Subject: RE: [rohc] Problem with RFC 3095 Extension Header encoding

From: "Lars-Erik Jonsson (LU/EAB)" <[email-masked]>
Date: Tue, 9 Sep 2003 13:28:54 +0200

Starting to catch up...

> -----Original Message-----
> From: Price, Richard [mailto:[email-masked]]
> Sent: den 4 juni 2003 18:19
> To: [email-masked]
> Subject: [rohc] Problem with RFC 3095 Extension Header encoding
> 
> 
> Hi,
> 
> I'm currently attempting to add support for IP Extension Headers to
> our ROHC-TCP implementation - the plan is to reuse the existing packet
> formats from RFC 3095. However, I'm having some difficulty 
> decoding the
> compressed versions of the AH, ESP and GRE Sequence Numbers.
> 
> What isn't clear from the standard is the following:
> 
> 1. What interpretation interval should I use to decode the sequence
>    number LSBs?

Looking at 5.8.5, I can not find any interpretation interval either,
so I would treat the LSB's as pure LSB's....i.e. with p=1.

> 2. Should the decoded sequence number be treated as an absolute value,
>    or as an offset from the RTP Sequence Number?

My interpretation of the text in 5.8.4 and 5.8.5 is that these sequence
numbers are NOT sent as offset from the RTP SN.

Any other opinions on these matters?

/L-E

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---

From: Kristofer Sandlund <[email-masked]>
Date: Tue, 09 Sep 2003 14:37:39 +0200

Hi!

Comments inlined below.

> so I would treat the LSB's as pure LSB's....i.e. with p=1.

Hmm... I'm not sure it is about LSB encoding at all here. The picture in 5.8.5.1:

      0   1   2   3   4   5   6   7       0   1   2   3   4   5   6   7
    +---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+
    | 0 |   LSB of sequence number  |   | 1 |                           |
    +---+---+---+---+---+---+---+---+   +---+                           +
                                        |                               |
                                        +     LSB of sequence number    +
                                        |                               |
                                        +                               +
                                        |                               |
                                        +---+---+---+---+---+---+---+---+

So it says "LSB of sequence number" and not "LSB encoded sequence number". Yes, 
this would be inefficient if AH/GRE/ESP SN jumps while we "wrap" the bits 
around, but I don't think the above can be interpreted as LSB encoding.
Also, the fact that 'p' is left out seems to support my interpretation.

> numbers are NOT sent as offset from the RTP SN.

I agree, it should be an absolute value.

Rgds,
	Kristofer Sandlund, Effnet AB