]
The following example advertises a subset of MEL support:
{
"capabilities": [
{
"capability-type": "FCI.SupportedMELFeatures",
"capability-value": {
"keywords": ["and", "or", "not", "nil", "true", "false"],
"operators" : ["==", "!=", "!", ">", "<", ">=",
"<=", "~=", " . ", "()" ],
"variables": ["req.h.<name>", "req.uri", "req.uri.path" ],
"built-in-functions": [ "string", "boolean", "upper",
"lower", "match", "match_replace", "path_element"]
}
}
]
}
Figure 4
11. Informative Examples
11.1. MI.ComputedCacheKey
The following examples illustrate usage of the Metadata Expression
Language to dynamically generate cache keys, as specified by the
MI.ComputedCacheKey object in the Cache Control Metadata
Specification [SVTA2033]
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Example setting the cache key to the value of the X-Cache-Key header
from the client HTTP request:
{
"generic-metadata-type": "MI.ComputedCacheKey",
"generic-metadata-value": {
"expression": "req.h.x-cache-key"
}
}
Figure 5
Example setting the cache key to the request URI, forced to lower-
case:
{
"generic-metadata-type": "MI.ComputedCacheKey",
"generic-metadata-value": {
"expression": "lower(req.uri)"
}
}
Figure 6
11.2. MI.ExpressionMatch
The following example illustrates usage of the Metadata Expression
Language to create a match expression, as specified by the Processing
Stages Metadata Specification [SVTA2032]
In this example, the expression is true if the user-agent (glob)
matches '*Safari*' and the referrer equals 'www.x.com'.
{
"generic-metadata-type": "MI.MatchExpression",
"generic-metadata-value": {
"expression": "req.h.user-agent *= '*Safari*' and
req.h.referer == 'www.x.com'"
}
}
Figure 7
11.3. MI.ResponseTransform
The following example illustrates usage of the Metadata Expression
Language to alter the headers of an HTTP response, as specified by
the Processing Stages Metadata Specification [SVTA2032]
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An HTTP response is transformed by adding a dynamically constructed
header with a value that uses the expression language to concatenate
the values of the user-agent and host header.
{
"generic-metadata-type": "MI.ResponseTransform",
"generic-metadata-value": {
"header-transform": {
"add": [
{
"name": "X-custom-response-header",
"value": "req.h.user-agent . '-' . req.h.host",
"value-is-expressions": true
}
]
}
}
}
Figure 8
12. Security Considerations
The FCI and MI objects defined in the present document are
transferred via the interfaces defined in CDNI [RFC8006] which
describes how to secure these interfaces protecting integrity and
confidentiality while ensuring the authenticity of the dCDN and uCDN.
13. IANA Considerations
13.1. CDNI Payload Types
This document requests the registration of the following entries
under the "CDNI Payload Types" registry hosted by IANA:
+--------------------------+---------------+
| Payload Type | Specification |
+--------------------------+---------------+
| MI.SetVariable | RFCthis |
+--------------------------+---------------+
| FCI.SupportedMELFeatures | RFCthis |
+--------------------------+---------------+
Table 8: CDNI Payload Types
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14. 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:
* Pankaj Chaudhari - Disney Streaming Services
* Rajeev RK - picoNETS
* Yoav Gressel - Qwilt
* Alfonso Siloniz - Telefonica
* Ben Rosenblum - Vecima
15. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC8006] Niven-Jenkins, B., Murray, R., Caulfield, M., and K. Ma,
"Content Delivery Network Interconnection (CDNI)
Metadata", RFC 8006, DOI 10.17487/RFC8006, December 2016,
<https://www.rfc-editor.org/info/rfc8006>.
[RFC8008] Seedorf, J., Peterson, J., Previdi, S., van Brandenburg,
R., and K. Ma, "Content Delivery Network Interconnection
(CDNI) Request Routing: Footprint and Capabilities
Semantics", RFC 8008, DOI 10.17487/RFC8008, December 2016,
<https://www.rfc-editor.org/info/rfc8008>.
16. Informative References
[PCRE] PCRE, "Perl Compatible Regular Expressions",
<https://www.pcre.org/>.
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[SVTA] SVTA, "Streaming Video Technology Alliance Home Page",
<https://www.svta.org>.
[SVTA2029] SVTA, "CDNI Metadata Model Extensions",
<https://svta.org/documents/SVTA2029>.
[SVTA2032] SVTA, "Processing Stages Metadata Specification",
<https://svta.org/documents/SVTA2032>.
[SVTA2033] SVTA, "Cache Control Metadata",
<https://svta.org/documents/SVTA2033>.
[SVTA2041] SVTA, "Metadata Capabilities",
<https://svta.org/documents/SVTA2041>.
Authors' Addresses
Will Power
Lumen Technologies
United States of America
Email: wrpower@gmail.com
Glenn Goldstein
Lumen Technologies
United States of America
Email: glenng1215@gmail.com
Arnon Warshavsky
Qwilt
Israel
Email: arnon@qwilt.com
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