RE: [rohc] query: encoding of IP-ID for RND=0 in extn3

[Remi Pelland <[e-mail]>]

My interpretation is as follows:

Because the field is called "IP-ID", and because the definition for that
field (at the bottom of page 84) refers to the beginning of section 5.7, I
believe the swapped/offset should be sent.

I know that the beginning of section 5.7 refers to the "compressed" IP-ID
field and that in this particular case, it does not seem appropriate to
think about the IP-ID field as being compressed since we're sending all 16
bits.  However, the fact that this field is called "IP-ID" and that an
explicit reference to the beginning of section 5.7 is made leads me to think
that the swapped/offset field should be sent.

Remi.

-----Original Message-----
From: Lars-Erik Jonsson (LU/EAB) [mailto:[e-mail]]
Sent: Wednesday, September 10, 2003 3:50 AM
To: ROHC mailing list
Subject: RE: [rohc] query: encoding of IP-ID for RND=0 in extn3


Alan and others,

This observation deserves some attention. Personally, I
do not have any strong opinions on this subject, and I
agree with Alan's PS that it seems like the RFC would
go towards (2). However, to me it looks like (1) had 
been much more straight forward.

Implementer's and others, please speak up!!

/L-E


> -----Original Message-----
> From: Alan Kennington [mailto:[e-mail]]
> Sent: den 30 augusti 2003 12:56
> To: ROHC mailing list
> Subject: [rohc] query: encoding of IP-ID for RND=0 in extn3
> 
> 
> I am having some difficulty in interpreting RFC 3095, 5.7, page 75.
> 
> ============================================================
>    IP-ID: A compressed IP-ID field.
> 
>       IP-ID fields in compressed base headers carry the 
> compressed IP-ID
>       of the innermost IPv4 header whose corresponding RND flag is not
>       1.  The rules below assume that the IP-ID is for the 
> innermost IP
>       header.  If it is for an outer IP header, the RND2 and 
> NBO2 flags
>       are used instead of RND and NBO.
> 
>       If value(RND) = 0, hdr(IP-ID) is compressed using Offset IP-ID
>       encoding (see section 4.5.5) using p = 0 and default-slope(IP-ID
>       offset) = 0.
> 
>       If value(RND) = 1, IP-ID is the uncompressed 
> hdr(IP-ID).  IP-ID is
>       then passed as additional octets at the end of the compressed
>       header, after any extensions.
> 
>       If value(NBO) = 0, the octets of hdr(IP-ID) are swapped before
>       compression and after decompression.  The value of NBO 
> is ignored
>       when value(RND) = 1.
> =============================================================
> 
> Suppose I have RND=0 and I send a UO-1-ID packet with extension 3.
> Suppose also that I include the 16-bit IP Identification field in
> the extension 3. This is in 5.7.5, p.85.
> 
> Then the IP-ID field is clearly not compressed. But RND = 0.
> But sometimes the IP-ID field in the IP packets does not follow the
> established pattern. Therefore I may have to send all 16 bits even
> when RND = 0.
> 
> The above rules tell me what happens when RND = 0 and the IP-ID is
> compressed. But here the IP-ID is _not_ compressed.
> 
> Question:
> Now should I swap the bytes of IP-ID according to NBO?
> Should I subtract SN from the IP Identification field before I write
> it into the type 3 extension?
> 
> I think that it is equally credible that the IP-ID is encoded
> (1) without SN offset and bytes swap (i.e. verbatim) or
> (2) with SN offset and bytes swap.
> 
> Can anyone tell me what the majority view on this is?
> 
> Cheers,
> Alan Kennington.
> 
> PS. The ROHC implementer's guide, version 3, 4.8, page 7, says that
> the 16-bit IP Identification field in an IR/IR-DYN packet must be
> sent verbatim because it is called "Identification", not IP-ID.
> This tends to make me think that the extn 3 IP-ID _may_ be the
> swapped/offset version of IP-ID. But it is still unclear to me.
> 
> _______________________________________________
> Rohc mailing list
> [e-mail]
> https://www1.ietf.org/mailman/listinfo/rohc
> 

_______________________________________________
Rohc mailing list
[e-mail]
https://www1.ietf.org/mailman/listinfo/rohc

_______________________________________________
Rohc mailing list
[e-mail]
https://www1.ietf.org/mailman/listinfo/rohc