Folks,
I am attempting to write a dissector for the Dynamic Channel Change
messages in DOCSIS. The DCC request message contains a number of
Type/Length/Value fields. One of the types that may be present is
an Upstream Channel Descriptor Message (for which there is already a
dissector written). The problem is that the length field is only
one byte long so that if a UCD message is greater than 254 bytes, it must
be split into multiple TLV Values.
My solution was to use a tvbuff composite type to dissect the
message:
tvbuff *ucd_message = NULL;
static void
append_to_ucdsubst_tvbuff (tvbuff_t * tvb, guint16 start, guint16
len)
{
tvbuff_t *msg_frag;
/* check to see if ucd_message is NULL. if
so
* then create a new composite tvbuff */
if ( ucd_message == NULL)
{
ucd_message =
tvb_new_composite();
}
msg_frag = tvb_new_subset(tvb, start, len, len);
tvb_composite_append(ucd_message, msg_frag);
}
static void
dissect_dccreq (tvbuff_t * tvb, packet_info * pinfo, proto_tree *
tree)
{
<snip>
/* pos is the current position in the message */
/* tlv_data_len is the total length of the TLV data */
while (pos < tlv_data_len) {
type = tvb_get_guint8 (tvb,
pos++);
length = tvb_get_guint8 (tvb,
pos++);
switch(type)
{
<snip>
case
UCD_SUBSITUTION:
append_to_tvbuff_composite(tvb,
pos, length);
<snip>
}
pos +=
length;
}
/* the entire TLV block is dissected, so
we have all fragments */
if (ucd_message != NULL)
{
tvb_composite_finalize(ucd_message);
call_dissector
(docsis_ucd_handle, ucd_message, pinfo, dccreq_tree);
}
}
Since I can find no other uses of Composite TVBuff's and I am
running into some issues testing this code, I am asking that you code
review this and let me know if there are any reasons why this may not
work.
Thanks in advance,
Anand
--
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(}o o{)
-------ooO-(_)-Ooo-------
Anand V. Narwani, CCIE 3892
Advanced Engineering Services
Cisco Systems, Inc.
Direct/Fax: 919.392.3404
