[ogpx] type-system : version tag and handling unknown tags

[Meadhbh Hamrick <[e-mail]>]

at least one person has commented privately about the need to add a
version tag to the binary serialization. i was initially resistant,
but now think it makes sense. the use case is we may want to have
future versions of the binary serialization with new tags.

so i propose we add this verbage to section 4.3 of the type system draft:

"version - this optional tag MUST be the first tag in a serialization
stream. it identifies the an optional version identifier, to be used
for future extension of the binary serialization. the version tag is
the lower case 'v' octet and is followed by a 32 bit value
representing the version."

related to the version tag is the idea of what to do with tags you
don't understand. many tools that process XML have a "ignore but
preserve" semantic that i think works quite well. our situation is
mildly complicated as we don't have a way to know the length of each
tag content defined in the future. ergo, i suggest we say that all
future tags consist of a tag octet followed by a 32 bit network order
unsigned integer representing the length of the tag contents in
octets. that way, if someone has inserted an experimental tag you
don't understand into a binary encoding, you can at least skip over
it.

so i also propose we add the following verbiage to section 4.3 of the
type system draft:

"implementers MAY choose to include experimental or private tags in a
binary serialization. implementations that receive tags they cannot
handle MUST preserve the tag in the serialization. to facilitate
processing of binary serializations with unknown tags, tags not
defined in this specification MUST be of the form: a single tag octet,
a 32 bit network order unsigned integer representing the length of the
tag's content, and the content."

-cheers
-meadhbh

--
meadhbh hamrick * it's pronounced "maeve"
@OhMeadhbh * http://meadhbh.org/ * [e-mail]