[Cake] [RFC PATCH 4/5] q_netem: support delivering packets in delayed time slots

[Dave Taht]

Slotting is a crude approximation of the behaviors of shared media such
as cable, wifi, and LTE, which gather up a bunch of packets within a
varying delay window and deliver them, relative to that, nearly all at
once.

It works within the existing loss, duplication, jitter and delay
parameters of netem. Some amount of inherent latency must be specified,
regardless.

The new "slot" parameter specifies a minimum and maximum delay between
transmission attempts.

The "bytes" and "packets" parameters can be used to limit the amount of
information transferred per slot.

Examples of use:

tc qdisc add dev eth0 root netem delay 200us \
        slot 800us 10ms bytes 64k packets 42

A more correct example, using stacked netem instances and a packet limit
to emulate a tail drop wifi queue with slots and variable packet
delivery, with a 200Mbit isochronous underlying rate, and 20ms path
delay:

tc qdisc add dev eth0 root handle 1: netem delay 20ms rate 200mbit \
         limit 10000
tc qdisc add dev eth0 parent 1:1 handle 10:1 netem delay 200us \
         slot 800us 10ms bytes 64k packets 42 limit 512
---
 tc/q_netem.c | 55 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 54 insertions(+), 1 deletion(-)

diff --git a/tc/q_netem.c b/tc/q_netem.c
index 82eb46f..1524788 100644
--- a/tc/q_netem.c
+++ b/tc/q_netem.c
@@ -40,7 +40,10 @@ static void explain(void)
 "                 [ loss gemodel PERCENT [R [1-H [1-K]]]\n" \
 "                 [ ecn ]\n" \
 "                 [ reorder PRECENT [CORRELATION] [ gap DISTANCE ]]\n" \
-"                 [ rate RATE [PACKETOVERHEAD] [CELLSIZE] [CELLOVERHEAD]]\n");
+"                 [ rate RATE [PACKETOVERHEAD] [CELLSIZE] [CELLOVERHEAD]]\n" \
+"                 [ slot MIN_DELAY MAX_DELAY [packets MAX_PACKETS]" \
+" [bytes MAX_BYTES]]\n" \
+		);
 }
 
 static void explain1(const char *arg)
@@ -178,6 +181,7 @@ static int netem_parse_opt(struct qdisc_util *qu, int argc, char **argv,
 	struct tc_netem_gimodel gimodel;
 	struct tc_netem_gemodel gemodel;
 	struct tc_netem_rate rate = {};
+	struct tc_netem_slot slot = {};
 	__s16 *dist_data = NULL;
 	__u16 loss_type = NETEM_LOSS_UNSPEC;
 	int present[__TCA_NETEM_MAX] = {};
@@ -421,6 +425,36 @@ static int netem_parse_opt(struct qdisc_util *qu, int argc, char **argv,
 					return -1;
 				}
 			}
+		} else if (matches(*argv, "slot") == 0) {
+			NEXT_ARG();
+			present[TCA_NETEM_SLOT] = 1;
+			if (get_time64(&slot.min_delay, *argv)) {
+				explain1("slot min_delay");
+				return -1;
+			}
+			if (NEXT_IS_NUMBER()) {
+				NEXT_ARG();
+				if (get_time64(&slot.max_delay, *argv)) {
+					explain1("slot min_delay max_delay");
+					return -1;
+				}
+			}
+			if (slot.max_delay < slot.min_delay)
+				slot.max_delay = slot.min_delay;
+		} else if (matches(*argv, "packets") == 0) {
+			NEXT_ARG();
+			if (get_s32(&slot.max_packets, *argv, 0)) {
+				explain1("slot packets");
+				return -1;
+			}
+		} else if (matches(*argv, "bytes") == 0) {
+			unsigned int max_bytes;
+			NEXT_ARG();
+			if (get_size(&max_bytes, *argv)) {
+				explain1("slot bytes");
+				return -1;
+			}
+			slot.max_bytes = (int) max_bytes;
 		} else if (strcmp(*argv, "help") == 0) {
 			explain();
 			return -1;
@@ -481,6 +515,10 @@ static int netem_parse_opt(struct qdisc_util *qu, int argc, char **argv,
 	    addattr_l(n, 1024, TCA_NETEM_CORRUPT, &corrupt, sizeof(corrupt)) < 0)
 		return -1;
 
+	if (present[TCA_NETEM_SLOT] &&
+	    addattr_l(n, 1024, TCA_NETEM_SLOT, &slot, sizeof(slot)) < 0)
+		return -1;
+
 	if (loss_type != NETEM_LOSS_UNSPEC) {
 		struct rtattr *start;
 
@@ -535,6 +573,7 @@ static int netem_print_opt(struct qdisc_util *qu, FILE *f, struct rtattr *opt)
 	int *ecn = NULL;
 	struct tc_netem_qopt qopt;
 	const struct tc_netem_rate *rate = NULL;
+	const struct tc_netem_slot *slot = NULL;
 	int len;
 	__u64 rate64 = 0;
 
@@ -595,6 +634,11 @@ static int netem_print_opt(struct qdisc_util *qu, FILE *f, struct rtattr *opt)
 				return -1;
 			rate64 = rta_getattr_u64(tb[TCA_NETEM_RATE64]);
 		}
+		if (tb[TCA_NETEM_SLOT]) {
+			if (RTA_PAYLOAD(tb[TCA_NETEM_SLOT]) < sizeof(*slot))
+				return -1;
+		        slot = RTA_DATA(tb[TCA_NETEM_SLOT]);
+		}
 	}
 
 	fprintf(f, "limit %d", qopt.limit);
@@ -668,6 +712,15 @@ static int netem_print_opt(struct qdisc_util *qu, FILE *f, struct rtattr *opt)
 			fprintf(f, " celloverhead %d", rate->cell_overhead);
 	}
 
+	if (slot) {
+		fprintf(f, " slot %s", sprint_time64(slot->min_delay, b1));
+		fprintf(f, " %s", sprint_time64(slot->max_delay, b1));
+		if(slot->max_packets)
+			fprintf(f, " packets %d", slot->max_packets);
+		if(slot->max_bytes)
+			fprintf(f, " bytes %d", slot->max_bytes);
+	}
+
 	if (ecn)
 		fprintf(f, " ecn ");
 
-- 
2.7.4