The atom:id element is mandatory, but clients (doing POST of a new entity) might not know an appropriate value

6.2 Element atom:id

"The atom:id element MUST contain the entity-id"

This is a problem for clients creating new entities. In the case of a server supporting canonical URLs, if some of the key properties are server-generated, the client won't know what is an appropriate entity-id.

Also, the ATOM Syndication spec makes it clear that atom:entry must have an atom:id.

For OData, we need to define what value a client should pass for atom:id (in POST) when atom:id is not knowable to the client.