diff --git a/ring.h b/ring.h
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+/*
+ * Copyright 2014 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * ring.c - packet ring buffer functions
+ */
+#ifndef __RING_H__
+#define __RING_H__
+
+#include <linux/if.h>
+#include <linux/if_packet.h>
+
+#include "clatd.h"
+
+struct tun_data;
+
+// Frame size. Must be a multiple of TPACKET_ALIGNMENT (=16)
+// Why the 16? http://lxr.free-electrons.com/source/net/packet/af_packet.c?v=3.4#L1764
+#define TP_FRAME_SIZE (TPACKET_ALIGN(MAXMTU) + TPACKET_ALIGN(TPACKET2_HDRLEN) + 16)
+
+// Block size. Must be a multiple of the page size, and a power of two for efficient memory use.
+#define TP_BLOCK_SIZE 65536
+
+// In order to save memory, our frames are not an exact divider of the block size. Therefore, the
+// mmaped region will have gaps corresponding to the empty space at the end of each block.
+#define TP_FRAMES (TP_BLOCK_SIZE / TP_FRAME_SIZE)
+#define TP_FRAME_GAP (TP_BLOCK_SIZE % TP_FRAME_SIZE)
+
+// TODO: Make this configurable. This requires some refactoring because the packet socket is
+// opened before we drop privileges, but the configuration file is read after. A value of 16
+// results in 656 frames (1048576 bytes).
+#define TP_NUM_BLOCKS 16
+
+struct packet_ring {
+  uint8_t *base;
+  struct tpacket2_hdr *next;
+  int slot, numslots;
+  int block, numblocks;
+};
+
+int ring_create(struct tun_data *tunnel);
+void ring_read(struct packet_ring *ring, int write_fd, int to_ipv6);
+
+#endif