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If a client has a partial copy of an object in its cache and wishes to have an up-to-date copy of the entire object in its cache, it can use the Range request-header with a conditional GET using either or both of If-Unmodified-Since and If-Match headers. If the condition fails because an aliased object was updated, the client must make a second request to obtain the entire current object. The If-Range header allows a client to "short-circuit" the second request. The meaning is ; "if the object is unchanged, send me the missing part(s) that I am missing; otherwise, send me the entire object."
If the client has no entity tag for an object but has a Last-Modified date, it can use that date in an If-Range header. Swarm can distinguish between a valid HTTP-date and any form of entity-tag by looking for double quotes. The If-Range header should be used together with a Range header, and is ignored if the request does not include a Range header.
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