netdev - [PATCH v2 net-next 2/2] netem: add cell concept to simulate special MAC behavior

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Date:	Fri, 25 Nov 2011 03:22:53 +0100
From:	Hagen Paul Pfeifer <hagen@...u.net>
To:	netdev@...r.kernel.org
Cc:	Stephen Hemminger <shemminger@...tta.com>,
	Hagen Paul Pfeifer <hagen@...u.net>
Subject: [PATCH v2 net-next 2/2] netem: add cell concept to simulate special MAC behavior

This extension can be used to simulate special link layer
characteristics. Simulate because packet data is not modified, only the
calculation base is changed to delay a packet based on the original
packet size and artificial cell information.

packet_overhead can be used to simulate a link layer header compression
scheme (e.g. set packet_overhead to -20) or with a positive
packet_overhead value an additional MAC header can be simulated. It is
also possible to "replace" the 14 byte Ethernet header with something
else.

cell_size and cell_overhead can be used to simulate link layer schemes,
based on cells, like some TDMA schemes. Another application area are MAC
schemes using a link layer fragmentation with a (small) header each.
Cell size is the maximum amount of data bytes within one cell. Cell
overhead is an additional variable to change the per-cell-overhead (e.g.
5 byte header per fragment).

Example (5 kbit/s, 20 byte per packet overhead, cellsize 100 byte, per
cell overhead 5 byte):

	tc qdisc add dev eth0 root netem rate 5kbit 20 100 5

Signed-off-by: Hagen Paul Pfeifer <hagen@...u.net>
---
 include/linux/pkt_sched.h |    3 +++
 net/sched/sch_netem.c     |   30 +++++++++++++++++++++++++++---
 2 files changed, 30 insertions(+), 3 deletions(-)

diff --git a/include/linux/pkt_sched.h b/include/linux/pkt_sched.h
index 26c37ca..63845cf 100644
--- a/include/linux/pkt_sched.h
+++ b/include/linux/pkt_sched.h
@@ -498,6 +498,9 @@ struct tc_netem_corrupt {
 
 struct tc_netem_rate {
 	__u32	rate;	/* byte/s */
+	__s32   packet_overhead;
+	__u32   cell_size;
+	__s32   cell_overhead;
 };
 
 enum {
diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 9b7af9f..11ca527 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -80,6 +80,9 @@ struct netem_sched_data {
 	u32 reorder;
 	u32 corrupt;
 	u32 rate;
+	s32 packet_overhead;
+	u32 cell_size;
+	s32 cell_overhead;
 
 	struct crndstate {
 		u32 last;
@@ -299,9 +302,24 @@ static psched_tdiff_t tabledist(psched_tdiff_t mu, psched_tdiff_t sigma,
 	return  x / NETEM_DIST_SCALE + (sigma / NETEM_DIST_SCALE) * t + mu;
 }
 
-static psched_time_t packet_len_2_sched_time(unsigned int len, u32 rate)
+static psched_time_t packet_len_2_sched_time(unsigned int len,
+					     struct netem_sched_data *q)
 {
-	return PSCHED_NS2TICKS((u64)len * NSEC_PER_SEC / rate);
+	len += q->packet_overhead;
+
+	if (q->cell_size) {
+		u32 carry = len % q->cell_size;
+		len += carry;
+
+		if (q->cell_overhead) {
+			u32 cells = len / q->cell_size;
+			if (carry)
+				cells += 1;
+			len += cells * q->cell_overhead;
+		}
+	}
+
+	return PSCHED_NS2TICKS((u64)len * NSEC_PER_SEC / q->rate);
 }
 
 /*
@@ -381,7 +399,7 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 		if (q->rate) {
 			struct sk_buff_head *list = &q->qdisc->q;
 
-			delay += packet_len_2_sched_time(skb->len, q->rate);
+			delay += packet_len_2_sched_time(skb->len, q);
 
 			if (!skb_queue_empty(list)) {
 				/*
@@ -565,6 +583,9 @@ static void get_rate(struct Qdisc *sch, const struct nlattr *attr)
 	const struct tc_netem_rate *r = nla_data(attr);
 
 	q->rate = r->rate;
+	q->packet_overhead = r->packet_overhead;
+	q->cell_size       = r->cell_size;
+	q->cell_overhead   = r->cell_overhead;
 }
 
 static int get_loss_clg(struct Qdisc *sch, const struct nlattr *attr)
@@ -906,6 +927,9 @@ static int netem_dump(struct Qdisc *sch, struct sk_buff *skb)
 	NLA_PUT(skb, TCA_NETEM_CORRUPT, sizeof(corrupt), &corrupt);
 
 	rate.rate = q->rate;
+	rate.packet_overhead = q->packet_overhead;
+	rate.cell_size       = q->cell_size;
+	rate.cell_overhead   = q->cell_overhead;
 	NLA_PUT(skb, TCA_NETEM_RATE, sizeof(rate), &rate);
 
 	if (dump_loss_model(q, skb) != 0)
-- 
1.7.7

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