Re: IP-ID encoding

Gurushant,

Gurushant wrote:
> Hello carl,
> 
> Correct me if my understanding is wrong.
> 
> Does this means Compressor can send the negative values ( offset ( -10) =
> ID( 20) - SN(30)) and W-LSB Decoder must reconstruct the value back.
> 
No, the compressor cannot send negative values. The IP-ID-Offset
calculations are done using unsigned arithmetics and the IP-ID-Offset is
always positive. I suggest reading RFC3095 section 4.4.5 and the FN
description for IP-ID-Offset calculations in RFC5225 for the details.

I commented this because you indicated that there is something wrong
with the IP-ID-Offset encoding. I wanted to make clear that there is no
need for corrections or modifications to the RFCs.

> I understand nowhere in RFC, they mention about negative offset value.
> 
True, there are only positive values for the IP-ID-Offset in RFC3095 and
RFC5225.


rgds,

/Calle


> Thanks
> Gurushant
> 
> -----Original Message-----
> From: Carl Knutsson [mailto:[email protected]]
> Sent: Monday, March 09, 2009 3:28 PM
> To: [email protected]; Gurushant G Kotagi; rohc
> Subject: Re: [rohc] IP-ID encoding
> 
> 
> Gurushant,
> 
> A new IP-ID offset can be established in the decompressor context
> either by using irregular encoding or W-LSB encoding. 16 bits irregular
> encoding covers the whole space of possible IP-ID offset values. A 16
> bit LSB (W-LSB encoded IP-ID offset) also covers the whole space of
> possible IP-ID offset values. The decompressor will only fail if the
> compressor sends to few lsb bits, ie if the "new" IP-ID offset is
> outside the interpretation interval of the decompressor.
> 
> The details of the ip-id encoding can be found in RFC0395 section 4.5.5
> for 0x0002 and in the formal notation part of RFC5225 for 0x0102.
> 
> 
> Best Regards,
> 
> Carl Knutsson
> 
> On Fri, 6 Mar 2009, Biplab Sarkar wrote:
> 
>> Hi Gurushant,
>>
>> The linear property of both IP-ID and the generated SN is what is
>> exploited during IP-ID encoding. Its encoded using Offset-IP-ID. As it
>> is using a sliding window with p=0, it can send across both a positive
>> or a negative offset.
>>
>> Thanks
>> Biplab
>>
>> Gurushant wrote:
>>> Hi all,
>>> Could you please tell me How the IP-ID encoding work in case of profile
>>> 0x0002 or 0x0102, where SN is generated inside the Compressor. And IP-ID
>>> jumps sequentially in the input packet and SN generated is greater than
> the
>>> IP-ID value.
>>>
>>> In this case if offset for the current packet is less than previous
> offset.
>>> I believe Decompressor will not be successful.
>>>
>>> Thanks
>>> Gurushant
>>>
>>>
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> 
> 
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