Subject: [rohc] Problem about list compression

From: tangzhenghua <[email-masked]>
Date: Wed, 29 Mar 2006 14:33:21 +0800

Hi,Rohcer's

I have a problem about list compression

Section 5.8.6.1 of RFC3095 define the compressed list format of generic scheme.there is a field of gen_id to express the reference list
But this field just exist at u/0 mode,if compressor and decompressor work at R mode,How decompressor find the correct reference list from the compressed list format.
    5.8.6.1.  Encoding Type 0 (generic scheme)

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | ET=0  |GP |PS |    CC = m     |
   +---+---+---+---+---+---+---+---+
   :            gen_id             :  1 octet, if GP = 1
   +---+---+---+---+---+---+---+---+
   |        XI 1, ..., XI m        |  m octets, or m * 4 bits
   /                --- --- --- ---/
   |               :    Padding    :  if PS = 0 and m is odd
   +---+---+---+---+---+---+---+---+
   |                               |
   /       item 1, ..., item n     /  variable
   |                               |
   +---+---+---+---+---+---+---+---+

For example,
(1)In IR state and R mode,compressor send 5 IR packet to decompressor,decompressor put this 5 reference list into sliding window and send ACK to compressor one by one.
(2)Compressor receive the ACK of NO.1 packet after send No.5 IR packet,So the list of No.1 can be used as reference list. 
(3)Compressor receive a new packet marked with No.6.It can be compressed according to above format .
(4)decompressor receive a compressed list,Because there is no gen_id in this format and there are 5 reference list in the sliding window,So how can decompressor choose the reference list to restore the origin list.

Because draft-ietf-rohc-rfc3095bis-improvements-01 demonstrate that the  List compression should only use the generic scheme.So i am confusing with this problem.

Can someone give me some suggestion

Thanks 

Regards

Zhenghua
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