Re: Compression of AH/NULL-ESP headers in ROHCv1

At Mon, 22 Feb 2010 22:23:03 +0800,
Jer-ming Lin wrote:
> 
> Dear ROHCers,
> 
> "The AH sequence number can be omitted from the compressed header
> when the offset from the sequence number (SN) of the compressed
> header is constant, ...."
> 
> When I read the above lines about the AH sequence number in RFC 4815
> 5.8, I got the some questions.  Please clarify the following
> probelms. Thanks
> 
> 1) Does the SN mean RTP SN or AH SN in the "offset sequence number
>    (SN) of the compressed header"?

It means the master SN, derived from/compressed to the transmitted,
compressed SN bits.  And the master SN depends on the profile used. If
the used profile is RTP, the master SN is the RTP-SN. In UDP profile
it is a counter. The AH is handled as an extension header and can
occur in all profiles, I think.

> 2) Does the offset mean the method like "offset IP-ID encoding" or
>    just calculating the delta respectively(RTP SN and AH SN) and
>    compare?  Since RTP SN is 2 bytes and AH is 4 bytes, the two
>    methods will be different at wraparound.

I think, it is "just calculating the delta", because you are not
transmitting a (compressed) AH-SN offset itself anywhere. If the AH-SN
is in step with the compressed master SN, it can be omitted. If not,
you have to transmitt it "compressed" here:

5.8.5.1.  Format of IP Extension Header(s) field

      +-----+-----+-----+-----+-----+-----+-----+-----+
      :    compressed AH Seq Number,  1 or 4 octets   :  if ASeq = 1
      +-----+-----+-----+-----+-----+-----+-----+-----+

"Compressed" means, either 7bit or 31 bits LSB. See below.

I think, if you are using a 4 byte variable for the AH and adding the
delta (offset) the is no need for a special wraparound handling, from
my point of view. If the AH SN wraps over before compression, but the
offset is constant and in step with the master SN, it will be wrap
over on de-compressor site also when adding the offset.

Gruß, Klaus