OVF file format: envelope and file references

Source: DMTF DSP0243 2.1.1 clauses 5 to 7 and Annex D, and the DSP8023 2.0.1 schema (both read in full). The descriptor "shall validate against DSP8023".

Namespaces

Prefix	Namespace
`ovf`	`http://schemas.dmtf.org/ovf/envelope/2` (the OVF 1.x namespace ends `/1`)
`ovfenv`	`http://schemas.dmtf.org/ovf/environment/1`
`rasd`	`http://schemas.dmtf.org/wbem/wscim/1/cim-schema/2/CIM_ResourceAllocationSettingData`
`vssd`	…`/CIM_VirtualSystemSettingData`
`epasd`	…`/CIM_EthernetPortAllocationSettingData`
`sasd`	…`/CIM_StorageAllocationSettingData`
`cim`	`http://schemas.dmtf.org/wbem/wscim/1/common`

The choice of prefix is arbitrary. The schema's other namespaces are the CIM boot settings (CIM_BootConfigSetting, CIM_BootSourceSetting) and XML Signature and Encryption.

The Envelope element

The outermost level consists of, in order (XSD sequence): References (required), any number of sections (package-level meta-data), one Content (a VirtualSystem or VirtualSystemCollection, required), and any number of Strings message bundles. The attribute xml:lang is optional and gives the default locale of the descriptor's messages (default en-US). The version is given by the namespace.

References and File

The References element lists the files of the package so that a tool can check a package without interpreting the descriptor. External string resource bundles shall be first. Each File:

Attribute	Required	Meaning and rules
`ovf:id`	yes	The reference key used elsewhere (`ovf:fileRef`); unique in the package.
`ovf:href`	yes	A URL: a relative path (relative to the descriptor, RFC 3986 syntax, no `..` segments) or `file`, `http`, `https` URL; other schemes should not be used. The referenced file shall exist. Two Files shall not refer to the same file.
`ovf:size`	no	The size in bytes (`xs:unsignedLong`); should match the real size; for a compressed file, the compressed size.
`ovf:compression`	no	`gzip` (RFC 1952), or `identity` or absent for none. (The XSD's documentation also names `bzip2`.) Over HTTP the server may state it with Content-Encoding instead.
`ovf:chunkSize`	no	The size of each chunk but the last, when the file is split. The File then names a chunk file: the real names are `href` + "." + a 9-digit number from 000000000.

When a file is compressed and chunked, the whole file is compressed first and the chunks are equal slices of the compressed data. Manifest entries use the chunk names. The manifest and certificate shall not be listed.

Content: VirtualSystem and VirtualSystemCollection

There shall be one or more Content elements. Each has an ovf:id (required; unique among the direct children of a collection), an Info (required: a human-readable description), an optional Name (localisable), and sections. A VirtualSystem describes one virtual system and shall have a VirtualHardwareSection. A VirtualSystemCollection holds zero or more VirtualSystem or VirtualSystemCollection elements, and the sections that apply to the whole collection (it shall not have a VirtualHardwareSection).

Extensibility and conformance

New sections may be defined (members of the Section substitution group); each shall contain an Info.
The schemas are open: types may be extended at the end with elements of another namespace, and attributes of another namespace may be added. Custom extensions shall not use the namespaces of the specification.
ovf:required on an extension element says whether it is mandatory (default true). A deployment function that meets a mandatory element it does not understand shall fail; information in custom attributes shall not be required for correct behaviour.
Conformance level 1: only elements and attributes of the specification. Level 2: custom ones, all optional. Level 3: at least one required custom extension, whose definition shall be public as an open XML Schema; avoid it for portable packages. The level is not stated in the file.

Message bundles (Strings)

Elements that can be localised carry an optional ovf:msgid; a Strings element (xml:lang, optional ovf:fileRef) holds Msg elements (ovf:msgid required, optional ovf:fileRef to a UTF-8 plain-text file that holds the message). Internal bundles are at the end of the Envelope; an external bundle file is listed first in References, holds exactly one Strings element, and that element has no ovf:fileRef. Later values of a msgid in a locale overwrite earlier ones.