Re: [Wireshark-commits] rev 39384: /trunk/epan/dissectors/ /trunk/epan/dissectors/: packet-pdcp-lte.c

[Martin Mathieson <martin.r.mathieson () googlemail com>]

On Wed, Oct 12, 2011 at 6:42 PM, Guy Harris <guy () alum mit edu> wrote:

> (Paging LTE experts here....)
>
> On Oct 12, 2011, at 8:02 AM, wmeier () wireshark org wrote:
>
> > http://anonsvn.wireshark.org/viewvc/viewvc.cgi?view=rev&revision=39384
> >
> > User: wmeier
> > Date: 2011/10/12 08:02 AM
> >
> > Log:
> > Fix a benign bug: Use correct proto_tree_add_item() encoding arg.
>
> At least as I read RFC 3095:
>
>   UOR-2-TS
>
>     0   1   2   3   4   5   6   7
>   +---+---+---+---+---+---+---+---+
>   | 1   1   0 |        TS         |
>   +===+===+===+===+===+===+===+===+
>   |T=1| M |          SN           |
>   +---+---+---+---+---+---+---+---+
>   | X |            CRC            |
>   +---+---+---+---+---+---+---+---+
>
> neither the old code nor the new code are correct - the "M" bit is in the
> octet after the TS field.
>
> I don't see anything obvious in 3GPP TS 36.323 itself that says the format
> is different; does something in a later RFC specify something different?

Guy, I'm sure to be missing something, but I don't see what is wrong with
the current code, pasted here:

    if (T) {
        /* UOR-2-TS format */

        /* TS */
        guint8 ts = tvb_get_guint8(tvb, offset) & 0x1f;
        proto_tree_add_uint(tree, hf_pdcp_lte_rohc_ts, tvb, offset, 1, ts);
        offset++;

        /* Large CID */
        if (p_pdcp_info->large_cid_present) {
            offset = dissect_large_cid(tree, tvb, offset);
        }

        /* m */
        proto_tree_add_item(tree, hf_pdcp_lte_rohc_m, tvb, offset, 1,
ENC_BIG_ENDIAN);

        /* TODO: */
    }


'offset' is incremented after TS is dissected.  The strange +====+=====+
notation indicates that if we have a large CID, thats where it appears.
hf_pdcp_lte_rohc_m has the bitmask 0x40, which is OK.

Note that the ROHC support in this file has been superceded by
packet-rohc.c.  Its been on my TODO list for a long time that I rip out
this implementation and call the one in packet-rohc.c instead (after
carefully checking that everything here was already there or merge it
across).  As far as I know, RFC 3095 is used as-is in PDCP (note that there
are corrections/clarifications for this RFC, don't remember where)

Regards,
Martin

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