From 3ada834e30a0742991209140049035c626ccea85 Mon Sep 17 00:00:00 2001
From: Wolfgang Denk <wd@pollux.(none)>
Date: Thu, 10 Nov 2005 20:59:46 +0100
Subject: [PATCH] Add explanation for CDP checksum algorithm
---
net/net.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/net/net.c b/net/net.c
index 2986924824..37c5fb698e 100644
--- 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)
--
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