Subject: [rohc] UDP checksum behaviourin a life of stream

From: "govinda reddy m" <[email-masked]>
Date: Thu, 15 May 2008 15:58:27 +0900

Dear All,
We know that if UDP checksum flag is disabled, its value is constantly zero
and could be ompressed away.
If enabled, its value depends on the payload, which for compression purposes
is equivalent to it changing randomly with every packet.

My question is:Will we need to examine UDP checksum for every packet in a
flow to enable or disable UDP checksum flag?
                      Why i am asking is, in my implementation while doing
heuristics process to find "Profile" and some fields "Behaviour", we are
examining for the first three packets only in the flow,
                      anf for the third packet we check the non-zero value
of UDP checksum and enable if if is non-zero, otherwise by default it will
be disabled.
                      After that we are never examining the UDP checksum.

                      So my doubt, is it true that for some packets UDP
checksum will be calculated and inserted by source for a while and insert
zero for another while in the same flow?

-- 
Cheers
Govinda Reddy M
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From: Vicknesan Ayadurai <[email-masked]>
Date: Thu, 15 May 2008 10:18:15 +0200 (CEST)

Hi Govinda,

I don't think any assumptions can be made about how the UDP checksum 
is enabled/disabled by the applications.

FYI, one of the test scenarios from ROHC3095 interop testing (years 
ago) involved UDP checksums being enabled/disabled mid-flow, and 
whether compressor implementations would convey this info correctly to 
the decompressors at the other end.

Regards,
vick