This patch adds support for correct computation of
variable length DCCP checksums as specified in
RFC 4340, section 9.
Previously wireshark was unable to compute these
checksums, only full-coverage checksums could be
validated.
This patch
* makes checksum computation dependent
upon the header CsCov field (cf. RFC 4340, 5.1)
* removes the case where checksums are zero
(unlike UDP/packet-udp, from which the code stems,
zero checksums are illegal in DCCP (as in TCP))
* fixes a minor typo - missing bitshift of the
CCVal field
The patch has been tested against latest automatic build,
correctness of checksum computation
has been manually validated; sample traces can be supplied.
I would like to see this merged, in particular since a
Linux kernel patch for DCCP partial checksums already exists.
Please CC: all feedback to this address, I am not subscribed.
Thanks,
Gerrit Renker
---
packet-dcp.c | 23 +++++++++++++++--------
1 file changed, 15 insertions(+), 8 deletions(-)
--- wireshark-0.99.4-SVN-19738/epan/dissectors/packet-dcp.c.orig 2006-10-30 14:52:53.000000000 +0000
+++ wireshark-0.99.4-SVN-19738/epan/dissectors/packet-dcp.c 2006-10-30 14:53:02.000000000 +0000
@@ -541,6 +541,16 @@
} /* end while() */
}
+/* compute DCCP checksum coverage according to RFC 4340, section 9 */
+static inline guint dccp_csum_coverage(const e_dcphdr *dcph, guint len)
+{
+ guint cov;
+
+ if (dcph->cscov == 0)
+ return len;
+ cov = (dcph->data_offset + dcph->cscov - 1) * sizeof(guint32);
+ return (cov > len)? len : cov;
+}
static void dissect_dcp(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
{
@@ -595,7 +605,8 @@
/* DBG("dcph->data_offset: %d\n", dcph->data_offset); */
dcph->cscov=tvb_get_guint8(tvb, offset+5)&0x0F;
/* DBG("dcph->cscov: %d\n", dcph->cscov); */
- dcph->ccval=tvb_get_guint8(tvb, offset+5)&0xF0;
+ dcph->ccval=tvb_get_guint8(tvb, offset+5) &0xF0;
+ dcph->ccval >>= 4;
/* DBG("dcph->ccval: %d\n", dcph->ccval); */
dcph->checksum=tvb_get_ntohs(tvb, offset+6);
/* DBG("dcph->checksum: %d\n", dcph->checksum); */
@@ -662,15 +673,11 @@
proto_tree_add_uint(dcp_tree, hf_dcp_ccval, tvb, offset + 5, 1, dcph->ccval);
proto_tree_add_uint(dcp_tree, hf_dcp_cscov, tvb, offset + 5, 1, dcph->cscov);
- /* checksum analisys taken from packet-udp */
+ /* checksum analysis taken from packet-udp (difference: mandatory checksums in DCCP) */
reported_len = tvb_reported_length(tvb);
len = tvb_length(tvb);
- if (dcph->checksum == 0) {
- /* No checksum supplied in the packet */
- proto_tree_add_uint_format_value(dcp_tree, hf_dcp_checksum, tvb,
- offset + 6, 2, dcph->checksum, "0x%04x (none)", dcph->checksum);
- } else if (!pinfo->fragmented && len >= reported_len) {
+ if (!pinfo->fragmented && len >= reported_len) {
/* The packet isn't part of a fragmented datagram and isn't
truncated, so we can checksum it.
@@ -703,7 +710,7 @@
break;
}
cksum_vec[3].ptr = tvb_get_ptr(tvb, offset, len);
- cksum_vec[3].len = reported_len;
+ cksum_vec[3].len = dccp_csum_coverage(dcph, reported_len);
computed_cksum = in_cksum(&cksum_vec[0], 4);
if (computed_cksum == 0) {
proto_tree_add_uint_format_value(dcp_tree, hf_dcp_checksum, tvb,
