RE: [rohc] linux constant IP ID field can't be compressed in profile 0x0001?

["Price, Richard" <richard.price@roke.co.uk>]

Hi,

I'm confused - is this a part of the final standard?

My interpretation of RFC 3095 is that the compressor subtracts the
sequence number from the IP-ID, and then sends the resulting offset
using W-LSB encoding. The decompressor reconstructs the original
IP-ID by decoding the offset and then adding the sequence number. In
particular, if the decompressor receives 0 LSBs of offset, it will
always assume that the offset is the same as the value stored in the
context - i.e. that the IP-ID is going up by 1 in every successive
packet.

Regards,

Richard

> -----Original Message-----
> From: zhigang.c.liu@nokia.com [mailto:zhigang.c.liu@nokia.com]
> Sent: Friday, May 30, 2003 8:14 PM
> To: micke@cs.arizona.edu; Lars-Erik.Jonsson@epl.ericsson.se;
> rohc@ietf.org
> Subject: RE: [rohc] linux constant IP ID field can't be compressed in
> profile 0x0001?
> 
> 
> > Since the IP ID field being constant is clearly a pattern, it seems
> > reasonable to demand that this pattern should be detected in the normal 
> > way patterns are detected. Thus, even if the existing WLSB encoding doesn't 
> > compress the constantly zero field very well, the compressor would still 
> > be able to send type 0 headers and the decompressor should be able do 
> > decompress them. Also for profile 0x0001.
> > 
> > I believe Zhigang said this about a year ago, and I don't recall any
> > protests at that time. Have we since decided to not recognize this pattern?
> 
> Thanks for pointing this out, Micke. I cannot remember when I said that
> (probably 2 years ago?) but you're certainly right. 
> 
> In this case, the pattern of (IP-ID - SN) is slope = -1. So, it only takes
> two headers (successfully decompressed) for the decompressor to detect the
> pattern. Considering possible packet loss, the compressor only need to send
> a few (>2) non-type-0 headers. The overhead due to inefficient IP-ID offset 
> encoding only occurs in those headers. In fact, since a few IR packets
> (carrying the full value of IP-ID) have to be sent at the beginning of a 
> packet flow anyway, there is no additional cost to handle this particular
> case.
> 
> BR,
> Zhigang
> 
> 
> 
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