[rohc] Questions on Context Replication

["Cho Chia Yuan" <[e-mail]>]

Hi Ghyslain,
 
I have a few questions regarding <draft-ietf-rohc-context-replication-01.txt>:
 
1) Is the reverse feedback channel a compulsory pre-requisite for context replication or is it optional?
 
2) When the decompressor validation of the context following decompression of an IR-CR packet fails, is the STATIC-NACK sent back to the compressor optional or compulsory?
 
The nature of my questions arises from the following:
(3.3.3.)
   Context replication is designed to operate over links where a
   feedback channel is available. This is necessary to ensure that the
   information used to create a new context is synchronized between the
   compressor and the decompressor. ...
 
-The above suggests that the feedback channel is compulsory?
 
(3.4.3.)
   Specifically, when the decompressor fails to validate the context
   following the decompression of one or more initial IR-CR packets, it
   MUST invalidate the context and remain in its initial state. In
   addition, the decompressor SHOULD send a STATIC-NACK.
 
-Since reverse (positive and negative) feedbacks are all optional, does it mean that the feedback channel is optional?
 

My opinion is that the reverse feedback channel must be present for context replication, so that STATIC-NACKS are sent back to the compressor when context validation fails upon decompression of an IR-CR packet. IR-CR context validation failures must be communicated back to the compressor. This is because 'valid' base contexts at the compressor may be already invalidated at the decompressor. 
 
The case for this - suppose the reverse feedback channel is not present (or STATIC-NACKs are not sent back upon context validation failure at the decompressor), and furthermore the decompressor has discarded/overwritten all the 'valid' base contexts still kept by the compressor. Then, the compressor has no way of knowing the truth. After upwards transition from the CR state in optimistic mode, the compressor periodically transits downwards to IR state, in which it finds seemingly 'valid' base contexts, transits to CR state, and after a period of time optimistically proceeds to the 'Higher Order State', ... , and all the time nothing has been received successfully at all.
 
Thus, I think STATIC-NACKs for IR-CR packets should not be optional, because an avenue for fallback to IR packets, in the event of failure, must be provided.
 
Furthermore, if the policy enforced is such that the most recent valid base context is chosen for replication always, then maybe it can be assumed that if context validation for an IR-CR packet fails at the decompressor, there is no need for the compressor to attempt context replication from another base context? Thus, can the decompressor simply send back a STATIC-NACK to signal fallback to conventional context initialization? This may mean that the sending of STATIC-NACK would be MUST instead of SHOULD in the above section.
 
Chia Yuan Cho
National University of Singapore
/ Institute for Infocomm Research
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