rsvp: explain why we don't worry about in_cksum() returning 0xFFFF.

Change-Id: I31ed0382b138042a7dcb4be2d4695b9a320c3efe
Reviewed-on: https://code.wireshark.org/review/37428
Reviewed-by: Guy Harris <gharris@sonic.net>

1 files changed, 27 insertions, 0 deletions

diff --git a/epan/dissectors/packet-rsvp.c b/epan/dissectors/packet-rsvp.c
index 1044a922ed..37623bfd8f 100644
--- a/epan/dissectors/packet-rsvp.c
+++ b/epan/dissectors/packet-rsvp.c
@@ -7813,6 +7813,33 @@ dissect_rsvp_msg_tree(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree,
            truncated, so we can checksum it. */
         SET_CKSUM_VEC_TVB(cksum_vec[0], tvb, 0, msg_length);
         computed_cksum = in_cksum(&cksum_vec[0], 1);
+        /*
+         * in_cksum() should never return 0xFFFF here, because, to quote
+         * RFC 1624 section 3 "Discussion":
+         *
+         *     In one's complement, there are two representations of
+         *     zero: the all zero and the all one bit values, often
+         *     referred to as +0 and -0.  One's complement addition
+         *     of non-zero inputs can produce -0 as a result, but
+         *     never +0.  Since there is guaranteed to be at least
+         *     one non-zero field in the IP header, and the checksum
+         *     field in the protocol header is the complement of the
+         *     sum, the checksum field can never contain ~(+0), which
+         *     is -0 (0xFFFF).  It can, however, contain ~(-0), which
+         *     is +0 (0x0000).
+         *
+         * RFC 1624 is discussing the checksum of the *IPv4* header,
+         * where the "version" field is 4, ensuring that, in a valid
+         * IPv4 header, there is at least one non-zero field, but it
+         * also applies to an RSVP packet, because header includes a
+         * version field with the value 1, so at least one field in
+         * the checksummed data is non-zero.
+         *
+         * in_cksum() returns the negation of the one's-complement
+         * sum of all the data handed to it, and that data won't be
+         * all zero, so the sum won't be 0 (+0), and thus the negation
+         * won't be -0, i.e. won't be 0xFFFF.
+         */
         if (computed_cksum == 0) {
             proto_item_append_text(cksum_item, " [correct]");
         } else if (cksum == 0 && have_integrity_object) {