q_netem: support delivering packets in delayed time slots

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

Signed-off-by: Yousuk Seung <ysseung@google.com>
Signed-off-by: Dave Taht <dave.taht@gmail.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David Ahern <dsahern@gmail.com>

diff --git a/man/man8/tc-netem.8 b/man/man8/tc-netem.8
index f2cd86b6ed8ae82b8cc2fbd2ecbe41d2fcbad507..8d485b0267518ced0ded2d3c138bb1c8b1a9556b 100644 (file)
--- a/man/man8/tc-netem.8
+++ b/man/man8/tc-netem.8
@@ -8,7 +8,8 @@ NetEm \- Network Emulator
 .I OPTIONS
 
 .IR OPTIONS " := [ " LIMIT " ] [ " DELAY " ] [ " LOSS \
-" ] [ " CORRUPT " ] [ " DUPLICATION " ] [ " REORDERING " ][ " RATE " ]"
+" ] [ " CORRUPT " ] [ " DUPLICATION " ] [ " REORDERING " ] [ " RATE \
+" ] [ " SLOT " ]"
 
 .IR LIMIT " := "
 .B limit
@@ -51,6 +52,14 @@ NetEm \- Network Emulator
 .B rate
 .IR RATE " [ " PACKETOVERHEAD " [ " CELLSIZE " [ " CELLOVERHEAD " ]]]]"
 
+.IR SLOT " := "
+.BR slot
+.IR MIN_DELAY " [ " MAX_DELAY " ] ["
+.BR packets
+.IR PACKETS " ] [ "
+.BR bytes
+.IR BYTES " ]"
+
 
 .SH DESCRIPTION
 NetEm is an enhancement of the Linux traffic control facilities
@@ -162,6 +171,27 @@ granularity avoid a perfect shaping at a specific level. This will show up in
 an artificial packet compression (bursts). Another influence factor are network
 adapter buffers which can also add artificial delay.
 
+.SS slot
+defer delivering accumulated packets to within a slot, with each available slot
+configured with a minimum delay to acquire, and an optional maximum delay.  Slot
+delays can be specified in nanoseconds, microseconds, milliseconds or seconds
+(e.g. 800us). Values for the optional parameters
+.I BYTES
+will limit the number of bytes delivered per slot, and/or
+.I PACKETS
+will limit the number of packets delivered per slot.
+
+These slot options can provide a crude approximation of bursty MACs such as
+DOCSIS, WiFi, and LTE.
+
+Note that slotting is limited by several factors: the kernel clock granularity,
+as with a rate, and attempts to deliver many packets within a slot will be
+smeared by the timer resolution, and by the underlying native bandwidth also.
+
+It is possible to combine slotting with a rate, in which case complex behaviors
+where either the rate, or the slot limits on bytes or packets per slot, govern
+the actual delivered rate.
+
 .SH LIMITATIONS
 The main known limitation of Netem are related to timer granularity, since
 Linux is not a real-time operating system.

diff --git a/tc/q_netem.c b/tc/q_netem.c
index 9f9a9b3df255f89fac5269c3f1a43ddb86a8cf71..824018b2d37db3ff500db61ad98f073fe89d9790 100644 (file)
--- 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)
@@ -164,6 +167,7 @@ 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] = {};
@@ -412,6 +416,45 @@ 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) ||
+                                   slot.max_delay < slot.min_delay) {
+                                       explain1("slot max_delay");
+                                       return -1;
+                               }
+                       } else {
+                               slot.max_delay = slot.min_delay;
+                       }
+                       if (NEXT_ARG_OK() &&
+                           matches(*(argv+1), "packets") == 0) {
+                               NEXT_ARG();
+                               if (!NEXT_ARG_OK() ||
+                                   get_s32(&slot.max_packets, *(argv+1), 0)) {
+                                       explain1("slot packets");
+                                       return -1;
+                               }
+                               NEXT_ARG();
+                       }
+                       if (NEXT_ARG_OK() &&
+                           matches(*(argv+1), "bytes") == 0) {
+                               unsigned int max_bytes;
+                               NEXT_ARG();
+                               if (!NEXT_ARG_OK() ||
+                                   get_size(&max_bytes, *(argv+1))) {
+                                       explain1("slot bytes");
+                                       return -1;
+                               }
+                               slot.max_bytes = (int) max_bytes;
+                               NEXT_ARG();
+                       }
                } else if (strcmp(*argv, "help") == 0) {
                        explain();
                        return -1;
@@ -472,6 +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;
 
@@ -526,6 +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;
 
@@ -586,6 +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);
@@ -659,6 +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 ");