Re: [rohc] Support of ROHC Piggyback Feedback in LTE

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Hello,

Kind reminder - please note that this mailing list is for questions concerning 
IETF specifications handled by the ROHC WG.

Questions related to 3GPP specifications may not receive any answers, and 
should be directed to the group responsible for those specifications.

Wrt to the total length of the entire feedback information, my understanding is 
that it is derived by inspecting the first two octets of each feedback element 
in the ROHC packet.

Cheers,

///Ghyslain

PS: Regarding your 3GPP specific questions, let's hope the following can 
provide you with some useful guidance. For further questions, please turn to 
the proper forum. The following is my understanding of the LTE specs.

For Rel-8 PDCP LTE as per TS36.323v860, if header compression is configured for 
the DRB by RRC at RB setup the following applies: the PDCP Data PDUs carry the 
entire ROHC packet (i.e. piggybacked feedback, if any, and the ROHC header and 
payload) in the data part of the PDCP PDU (see subclause 6.2.3 and 6.2.4 in 
particular); the PDCP Control PDU carries only interspersed ROHC feedback 
packets (see subclause 5.5.4 and 6.2.5 in particular).

Ciphering in Rel-8 LTE PDCP is not applicable to PDCP Control PDUs (there is no 
associated PDCP SN), but ciphering is always applied to PDCP Data PDUs for DRBs 
(thus always covering both the piggybacked feedback, if any, and the rest of 
the ROHC packet). The compressor in the PDCP entity first assemble the ROHC 
packet (including any piggyback feedback from the decompressor), then PDCP 
ciphers the result as the data part of the PDCP PDU with the COUNT associated 
to the original, uncompressed SDU.

ROHC is not applicable to SRBs.


Quoting Karthik Balaguru <[e-mail]>:

> Hi,
> 
> Need clarifications on ROHC Piggyback feedback support in LTE.
> 
> LTE standrad states that PDCP Control Packet (Feedback) shall not be 
> ciphered (Reference - 3GPP TS 36.323 V8.5.0, Section 5.5.4 ) and PDCP data 
> 
> packets shall be ciphered by configured algorithm (Reference - Section 
> 5.2.1, 5.2.2 ) . 
> In case of Piggyback feedback packet, DataPacket will be attatched at the 
> end of feedback packet.  (RFC 3095 - Page no 43,44 & Section 5.2.2)
> 1. In LTE PDCP, If the Data Packet should be ciphered for these piggyback 
> packets , then how will the Peer entity generate COUNT input for ciphering 
> algorithm as PDCP-SN is not available ?
> 2. In LTE PDCP, Can the data Packet be sent without ciphering in Piggyback 
> Feedback packets ?
> 
> Further reference w.r.t RFC 3095 - Page no 44
> d) To allow piggybacking, 5), it is possible to deduce the length of 
> feedback information by examining the first few octets of the
>     feedback.  This allows the decompressor to pass piggybacked feedback 
> information to the associated same-side compressor
>     without understanding its format.  The length information decouples 
> the decompressor from the compressor in the sense that
>     the decompressor can process the compressed header immediately without 
> waiting for the compressor to hand it back after parsing
>     the feedback information.
> 
> Kindly clarify regarding the support of RoHC piggyback Feedback in LTE ?
> 
> Thx in advans,
> Karthik Balaguru
> 
> ______________________________________________________________________