Define an X-TrackbackVersion header to communicate the version of the Trackback spec
It should be possible for a trackback implementation to implement both the original and the new trackback spec. The X-TrackbackVersion header would allow the client to specify which spec the ping conforms to. The header must be used for all implementations of the new spec. Older implementations either won't have the header or will specify X-TrackbackVersion: 1.0, making it clear which version of the spec is being used.
Create Section 2.x: The X-TrackbackVersion HTTP Request Header
The X-TrackbackVersion HTTP Request Header tells the server which version of the Trackback specification the Ping conforms to. The version number for this Trackback protocol specification is "2.0". Client implementations of this specification MUST include the X-TrackbackVersion HTTP header with all Pings.
POST /trackback-url HTTP/1.1 Host: example.org X-TrackbackVersion: 2.0 ...
Backwards compatibility with previous TrackBack implementations.