1 files changed, 66 insertions, 0 deletions
diff --git a/src/daemon/frame.c b/src/daemon/frame.c
new file mode 100644
index 0000000..adc4629
--- /dev/null
+++ b/src/daemon/frame.c
@@ -0,0 +1,66 @@
+/* -*- mode: c; c-file-style: "openbsd" -*- */
+/*
+ * Copyright (c) 2009 Vincent Bernat <bernat@luffy.cx>
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include "lldpd.h"
+
+/**
+ * Compute the checksum as 16-bit word.
+ */
+u_int16_t
+frame_checksum(const u_char *cp, int len, int cisco)
+{
+	unsigned int sum = 0, v = 0;
+	int oddbyte = 0;
+
+	while ((len -= 2) >= 0) {
+		sum += *cp++ << 8;
+		sum += *cp++;
+	}
+	if ((oddbyte = len & 1) != 0) v = *cp;
+
+	/* The remaining byte seems to be handled oddly by Cisco. From function
+	 * dissect_cdp() in wireshark. 2014/6/14,zhengy@yealink.com:
+	 *
+	 * CDP doesn't adhere to RFC 1071 section 2. (B). It incorrectly assumes
+	 * checksums are calculated on a big endian platform, therefore i.s.o.
+	 * padding odd sized data with a zero byte _at the end_ it sets the last
+	 * big endian _word_ to contain the last network _octet_. This byteswap
+	 * has to be done on the last octet of network data before feeding it to
+	 * the Internet checksum routine.
+	 * CDP checksumming code has a bug in the addition of this last _word_
+	 * as a signed number into the long word intermediate checksum. When
+	 * reducing this long to word size checksum an off-by-one error can be
+	 * made. This off-by-one error is compensated for in the last _word_ of
+	 * the network data.
+	 */
+	if (oddbyte) {
+		if (cisco) {
+			if (v & 0x80) {
+				sum += 0xff << 8;
+				sum += v - 1;
+			} else {
+				sum += v;
+			}
+		} else {
+			sum += v << 8;
+		}
+	}
+
+	sum = (sum >> 16) + (sum & 0xffff);
+	sum += sum >> 16;
+	return (0xffff & ~sum);
+}