Re: [rohc] Clarification regarding the ESP SN Encoding

[Anil Maguluri <[e-mail]>]

Dear Klaus,

Thanks for your response.
Comments/queries are inline.

Regards,
Anil Kumar Maguluri

-----Original Message-----
From: Klaus Warnke [mailto:[e-mail]]
Sent: Thursday, June 17, 2010 2:42 PM
To: Anil Maguluri
Cc: Klaus Warnke; [e-mail]; 'Anil Kumar Maguluri'
Subject: Re: [rohc] Clarification regarding the ESP SN Encoding

At Wed, 16 Jun 2010 16:51:30 +0530,
Anil Maguluri wrote:
>
> Dear Klaus,
>
> Thanks for your response.
> Please clarify me whether my understanding is right or wrong.
>
> As per the RFC 3095, ROHCv1 won't support reordering between compressor
> and decompressor. Right?
Yes. See

4.1.  Operating assumptions

   Reordering

      The channel between compressor and decompressor is required to
      maintain packet ordering, i.e., the decompressor must receive
      packets in the same order as the compressor sent them.
[ANIL] Ok.

> But, the compressor can receive packets in out-of-order from the
> upper layers. Right?
Yes. If I remeber correct, the "-1" in the formular of p takes this
into account. Then the sliding window contains SN values that are
"-1" from v_ref. See 4.5.1. "v_ref - p". For UDP profile, where the SN
is generated by compressor, there is reorder possible, sure, the
interpretation interval contains no "-1" SN value.

[ANIL]
As per RFC 3095, 4.5.1

The parameter p is introduced so that the interpretation interval can
be shifted with respect to v_ref. Choosing a good value for p will
yield a more efficient encoding for fields with certain
characteristics. Below are some examples:
a) For field values that are expected always to increase, p can be
set to -1. The interpretation interval becomes
[v_ref + 1, v_ref + 2^k].
b) For field values that stay the same or increase, p can be set to
0. The interpretation interval becomes [v_ref, v_ref + 2^k - 1].
c) For field values that are expected to deviate only slightly from a
constant value, p can be set to 2^(k-1) - 1. The interpretation
interval becomes [v_ref - 2^(k-1) + 1, v_ref + 2^(k-1)].
d) For field values that are expected to undergo small negative
changes and larger positive changes, such as the RTP TS for video,
or RTP SN when there is misordering, p can be set to 2^(k-2) - 1.
The interval becomes [v_ref - 2^(k-2) + 1, v_ref + 3 * 2^(k-2)],
i.e., 3/4 of the interval is used for positive changes.

As per my understanding, we have to use "a" in case if the compressor
received packets in-order [RTP SN/ESP SN] (sequential) and "d" in case
if the compressor received packets in out-of-order. Is it right?

Also RFC 3095, 5.7,

"SN: The compressed RTP Sequence Number.
Compressed with W-LSB. The interpretation intervals, see section
4.5.1, are defined as follows:

p = 1                  if bits(SN) <= 4
p = 2^(bits(SN)-5) - 1 if bits(SN) > 4 "

The above "p" values should use at the decompressor for RTP SN/ESP SN.
Is it right?

> As per the RFC 5225, ROHCv2 supports the reordering between the
> compressor and decompressor. This is one of the advantage for
> ROHCv2.
Yes. There is a "reorder_ratio_value" defined.
[ANIL] Ok.


> Regards,
> Anil Kumar Maguluri
br, Klaus

>
> -----Original Message-----
> From: Klaus Warnke [mailto:[e-mail]]
> Sent: Wednesday, June 16, 2010 4:34 PM
> To: Anil Maguluri
> Cc: [e-mail]; 'Anil Kumar Maguluri'
> Subject: Re: [rohc] Clarification regarding the ESP SN Encoding
>
> Hello Anil,
>
> if I remember correct, RoHCv1 requires, that the link layer delivers
> all packets in order (RoHCv2 not).
>
> For p see 5.12.:
>
>   The interpretation interval (value of p) for the ESP-based SN is as
>   with ROHC RTP (profile 0x0001).
>
> Therefore p is as 5.7.:
>
>   p = 1                   if bits(SN) <= 4
>   p = 2^(bits(SN)-5) - 1  if bits(SN) >  4
>
> Regards, Klaus
>
> At Wed, 16 Jun 2010 12:32:18 +0530,
> Anil Maguluri wrote:
> > Currently I am working on ROHCv1 Profile-3 (ESP/IP).
> >
> > Please let me know whether my understanding is right or not
> > regarding the ESP SN Encoding.
> >
> > As per RFC 3095, 4.5.1, " The parameter p is introduced so that the
> > interpretation interval can be shifted with respect to v_ref.
> > Choosing a good value for p will yield a more efficient encoding for
> > fields with certain characteristics. Below are some examples:
> >
> > - P = -1           if the field value is expected to increase always
> >
> > - p = 0            if the field value is constant or increase
> >
> > - p = 2^(k-1) - 1, if the field value is expected to deviate
> >                    slightly from a constant
> >
> > - p = 2^(k-2) - 1, if the field value is expected to undergo small
> >                    negative changes or larger positive changes "
> >
> > Also RFC 3095, 5.7, " SN: The compressed RTP Sequence Number.
> > Compressed with W-LSB. The interpretation intervals, see section
> > 4.5.1, are defined as follows:
> >
> > p = 1                  if bits(SN) <= 4
> > p = 2^(bits(SN)-5) - 1 if bits(SN) > 4 "
> >
> > As per my understanding, we should use the below 'p' values at
> > compressor [for SN Encoding]
> >
> > - p = -1,                if packets are received in-order
> >
> > - p = 2^(k-2)-1,         if packets are received out-of-order at
> >                          decompressor
> >
> > - p = 1                  if bits(SN) <= 4
> >
> > - p = 2^(bits(SN)-5) - 1 if bits(SN) > 4
> >
> >
> > Queries
> >
> > 1. Is my understanding is right?
> >
> > 2. What is the value of 'p' for Profile-3 at compressor and
> >    decompressor?  Is it same as above?
> >
> >
> > Thanks for your support.
> >
> > Regards,
> > Anil Kumar Maguluri
>
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