Subject: [rohc] IP-ID encoding

From: "Gurushant" <[email-masked]>
Date: Fri, 6 Mar 2009 17:57:31 +0530

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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From: Biplab Sarkar <[email-masked]>
Date: Fri, 06 Mar 2009 22:14:38 +0530

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:
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From: Carl Knutsson <[email-masked]>
Date: Mon, 09 Mar 2009 10:58:18 +0100

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

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From: "Gurushant" <[email-masked]>
Date: Mon, 9 Mar 2009 17:54:43 +0530

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.

I understand nowhere in RFC, they mention about negative offset value.

Thanks
Gurushant

-----Original Message-----
From: Carl Knutsson [mailto:[email-masked]]
Sent: Monday, March 09, 2009 3:28 PM
To: [email-masked]; Gurushant G Kotagi; rohc
Subject: Re: [rohc] IP-ID encoding

Gurushant,

Best Regards,

Carl Knutsson

On Fri, 6 Mar 2009, Biplab Sarkar wrote:

> > The information contained in this electronic message and any attachments
to this message are intended for the exclusive use of the addressee(s) and
may contain proprietary, confidential or privileged information. If you are
not the intended recipient, you should not disseminate, distribute or copy
this e-mail. Please notify the sender immediately and destroy all copies of
this message and any attachments contained in it.

---

From: Carl Knutsson <[email-masked]>
Date: Wed, 11 Mar 2009 16:01:48 +0100

Gurushant,

> 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