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

Alan Kennington <ak1.rohc@topology.org> Sat, 30 August 2003 15:37 UTC

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Date: Sat, 30 Aug 2003 20:26:26 +0930
From: Alan Kennington <ak1.rohc@topology.org>
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Subject: [rohc] query: encoding of IP-ID for RND=0 in extn3
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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.

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