Subject: Re: [rohc] UOR-2 packets with TS_STRIDE and Tsc equal to 1 in Extension 3

From: "Kavitha KR" <[email-masked]>
Date: Mon, 5 Apr 2004 19:17:21 +0530

Hi Pawel,

>I have no doubt what decompressor should do when it receives UOR-2*
>with TS is unscaled form (i.e. Tsc = 0) and TS_STRIDE present (i.e. TSS =
1).
>But I still don't know what is correct decompressor's behaviour in case it

>receives UOR-2* packet with scaled TS (i.e. Tsc = 1) and TS_STRIDE present

>(i.e. TSS =1). How it can calculate TS_OFFSET needed to decompress the
received
>scaled TS value ?

====> Yeah. I see your point.It is not possible to decompress without the
new
      TS_OFFSET value. But new TS_OFFSET can be obtained only if you
decompress
      the TS field. So there is a confusion of whether to use context(TS)
      or the last successfully decompressed value of TS to get the
TS_OFFSET.

>Note that the following text from RFC 3095 section 5.7.5 page 85:
>" Tsc: Tsc = 0 indicates that TS is not scaled;
>  Tsc = 1 indicates that TS is scaled according to section
>  4.5.3, using value(TS_STRIDE). "
>For me the fact that the TS is scaled using "value(TS_STRIDE)" and not
context
>(TS_STRIDE) suggests that the TS can be scaled using TS_STRIDE sent in the
same
>packet as the TS field.

====>Yes,it seems that ROHC allows sending of packets that contain
TS_STRIDE and
     the TS_SCALED value, that is compressed using the same TS_STRIDE in a
single
     packet.
     But on the other hand, when we want to communicate TS_STRIDE, we
     are basically initializing the timestamp context. In which we should
explicitly
     communicate TS_STRIDE and implicitly communicate TS_OFFSET as well.
Which means
     we must send unscaled TS and TS_STRIDE for some packets until
decompressor
     confidence is obtained. [Please refer section 4.5.3 Page 29, Point 1
of RFC3095.]

Regards,
Kavitha.

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From: "Pawel Szymanski" <[email-masked]>
Date: Mon, 5 Apr 2004 16:25:49 +0200

Hi Kavitha,

See my comment below.

>      confidence is obtained. [Please refer section 4.5.3 Page 29, Point 1

Do you think that section 4.5.3 Page 29, Point 1 means that sending packets 
containing both scaled TS field and TS_STRIDE is illegal ?

In my opinion, the point you are refering to only describes the most common 
scenario of initializing TS_STRIDE and TS_OFFSET parameters. However it does 
not state clearly that other scenarios are illegal.

I know that in most cases it does not make sense to send such packets, but from 
the implementors point of view I would expect a clear statemenet (for example 
in the implementation guide) whether such packets are legal and what is the 
correct behaviour of the decompressor when it receives such a packet.

Regards 

Pawel

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