Subject: [rohc] RFC4815 no TS bits

From: Jer-ming Lin <[email-masked]>
Date: Wed, 03 Feb 2010 21:07:41 +0800

Dear ROHCer,

     In the RFC4815 4.2 "When no TS bits are received in the  
compressed header, the scaled TS value is reconstructed assuming a  
linear extrapolation from the SN, i.e., delta_TS = delta_SN *  
default-slope, where delta_SN and delta_TS are both signed integers."

     Does that mean the decompressor must receive at least one scaled  
TS before get no-TS bits? How about absolute TS or unscaled TS? Since  
the decompressor can derive the scaled TS directly when it gets the  
unscaled TS, the compressor can send several unscaled TS to establish  
the TS_STRIDE and the relationship of delta_TS. Is that right?

Sincerely,
Jer-ming

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From: Carl Knutsson <[email-masked]>
Date: Thu, 04 Feb 2010 09:57:11 +0100

Jer-ming,

Jer-ming Lin wrote:
> Dear ROHCer,
> 
>     In the RFC4815 4.2 "When no TS bits are received in the compressed
> header, the scaled TS value is reconstructed assuming a linear
> extrapolation from the SN, i.e., delta_TS = delta_SN * default-slope,
> where delta_SN and delta_TS are both signed integers."
> 
>     Does that mean the decompressor must receive at least one scaled TS
> before get no-TS bits? How about absolute TS or unscaled TS? 

No, at minimum it must have received TS_STRIDE (default-slope in this
text) and 32 bits (unscaled) TS, for example an in IR packet.

>                                                               Since the
> decompressor can derive the scaled TS directly when it gets the unscaled
> TS, the compressor can send several unscaled TS to establish the
> TS_STRIDE and the relationship of delta_TS. Is that right?
> 

No, the compressor must first establish the TS_STRIDE by sending it
as-is together with unscaled TS. From those two values, TS_SCALED and
TS_OFFSET can be derived and used for later for compression.

cheers,

/Calle

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From: Jer-ming Lin <[email-masked]>
Date: Thu, 04 Feb 2010 18:34:04 +0800

Dear Sir,

Quoting Carl Knutsson <[email-masked]>:
>> ..

    After establishing the TS_STRIDE and TS_OFFSET, the compressor  
sends scaled TS bits or no TS bits if scaled TS has the relationship  
of "delta_TS=delta_SN*default-slope(TS)."

    For performance issue, if the scaled TS(derived from unscaled  
TS/TS_STRIDE) also have the relationship of delta_TS in consecutive  
IR/IR-DYN/UOR-ext3 packets. Can the compressor send no-TS bits  
directly after the establishing packets? The RFC did not mention this.  
I think this can reduce the compressed header size since no-TS bits  
are sent.

For example,

SN,     TS, TS_STRIDE, scaled TS,  Type,  transmit TS bits,
  1,  26189,       280,        93,    IR,                32
  2,  26469,       280,        94,    IR,                32
  3,  26749,       280,        95,    IR,                32
  4,  27029,       280,        96,  UO-0,                 0

The first, sencond, third construct the TS_STRIDE and the relationship  
of delta_TS. The forth cand send no-TS bits.

Sincerely yours,
Jer-ming