Add explanation for CDP checksum algorithm

diff --git a/net/net.c b/net/net.c
index 2986924824b1a166b2f210ba6a4444ace9f30758..37c5fb698e1b18ef1a4bc925a3685b1fe9dbddb1 100644 (file)
--- a/net/net.c
+++ b/net/net.c
@@ -829,7 +829,10 @@ static ushort CDP_compute_csum(const uchar *buff, ushort len)
                }
                if (len) {
                        leftover = (signed short)(*(const signed char *)buff);
-                       /* * XXX CISCO SUCKS big time! (and blows too) */
+                       /* CISCO SUCKS big time! (and blows too):
+                        * CDP uses the IP checksum algorithm with a twist;
+                        * for the last byte it *sign* extends and sums.
+                        */
                        result = (result & 0xffff0000) | ((result + leftover) & 0x0000ffff);
                }
                while (result >> 16)