Index: sys/net/if_var.h
===================================================================
--- sys/net/if_var.h
+++ sys/net/if_var.h
@@ -183,7 +183,8 @@
 struct m_snd_tag;
 
 #define	IF_SND_TAG_TYPE_RATE_LIMIT 0
-#define	IF_SND_TAG_TYPE_MAX 1
+#define	IF_SND_TAG_TYPE_UNLIMITED 1
+#define	IF_SND_TAG_TYPE_MAX 2
 
 struct if_snd_tag_alloc_header {
 	uint32_t type;		/* send tag type, see IF_SND_TAG_XXX */
@@ -198,19 +199,25 @@
 
 struct if_snd_tag_rate_limit_params {
 	uint64_t max_rate;	/* in bytes/s */
+	uint32_t queue_level;	/* 0 (empty) .. 65535 (full) */
+#define	IF_SND_QUEUE_LEVEL_MIN 0
+#define	IF_SND_QUEUE_LEVEL_MAX 65535
 };
 
 union if_snd_tag_alloc_params {
 	struct if_snd_tag_alloc_header hdr;
 	struct if_snd_tag_alloc_rate_limit rate_limit;
+	struct if_snd_tag_alloc_rate_limit unlimited;
 };
 
 union if_snd_tag_modify_params {
 	struct if_snd_tag_rate_limit_params rate_limit;
+	struct if_snd_tag_rate_limit_params unlimited;
 };
 
 union if_snd_tag_query_params {
 	struct if_snd_tag_rate_limit_params rate_limit;
+	struct if_snd_tag_rate_limit_params unlimited;
 };
 
 typedef int (if_snd_tag_alloc_t)(struct ifnet *, union if_snd_tag_alloc_params *,
Index: sys/netinet/in_pcb.h
===================================================================
--- sys/netinet/in_pcb.h
+++ sys/netinet/in_pcb.h
@@ -761,6 +761,7 @@
 void	in_pcbdetach_txrtlmt(struct inpcb *);
 int	in_pcbmodify_txrtlmt(struct inpcb *, uint32_t);
 int	in_pcbquery_txrtlmt(struct inpcb *, uint32_t *);
+int	in_pcbquery_txrlevel(struct inpcb *, uint32_t *);
 void	in_pcboutput_txrtlmt(struct inpcb *, struct ifnet *, struct mbuf *);
 void	in_pcboutput_eagain(struct inpcb *);
 #endif
Index: sys/netinet/in_pcb.c
===================================================================
--- sys/netinet/in_pcb.c
+++ sys/netinet/in_pcb.c
@@ -2797,6 +2797,36 @@
 }
 
 /*
+ * Query existing TX queue level based on the existing
+ * "inp->inp_snd_tag", if any.
+ */
+int
+in_pcbquery_txrlevel(struct inpcb *inp, uint32_t *p_txqueue_level)
+{
+	union if_snd_tag_query_params params = { };
+	struct m_snd_tag *mst;
+	struct ifnet *ifp;
+	int error;
+
+	mst = inp->inp_snd_tag;
+	if (mst == NULL)
+		return (EINVAL);
+
+	ifp = mst->ifp;
+	if (ifp == NULL)
+		return (EINVAL);
+
+	if (ifp->if_snd_tag_query == NULL) {
+		error = EOPNOTSUPP;
+	} else {
+		error = ifp->if_snd_tag_query(mst, &params);
+		if (error == 0 &&  p_txqueue_level != NULL)
+			*p_txqueue_level = params.rate_limit.queue_level;
+	}
+	return (error);
+}
+
+/*
  * Allocate a new TX rate limit send tag from the network interface
  * given by the "ifp" argument and save it in "inp->inp_snd_tag":
  */
@@ -2805,7 +2835,8 @@
     uint32_t flowtype, uint32_t flowid, uint32_t max_pacing_rate)
 {
 	union if_snd_tag_alloc_params params = {
-		.rate_limit.hdr.type = IF_SND_TAG_TYPE_RATE_LIMIT,
+		.rate_limit.hdr.type = (max_pacing_rate == -1U) ?
+		    IF_SND_TAG_TYPE_UNLIMITED : IF_SND_TAG_TYPE_RATE_LIMIT,
 		.rate_limit.hdr.flowid = flowid,
 		.rate_limit.hdr.flowtype = flowtype,
 		.rate_limit.max_rate = max_pacing_rate,