Re: [MMUSIC] File transfer Issue 51: transfer identifier

[Paul Kyzivat <pkyzivat at cisco.com>]

inline

Miguel Garcia wrote:
> 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.

This was a major concern of mine when discussing comedia.
I tried very hard to come up with a mechanism that had this property. 
But in the end I was unsuccessful. What is proposed here is what was 
used in comedia.

I will have to dig back through all the discussion of comedia to 
discover what the major impediments were to a mechanism with this 
property. I just don't remember any longer.

> 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'm quite sure we investigated mechanisms like this for comedia and 
found them problematic for some reason. Perhaps Gonzalo remembers - he 
was the other person involved in that I think.

	Paul

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

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