[PATCH net-next v4 06/16] bpf: Sample bpf program to set initial window

[Lawrence Brakmo <brakmo@fb.com>]

The sample bpf program, tcp_rwnd_kern.c, sets the initial
advertized window to 40 packets in an environment where
distinct IPv6 prefixes indicate that both hosts are not
in the same data center.

Signed-off-by: Lawrence Brakmo <brakmo@fb.com>
---
 samples/bpf/Makefile        |  1 +
 samples/bpf/tcp_rwnd_kern.c | 61 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 62 insertions(+)
 create mode 100644 samples/bpf/tcp_rwnd_kern.c

diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
index e29370a..ca95528 100644
--- a/samples/bpf/Makefile
+++ b/samples/bpf/Makefile
@@ -114,6 +114,7 @@ always += xdp_tx_iptunnel_kern.o
 always += test_map_in_map_kern.o
 always += cookie_uid_helper_example.o
 always += tcp_synrto_kern.o
+always += tcp_rwnd_kern.o
 
 HOSTCFLAGS += -I$(objtree)/usr/include
 HOSTCFLAGS += -I$(srctree)/tools/lib/
diff --git a/samples/bpf/tcp_rwnd_kern.c b/samples/bpf/tcp_rwnd_kern.c
new file mode 100644
index 0000000..5daa649
--- /dev/null
+++ b/samples/bpf/tcp_rwnd_kern.c
@@ -0,0 +1,61 @@
+/* Copyright (c) 2017 Facebook
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of version 2 of the GNU General Public
+ * License as published by the Free Software Foundation.
+ *
+ * BPF program to set initial receive window to 40 packets when using IPv6
+ * and the first 5.5 bytes of the IPv6 addresses are not the same (in this
+ * example that means both hosts are not the same datacenter.
+ */
+
+#include <uapi/linux/bpf.h>
+#include <uapi/linux/if_ether.h>
+#include <uapi/linux/if_packet.h>
+#include <uapi/linux/ip.h>
+#include <linux/socket.h>
+#include "bpf_helpers.h"
+
+#define DEBUG 1
+
+SEC("sockops")
+int bpf_rwnd(struct bpf_sock_ops *skops)
+{
+	char fmt1[] = "BPF command: %d\n";
+	char fmt2[] = "  Returning %d\n";
+	int rv = -1;
+	int op;
+
+	/* For testing purposes, only execute rest of BPF program
+	 * if neither port numberis 55601
+	 */
+	if (skops->remote_port != 55601 && skops->local_port != 55601)
+		return -1;
+
+	op = (int) skops->op;
+
+#ifdef DEBUG
+	bpf_trace_printk(fmt1, sizeof(fmt1), op);
+#endif
+
+	/* Check for RWND_INIT operation and IPv6 addresses */
+	if (op == BPF_SOCK_OPS_RWND_INIT &&
+		skops->family == AF_INET6) {
+
+		/* If the first 5.5 bytes of the IPv6 address are not the same
+		 * then both hosts are not in the same datacenter
+		 * so use a larger initial advertized window (40 packets)
+		 */
+		if (skops->local_ip6[0] != skops->remote_ip6[0] ||
+		    (skops->local_ip6[1] & 0xfffff000) !=
+		    (skops->remote_ip6[1] & 0xfffff000))
+			bpf_trace_printk(fmt2, sizeof(fmt2), -1);
+			rv = 40;
+	}
+#ifdef DEBUG
+	bpf_trace_printk(fmt2, sizeof(fmt2), rv);
+#endif
+	skops->reply = rv;
+	return 1;
+}
+char _license[] SEC("license") = "GPL";
-- 
2.9.3