Date:   Tue,  1 May 2018 11:32:41 -0700
From:   Dave Taht <dave.taht@...il.com>
To:     netdev@...r.kernel.org
Cc:     Dave Taht <dave.taht@...il.com>
Subject: [PATCH iproute2-next] Add tc-BPF example for a TCP pure ack recognizer

ack_recognize can shift pure tcp acks into another flowid.
---
 examples/bpf/ack_recognize.c | 98 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 98 insertions(+)
 create mode 100644 examples/bpf/ack_recognize.c

diff --git a/examples/bpf/ack_recognize.c b/examples/bpf/ack_recognize.c
new file mode 100644
index 0000000..5da620c
--- /dev/null
+++ b/examples/bpf/ack_recognize.c
@@ -0,0 +1,98 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright 2017 Google Inc.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ */
+
+/*
+ * Author: dave.taht@...il.com (Dave Taht)
+ *
+ * ack_recognizer: An eBPF program that correctly recognizes modern TCP ACKs,
+ * with tcp option fields like SACK and timestamps, and no additional data.
+ *
+ * ack_match call: Recognize "pure acks" with no data payload
+ *
+ */
+
+#include "bpf_api.h"
+#include "linux/if_ether.h"
+#include "linux/ip.h"
+#include "linux/in.h"
+#include "linux/ipv6.h"
+#include "linux/tcp.h"
+
+/*
+ * A pure ack contains the ip header, the tcp header + options, flags with the
+ * ack field set, and no additional payload. That last bit is what every prior
+ * ack filter gets wrong, they typically assume an obsolete 64 bytes, and don't
+ * calculate the options (like sack or timestamps) to subtract from the payload.
+ */
+
+__section_cls_entry
+int ack_match(struct __sk_buff *skb)
+{
+	void *data = (void *)(long)skb->data;
+	void *data_end = (void *)(long)skb->data_end;
+	struct ethhdr *eth = data;
+	struct iphdr *iph = data + sizeof(*eth);
+	struct tcphdr *tcp;
+
+	if (data + sizeof(*eth) + sizeof(*iph) + sizeof(*tcp) > data_end)
+		return 0;
+
+	if (eth->h_proto == htons(ETH_P_IP) &&
+	    iph->version == 4) {
+		if(iph->protocol == IPPROTO_TCP &&
+		   iph->ihl == 5 &&
+		   data + sizeof(*eth) + 20 + sizeof(*tcp) <= data_end) {
+			tcp = data + sizeof(*eth) + 20;
+			if (tcp->ack &&
+			    htons(iph->tot_len) == 20 + tcp->doff*4)
+				return -1;
+		}
+	} else if (eth->h_proto == htons(ETH_P_IPV6) &&
+		   iph->version == 6) {
+		struct ipv6hdr *iph6 = (struct ipv6hdr *) iph;
+		if(iph6->nexthdr == IPPROTO_TCP &&
+		   data + sizeof(*eth) + 40 + sizeof(*tcp) <= data_end ) {
+			tcp = data + sizeof(*eth) + 40;
+			if (tcp->ack &&
+			    tcp->doff*4 == htons(iph6->payload_len))
+				return -1;
+		}
+	}
+
+	return 0;
+}
+
+/* Example: Move acks into a priority queue:
+
+IFACE=eth0
+tc qdisc del dev $IFACE root 2> /dev/null
+tc qdisc add dev $IFACE root handle 1: prio bands 3 \
+	priomap 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
+tc qdisc add dev $IFACE parent 1:1 handle 10:1 sfq headdrop # acks only
+tc qdisc add dev $IFACE parent 1:2 handle 20:1 fq_codel # all other traffic
+tc qdisc add dev $IFACE parent 1:3 handle 30:1 fq_codel # unused
+tc filter add dev $IFACE parent 1: prio 1 bpf \
+	object-file ack_recognize.o flowid 1:1
+
+Please note that a strict priority queue is not a good idea (drr would be
+better), nor is doing any level of prioritization on acks at all....
+*/
+
+BPF_LICENSE("GPL");
-- 
2.7.4

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