Put in a long comment explaining what's confusing about this routine,

and explaining that we leave the "unused argument" warning in place as a
reminder that we need to fix this routine someday, when we actually have
a capture against which to test it.

diff --git a/print-vjc.c b/print-vjc.c
index c9dcc6dbf7ffd415d3e96fd0f7679962b62d97bf..1b6c6a428b555ffcd092cba13af2a7f2a64cb55a 100644 (file)
--- a/print-vjc.c
+++ b/print-vjc.c
@@ -25,7 +25,7 @@
 
 #ifndef lint
 static const char rcsid[] _U_ =
-    "@(#) $Header: /tcpdump/master/tcpdump/print-vjc.c,v 1.13 2003-11-16 09:36:41 guy Exp $ (LBL)";
+    "@(#) $Header: /tcpdump/master/tcpdump/print-vjc.c,v 1.14 2003-11-19 01:09:48 guy Exp $ (LBL)";
 #endif
 
 #include <tcpdump-stdinc.h>
@@ -39,6 +39,48 @@ static const char rcsid[] _U_ =
 #include "slcompress.h"
 #include "ppp.h"
 
+/*
+ * XXX - for BSD/OS PPP, what packets get supplied with a PPP header type
+ * of PPP_VJC and what packets get supplied with a PPP header type of
+ * PPP_VJNC?  PPP_VJNC is for "UNCOMPRESSED_TCP" packets, and PPP_VJC
+ * is for COMPRESSED_TCP packets (PPP_IP is used for TYPE_IP packets).
+ *
+ * RFC 1144 implies that, on the wire, the packet type is *not* needed
+ * for PPP, as different PPP protocol types can be used; it only needs
+ * to be put on the wire for SLIP.
+ *
+ * It also indicates that, for compressed SLIP:
+ *
+ *     If the COMPRESSED_TCP bit is set in the first byte, it's
+ *     a COMPRESSED_TCP packet; that byte is the change byte, and
+ *     the COMPRESSED_TCP bit, 0x80, isn't used in the change byte.
+ *
+ *     If the upper 4 bits of the first byte are 7, it's an
+ *     UNCOMPRESSED_TCP packet; that byte is the first byte of
+ *     the UNCOMPRESSED_TCP modified IP header, with a connection
+ *     number in the protocol field, and with the version field
+ *     being 7, not 4.
+ *
+ *     Otherwise, the packet is an IPv4 packet (where the upper 4 bits
+ *     of the packet are 4).
+ *
+ * So this routine looks as if it's sort-of intended to handle
+ * compressed SLIP, although it doesn't handle UNCOMPRESSED_TCP
+ * correctly for that (it doesn't fix the version number and doesn't
+ * do anything to the protocol field), and doesn't check for COMPRESSED_TCP
+ * packets correctly for that (you only check the first bit - see
+ * B.1 in RFC 1144).
+ *
+ * But it's called for BSD/OS PPP, not SLIP - perhaps BSD/OS does weird
+ * things with the headers?
+ *
+ * Without a BSD/OS VJC-compressed PPP trace, or knowledge of what the
+ * BSD/OS VJC code does, we can't say what's the case.
+ *
+ * We therefore leave "proto" - which is the PPP protocol type - in place,
+ * *not* marked as unused, for now, so that GCC warnings about the
+ * unused argument remind us that we should fix this some day.
+ */
 int
 vjc_print(register const char *bp, u_short proto)
 {