[rohc] ROHC context reuse & mode

[Kristofer Sandlund <kristofer.sandlund at effnet.com>]

Hi all!

I think I have found an ambiguity in RFC3095 that I would like to have 
clarified. I will explain the situation with examples.

Example 1:
A ROHC compressor is running in O-mode on context #1 (say for example 
IPv4/UDP/RTP, profile 1), when it decides to reuse this this context for a 
completely different flow (say IPv6/UDP/RTP, profile 1).
The compressor will of course have to send an IR packet to the decompressor, but 
what mode must it set in the IR packet? Do I inherit the mode of the old flow or 
do I go back to U-mode?

RFC3095 Section 4.4.1.  "Unidirectional mode -- U-mode" says:
   Compression with ROHC MUST start in the Unidirectional mode.

Does this mean that the compressor starts in U-mode "when the channel is set-up" 
or "whenever the compressor re-initializes a context"?
If I interpret it as the latter, then the decompressor will see an IR with 
U-mode while being in O-mode, i.e. a mode change occurs without the 3-way handshake.

Example 2:
The compressor has received the CONTEXT_REINITIALIZATION signal (section 6.3.1), 
but the channel is kept up and running. Then it seems most likely that 
compression must start in U-mode since you are supposed to re-establish 
everything (true?). The decompressor is in O-mode in this example as well. So it 
seems we cannot completely avoid a situation where the compressor could be 
forced to send U-mode IRs to an O-mode decompressor.

"Bonus questions":
* How does this work in profiles 2 & 3 where there is no mode parameter to send 
in the IR packets? Do I inherit "current mode" or do I inherit "initial mode"?
* If I reuse a profile 1 context in R-mode and make it a profile 0 or TCP 
profile context (these do not have R-mode), what does my compressor send? I 
cannot inherit "current mode" (R-mode), since it does not exist, and it seems 
like inheriting "initial mode" for the selected profile would be the only choice 
here.


So, this might seem like a pure implementation choice at first, but I think this 
needs to be clarified, since the questions posed above seem pretty unclear to me.

Regards,
    Kristofer Sandlund, Effnet AB


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