Kernel support for a dummynet option:

When a pipe or queue has the "noerror" attribute, do not report
drops to the caller (ip_output() and friends).
(2 lines to implement it, 2 lines to document it.)

This will let you simulate losses on the sender side as if they
happened in the middle of the network, i.e. with no explicit feedback
to the sender.

manpage and ipfw2.c changes to follow shortly, together with other
ipfw2 changes.

Requested by: silby
MFC after: 3 days

sys/netinet/ip_dummynet.c

@@ -1246,7 +1246,7 @@ dummynet_io(struct mbuf *m, int pipe_nr, int dir, struct ip_fw_args *fwa)
     if (q)
 	q->drops++ ;
     m_freem(m);
-    return ENOBUFS ;
+    return ( (fs && (fs->flags_fs & DN_NOERROR)) ? 0 : ENOBUFS);
 }
 
 /*

sys/netinet/ip_dummynet.h

@@ -259,6 +259,7 @@ struct dn_flow_set {
 #define DN_IS_RED		0x0002
 #define DN_IS_GENTLE_RED	0x0004
 #define DN_QSIZE_IS_BYTES	0x0008	/* queue size is measured in bytes */
+#define DN_NOERROR		0x0010	/* do not report ENOBUFS on drops  */
 #define DN_IS_PIPE		0x4000
 #define DN_IS_QUEUE		0x8000