Subject: [rohc] question about Dynamic Compression

From: "Carrie Zhang" <[email-masked]>
Date: Wed, 21 Sep 2005 09:56:13 -0500

Hi, experts,

Here is a question regarding Dynamic Compression, I could not
find out the answer via reading RFC3321.

Thanks a lot in advance for your patient to read through this email.

Here is the Dynamic Compression message flow from RFC3321, assume
that the "Remote Compressor Initiated Acknowledgements" are used
for explicit acknowledgement:
         Compressor 1                    Decompressor 2

Step  Saved       Acked    |            |   Saved
 #   State(s)    State(s)  |            |  State(s)
    -----------------------+------------+--------------
1)  s0             s0      |            |    s0
2)  s1=s0+m1               | --m1(s0)-->|
3)                         | <--ack(s1) |  s0,s1
4)  s0,s1        s0,s1     |            |
                           |            |
5)  s0,s1        s0,s1     | --m2(s1)-->| 
6)  s2=s1+m1               |            |

In step#2, state s1 is saved by compressor 1, assume the 
state ID is Sc1. Compressor 1 also includes requested 
feedback in m1 (it may put the Sc1 in requested feedback, 
or put an arbitrary identifier)

And in step#3, state s1 is also saved by decompressor 2, assume the 
state ID is Sd1. Actually, Sd1 correlates to Sc1, but they are not equal. 

Still in step#3,  state s1 is Acked by decompressor 2 to compressor 1 via
returned feedback (decompressor 2 shall put whatever data it received in
the requested feedback from step#2, into returned feedback according to 
RFC3320, page#19, the first paragraph). Now, compressor 1 knows that state 
s1 has been created in decompressor 2. 

So, in step#5, compressor 1 can make use of state s1 for m2. in SigComp
message m2, compressor 1 shall specify the PSI (partial state ID), which
should be Sc1. But, when decompressor 2 receives m2, how decompressor 2 
knows what is Sc1, it only knows Sd1. Should decompressor 2 remember the 
correlation between Sc1 and Sd1? Or, should decompressor 2 put Sc1 plus
Sd1 in the returned feedback in step#3, and then compressor 1 shall put
Sd1 in the PSI field in m2, instead of Sc1?

Thanks a lot for your patient to read through this email.

Regards,
Carrie

                  
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