Re: Question related to ROHC-TCP Profile

Hello Raffle,

Thank for providing good info, But I have one question. In case of #2, if it
is not possible to receive the sequence number( for scaling factor) from
other end link, since it implementation specific. Is it OK if Compressor use
any other suitable parameter as scaling factor. In this case any suggestion
to use any other parameter as scaling factor?

Regards,
Gurushant
  -----Original Message-----
  From: [email protected] [mailto:[email protected]]On Behalf Of
Raffles
  Sent: Thursday, April 02, 2009 11:29 PM
  To: pradeep shambhu
  Cc: [email protected]
  Subject: Re: [rohc] Question related to ROHC-TCP Profile


  Hi,

  Thanks for the question. I must confess I was only involved in RFC 4996
peripherally, RFC 4997 was my main focus. However, my understanding is that
you are half right. The scaled sequence number cannot be used until the
scaling factor is known. This means that any formats which use the scaled
sequence number cannot be used until the transmitted scaling factor has been
received from the other end of the link. Instead a format which encodes the
sequence number directly must be used. For example compressed format rnd_1
may be suitable if all the other encodings succeed:

   // Send LSBs of sequence number
   COMPRESSED rnd_1 {
     discriminator =:= '101110'                        [ 6 ];
     seq_number    =:= lsb(18, 65535)                  [ 18 ];
     msn           =:= lsb(4, 4)                       [ 4 ];
     psh_flag      =:= irregular(1)                    [ 1 ];
     header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
     ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM) ||
             (ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_ZERO));
   }
However, at the very start, even the above encoding won't work. The "lsb"
encoding won't work because the sequence number is not given an initial
value (see the INITIAL section), so the context will be undefined. "lsb"
encoding needs valid context in order to work.  So, there are three phases
  1. Prior to context, a random encoding of sequence number must be used,
such as co_common, which encodes the sequence number without needing any
context
  2. After context has been established but prior to receiving the
transmitted scaling factor from the other end of the link, any encoding can
be used that encodes the sequence number directly
  3. After the scaling factor has been received from the other end of the
link, any encoding method can be used (of course a good implementation would
aim to use the most efficient encoding)

  This is my best understanding after a quick look at 4996, but it is not a
definitive answer. Others on the list who know 4997 and/or TCP better than I
do may be able to confirm or deny whether the above is correct.

  I hope this helps.

  Regards

  Raffles

  pradeep shambhu wrote:
    Hi,

         In ROHC-TCP [4996] page no-33, Scaling factor will not be available
for scaled acknowledgement number , because packets are flowing in the
opposite direction. but how to use the explicit scaling factor ? Do we need
to take the random scaling factor value initially ?

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