[MMUSIC] File transfer Issue 51: transfer identifier

[Miguel Garcia <Miguel.Garcia at nsn.com>]

In the file transfer draft, the current a=file-transfer attribute is 
used to indicate to the remote endpoint whether a new file transfer is 
requested or not. This allows to keep the file description without 
provoking a new file transfer, if, for example, the SDP is repeated 
(imagine, due to session timer re-invite).

The problem is: in order to keep the existing ongoing file transfer, the 
 SDP creator has to change the SDP and set a=file-transfer to 
"existing". This is a fragile mechanism: if the application just ships 
the last SDP without any changes, then it will provoke a restart of the 
file, due to the presence of a=file-transfer:new.

Ideally we would like a mechanism so that, in case the application 
repeats the SDP, nothing happens, i.e., if the file transfer is still 
going on, remains. If it is concluded, it is not restarted.

Jonathan suggested to use a transfer identifier instead. This looks 
interesting: When the endpoint repeats the SDP, if a transfer is still 
taking place, it continues. If it already concluded, nothing happens. If 
it is a new identifier, it will start at this point.

I propose to replace the current file-transfer attribute with a 
file-transfer-id attribute, according to Jonathan's suggestions.

/Miguel
--
Miguel A. Garcia           tel:+358-50-4804586
Nokia Siemens Networks     Espoo, Finland


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