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+/*
+ * OpenVPN -- An application to securely tunnel IP networks
+ * over a single TCP/UDP port, with support for SSL/TLS-based
+ * session authentication and key exchange,
+ * packet encryption, packet authentication, and
+ * packet compression.
+ *
+ * Copyright (C) 2002-2010 OpenVPN Technologies, Inc. <sales@openvpn.net>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2
+ * as published by the Free Software Foundation.
+ *
+ * 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 (see the file COPYING included with this
+ * distribution); if not, write to the Free Software Foundation, Inc.,
+ * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ */
+
+#ifndef MTU_H
+#define MTU_H
+
+#include "buffer.h"
+
+/*
+ *
+ * Packet maninipulation routes such as encrypt, decrypt, compress, decompress
+ * are passed a frame buffer that looks like this:
+ *
+ * [extra_frame bytes] [mtu bytes] [extra_frame_bytes] [compression overflow bytes]
+ * ^
+ * Pointer passed to function points here so that routine
+ * can make use of extra_frame bytes before pointer
+ * to prepend headers, etc.
+ *
+ * extra_frame bytes is large enough for all encryption related overhead.
+ *
+ * mtu bytes will be the MTU size set in the ifconfig statement that configures
+ * the TUN or TAP device such as:
+ *
+ * ifconfig $1 10.1.0.2 pointopoint 10.1.0.1 mtu 1450
+ *
+ * Compression overflow bytes is the worst-case size expansion that would be
+ * expected if we tried to compress mtu + extra_frame bytes of uncompressible data.
+ */
+
+/*
+ * Standard ethernet MTU
+ */
+#define ETHERNET_MTU 1500
+
+/*
+ * It is a fatal error if mtu is less than
+ * this value for tun device.
+ */
+#define TUN_MTU_MIN 100
+
+/*
+ * Default MTU of network over which tunnel data will pass by TCP/UDP.
+ */
+#define LINK_MTU_DEFAULT 1500
+
+/*
+ * Default MTU of tunnel device.
+ */
+#define TUN_MTU_DEFAULT 1500
+
+/*
+ * MTU Defaults for TAP devices
+ */
+#define TAP_MTU_EXTRA_DEFAULT 32
+
+/*
+ * Default MSSFIX value, used for reducing TCP MTU size
+ */
+#define MSSFIX_DEFAULT 1450
+
+/*
+ * Alignment of payload data such as IP packet or
+ * ethernet frame.
+ */
+#define PAYLOAD_ALIGN 4
+
+
+/**************************************************************************/
+/**
+ * Packet geometry parameters.
+ */
+struct frame {
+ int link_mtu; /**< Maximum packet size to be sent over
+ * the external network interface. */
+
+ int link_mtu_dynamic; /**< Dynamic MTU value for the external
+ * network interface. */
+
+ int extra_frame; /**< Maximum number of bytes that all
+ * processing steps together could add.
+ * @code
+ * frame.link_mtu = "socket MTU" - extra_frame;
+ * @endcode
+ */
+
+ int extra_buffer; /**< Maximum number of bytes that
+ * processing steps could expand the
+ * internal work buffer.
+ *
+ * This is used by the \link compression
+ * Data Channel Compression
+ * module\endlink to give enough working
+ * space for worst-case expansion of
+ * incompressible content. */
+
+ int extra_tun; /**< Maximum number of bytes in excess of
+ * the tun/tap MTU that might be read
+ * from or written to the virtual
+ * tun/tap network interface. */
+
+ int extra_link; /**< Maximum number of bytes in excess of
+ * external network interface's MTU that
+ * might be read from or written to it. */
+
+ /*
+ * Alignment control
+ */
+# define FRAME_HEADROOM_MARKER_DECRYPT (1<<0)
+# define FRAME_HEADROOM_MARKER_FRAGMENT (1<<1)
+# define FRAME_HEADROOM_MARKER_READ_LINK (1<<2)
+# define FRAME_HEADROOM_MARKER_READ_STREAM (1<<3)
+ unsigned int align_flags;
+ int align_adjust;
+};
+
+/* Routines which read struct frame should use the macros below */
+
+/*
+ * Overhead added to packet payload due to encapsulation
+ */
+#define EXTRA_FRAME(f) ((f)->extra_frame)
+
+/*
+ * Delta between tun payload size and final TCP/UDP datagram size
+ * (not including extra_link additions)
+ */
+#define TUN_LINK_DELTA(f) ((f)->extra_frame + (f)->extra_tun)
+
+/*
+ * This is the size to "ifconfig" the tun or tap device.
+ */
+#define TUN_MTU_SIZE(f) ((f)->link_mtu - TUN_LINK_DELTA(f))
+#define TUN_MTU_SIZE_DYNAMIC(f) ((f)->link_mtu_dynamic - TUN_LINK_DELTA(f))
+
+/*
+ * This is the maximum packet size that we need to be able to
+ * read from or write to a tun or tap device. For example,
+ * a tap device ifconfiged to an MTU of 1200 might actually want
+ * to return a packet size of 1214 on a read().
