Subject: RE: [rohc] RE: I-D ACTION:draft-ietf-rohc-context-replication-04. txt

From: [email-masked]
Date: Fri, 24 Sep 2004 10:41:01 -0400

Hi Ghyslain,

Please see my comments below.

Br, Zhigang

> > Another question on the state transition in general: why ACK is 
> > optional for transition from CR state to higher state? 
> > 
> > RFC 3095 specifies that ACK is used to acknowledge successful 
> > decompression of an IR packet. Considering CR involves more factors 
> > than IR and thus is subject to more risks of robustness, shouldn't 
> > ACK be mandatory for IR-CR packets?
> 
> As long as the base context selected meets the requirement, 
> i.e. that is has been ACKed before and is still valid, there 
> is no more risks to the IR-CR packet than sending any other  
> compressed packet. 

Sorry, I missed the requirement that the base context MUST has been
ACKed. So I should correct myself to say IR-CR packets have the same
risks as IR packets.

> Thus having ACKs optional for that 
> transition and having the possibility to rely on the 
> optimistic approach is perfectly proper. 

I have to disagree. If IR-CR packets essentially replace IR packets for 
establishing a context and IR packets are ACKed per RFC 3095, it seems 
to make sense to have IR-CR packets ACKed too. The rational behind 
ACKing IR packets even in O-mode in RFC 3095 is that IR packets are
obviously very important. Another reason is that since IR packets
only happen at the beginning of a new context, the cost of sending ACKs
are negligible for the gained robustness, even in a perfect channel. 
I think the same applies to IR-CR packets. In addition, it makes the 
logic crisp clear (important for implementations as we learned from
work in ROHC): i.e. the decompressor must ACKs and the compressor must 
receive ACKs before moving to higher state. Once the compressor moves 
to higher state, it can then operate according the desired mode, either 
O-, R-, or any other modes specified for the profile. What do you think?

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From: "Lars-Erik Jonsson" <[email-masked]>
Date: Fri, 24 Sep 2004 18:07:59 +0200

> obviously very important.

Well the reason for acking an IR is basically that you want to move to
O-mode
from the initial U-mode. However, this is all optional in 3095 and there is
no
reason to do different because of replication.

/L-E