/* 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).
 *
 * Use "bpftool cgroup attach $cg sock_ops $prog" to load this BPF program.
 */

#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/bpf_helpers.h>
#include <bpf/bpf_endian.h>

#define DEBUG 1

SEC("sockops")
int bpf_rwnd(struct bpf_sock_ops *skops)
{
        int rv = -1;
        int op;

        /* For testing purposes, only execute rest of BPF program
         * if neither port numberis 55601
         */
        if (bpf_ntohl(skops->remote_port) !=
            55601 && skops->local_port != 55601) {
                skops->reply = -1;
                return 1;
        }

        op = (int) skops->op;

#ifdef DEBUG
        bpf_printk("BPF command: %d\n", 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] ||
                    (bpf_ntohl(skops->local_ip6[1]) & 0xfffff000) !=
                    (bpf_ntohl(skops->remote_ip6[1]) & 0xfffff000))
                        rv = 40;
        }
#ifdef DEBUG
        bpf_printk("Returning %d\n", rv);
#endif
        skops->reply = rv;
        return 1;
}
char _license[] SEC("license") = "GPL";