+ */
+#define PAYLOAD_SIZE(f) ((f)->link_mtu - (f)->extra_frame)
+#define PAYLOAD_SIZE_DYNAMIC(f) ((f)->link_mtu_dynamic - (f)->extra_frame)
+
+/*
+ * Max size of a payload packet after encryption, compression, etc.
+ * overhead is added.
+ */
+#define EXPANDED_SIZE(f) ((f)->link_mtu)
+#define EXPANDED_SIZE_DYNAMIC(f) ((f)->link_mtu_dynamic)
+#define EXPANDED_SIZE_MIN(f) (TUN_MTU_MIN + TUN_LINK_DELTA(f))
+
+/*
+ * These values are used as maximum size constraints
+ * on read() or write() from TUN/TAP device or TCP/UDP port.
+ */
+#define MAX_RW_SIZE_TUN(f) (PAYLOAD_SIZE(f))
+#define MAX_RW_SIZE_LINK(f) (EXPANDED_SIZE(f) + (f)->extra_link)
+
+/*
+ * Control buffer headroom allocations to allow for efficient prepending.
+ */
+#define FRAME_HEADROOM_BASE(f) (TUN_LINK_DELTA(f) + (f)->extra_buffer + (f)->extra_link)
+#define FRAME_HEADROOM(f) frame_headroom(f, 0)
+#define FRAME_HEADROOM_ADJ(f, fm) frame_headroom(f, fm)
+
+/*
+ * Max size of a buffer used to build a packet for output to
+ * the TCP/UDP port.
+ */
+#define BUF_SIZE(f) (TUN_MTU_SIZE(f) + FRAME_HEADROOM_BASE(f) * 2)
+
+/*
+ * Function prototypes.
+ */
+
+void frame_finalize (struct frame *frame,
+ bool link_mtu_defined,
+ int link_mtu,
+ bool tun_mtu_defined,
+ int tun_mtu);
+
+void frame_subtract_extra (struct frame *frame, const struct frame *src);
+
+void frame_print (const struct frame *frame,
+ int level,
+ const char *prefix);
+
+void set_mtu_discover_type (int sd, int mtu_type);
+int translate_mtu_discover_type_name (const char *name);
+
+/*
+ * frame_set_mtu_dynamic and flags
+ */
+
+#define SET_MTU_TUN (1<<0) /* use tun/tap rather than link sizing */
+#define SET_MTU_UPPER_BOUND (1<<1) /* only decrease dynamic MTU */
+
+void frame_set_mtu_dynamic (struct frame *frame, int mtu, unsigned int flags);
+
+/*
+ * allocate a buffer for socket or tun layer
+ */
+void alloc_buf_sock_tun (struct buffer *buf,
+ const struct frame *frame,
+ const bool tuntap_buffer,
+ const unsigned int align_mask);
+
+/*
+ * EXTENDED_SOCKET_ERROR_CAPABILITY functions -- print extra error info
+ * on socket errors, such as PMTU size. As of 2003.05.11, only works
+ * on Linux 2.4+.
+ */
+
+#if EXTENDED_SOCKET_ERROR_CAPABILITY
+
+void set_sock_extended_error_passing (int sd);
+const char *format_extended_socket_error (int fd, int *mtu, struct gc_arena *gc);
+
+#endif
+
+/*
+ * Calculate a starting offset into a buffer object, dealing with
+ * headroom and alignment issues.
+ */
+static inline int
+frame_headroom (const struct frame *f, const unsigned int flag_mask)
+{
+ const int offset = FRAME_HEADROOM_BASE (f);
+ const int adjust = (flag_mask & f->align_flags) ? f->align_adjust : 0;
+ const int delta = ((PAYLOAD_ALIGN << 24) - (offset + adjust)) & (PAYLOAD_ALIGN - 1);
+ return offset + delta;
+}
+
+/*
+ * frame member adjustment functions
+ */
+
+static inline void
+frame_add_to_extra_frame (struct frame *frame, const int increment)
+{
+ frame->extra_frame += increment;
+}
+
+static inline void
+frame_add_to_extra_tun (struct frame *frame, const int increment)
+{
+ frame->extra_tun += increment;
+}
+
+static inline void
+frame_add_to_extra_link (struct frame *frame, const int increment)
+{
+ frame->extra_link += increment;
+}
+
+static inline void
+frame_add_to_extra_buffer (struct frame *frame, const int increment)
+{
+ frame->extra_buffer += increment;
+}
+
+static inline void
+frame_add_to_align_adjust (struct frame *frame, const int increment)
+{
+ frame->align_adjust += increment;
+}
+
+static inline void
+frame_align_to_extra_frame (struct frame *frame)
+{
+ frame->align_adjust = frame->extra_frame + frame->extra_link;
+}
+
+static inline void
+frame_or_align_flags (struct frame *frame, const unsigned int flag_mask)
+{
+ frame->align_flags |= flag_mask;
+}
+
+static inline bool
+frame_defined (const struct frame *frame)
+{
+ return frame->link_mtu > 0;
+}
+
+#endif