Subject: [rohc] RE: No CRC bits in R-0 packet, why?

From: "Lars-Erik Jonsson (EAB)" <[email-masked]>
Date: Mon, 22 Jul 2002 11:35:40 +0200

> I am implementing the ROHC with reference to RFC3095.  Now I 
> am do with R mode stuffs. I find there is no CRC in R-O packets.
> Why?

This has been justified by the fact that R-0 packets do not 
update the decompressor context, and potential errors would
therefore not propagate to subsequent decompressed packets. 

Cheers,
/L-E

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From: [email-masked]
Date: Mon, 22 Jul 2002 10:51:06 -0500

> am do with R
> mode stuffs. I find there is no CRC in R-O packets.  Why?
> We neednt do CRC with R-O packets?  Why not use the UO-0 
> packet format instead
>  of R-0 packet format?

Because there is no need to have CRC in R-0 packets. Behind this
difference is the different design philosophy between R-mode 
and UO-mode. 

R-mode has strict (or pessimistic) logic for establishing reference or
upward state transition. (see section 5.5.1.1, secure reference
principle.) Once compressor gets in SO state, however, it focuses
on how to encode SN information with respect to the secure reference
established previously. R-0 has 6-bit SN so it can cover [-1, 62]. 
This makes it more robust to long packet loss. In addition, as Lars-Erik
already mentioned, R-0 is purposely designed not to update decompressor. 
A R-0 packet therefore has absolutely no effect on any other packet. 
No CRC in R-0 also saves computation power.

UO-mode, on the other hand, has more optimistic procedure for
upward state transition (see section 5.3.1.1.1). In particular, 
ACK is not required. Instead, the compressor normally obtains its 
confidence about decompressor status by sending several packets 
with the same information. In order to detect context 
desynchronization when all of those update packets are lost,
UO-mode needs to carry CRC in every packet, including UO-0.

Hope this helps.

Zhigang