source: trunk/third-party/libutp/utp.cpp @ 12663

#include <StdAfx.h>

#include "utp.h"
#include "templates.h"

#include <stdio.h>
#include <assert.h>
#include <string.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>
#include <limits.h> // for UINT_MAX

#ifdef WIN32
#include "win32_inet_ntop.h"

// newer versions of MSVC define these in errno.h
#ifndef ECONNRESET
#define ECONNRESET WSAECONNRESET
#define EMSGSIZE WSAEMSGSIZE
#define ECONNREFUSED WSAECONNREFUSED
#define ETIMEDOUT WSAETIMEDOUT
#endif
#endif

#ifdef POSIX
typedef sockaddr_storage SOCKADDR_STORAGE;
#endif // POSIX

// number of bytes to increase max window size by, per RTT. This is
// scaled down linearly proportional to off_target. i.e. if all packets
// in one window have 0 delay, window size will increase by this number.
// Typically it's less. TCP increases one MSS per RTT, which is 1500
#define MAX_CWND_INCREASE_BYTES_PER_RTT 3000
#define CUR_DELAY_SIZE 3
// experiments suggest that a clock skew of 10 ms per 325 seconds
// is not impossible. Reset delay_base every 13 minutes. The clock
// skew is dealt with by observing the delay base in the other
// direction, and adjusting our own upwards if the opposite direction
// delay base keeps going down
#define DELAY_BASE_HISTORY 13
#define MAX_WINDOW_DECAY 100 // ms

#define REORDER_BUFFER_SIZE 32
#define REORDER_BUFFER_MAX_SIZE 511
#define OUTGOING_BUFFER_MAX_SIZE 511

#define PACKET_SIZE 350

// this is the minimum max_window value. It can never drop below this
#define MIN_WINDOW_SIZE 10

// when window sizes are smaller than one packet_size, this
// will pace the packets to average at the given window size
// if it's not set, it will simply not send anything until
// there's a timeout
#define USE_PACKET_PACING 1

// if we receive 4 or more duplicate acks, we resend the packet
// that hasn't been acked yet
#define DUPLICATE_ACKS_BEFORE_RESEND 3

#define DELAYED_ACK_BYTE_THRESHOLD 2400 // bytes
#define DELAYED_ACK_TIME_THRESHOLD 100 // milliseconds

#define RST_INFO_TIMEOUT 10000
#define RST_INFO_LIMIT 1000
// 29 seconds determined from measuring many home NAT devices
#define KEEPALIVE_INTERVAL 29000


#define SEQ_NR_MASK 0xFFFF
#define ACK_NR_MASK 0xFFFF

#define DIV_ROUND_UP(num, denom) ((num + denom - 1) / denom)

#include "utp_utils.h"
#include "utp_config.h"

#define LOG_UTP if (g_log_utp) utp_log
#define LOG_UTPV if (g_log_utp_verbose) utp_log

uint32 g_current_ms;

// The totals are derived from the following data:
//  45: IPv6 address including embedded IPv4 address
//  11: Scope Id
//   2: Brackets around IPv6 address when port is present
//   6: Port (including colon)
//   1: Terminating null byte
char addrbuf[65];
char addrbuf2[65];
#define addrfmt(x, s) x.fmt(s, sizeof(s))

#pragma pack(push,1)

struct PACKED_ATTRIBUTE PackedSockAddr {

        // The values are always stored here in network byte order
        union {
                byte _in6[16];          // IPv6
                uint16 _in6w[8];        // IPv6, word based (for convenience)
                uint32 _in6d[4];        // Dword access
                in6_addr _in6addr;      // For convenience
        } _in;

        // Host byte order
        uint16 _port;

#define _sin4 _in._in6d[3]      // IPv4 is stored where it goes if mapped

#define _sin6 _in._in6
#define _sin6w _in._in6w
#define _sin6d _in._in6d

        byte get_family() const
        {
                return (IN6_IS_ADDR_V4MAPPED(&_in._in6addr) != 0) ? AF_INET : AF_INET6;
        }

        bool operator==(const PackedSockAddr& rhs) const
        {
                if (&rhs == this)
                        return true;
                if (_port != rhs._port)
                        return false;
                return memcmp(_sin6, rhs._sin6, sizeof(_sin6)) == 0;
        }
        bool operator!=(const PackedSockAddr& rhs) const { return !(*this == rhs); }

        PackedSockAddr(const SOCKADDR_STORAGE* sa, socklen_t len)
        {
                if (sa->ss_family == AF_INET) {
                        assert(len >= sizeof(sockaddr_in));
                        const sockaddr_in *sin = (sockaddr_in*)sa;
                        _sin6w[0] = 0;
                        _sin6w[1] = 0;
                        _sin6w[2] = 0;
                        _sin6w[3] = 0;
                        _sin6w[4] = 0;
                        _sin6w[5] = 0xffff;
                        _sin4 = sin->sin_addr.s_addr;
                        _port = ntohs(sin->sin_port);
                } else {
                        assert(len >= sizeof(sockaddr_in6));
                        const sockaddr_in6 *sin6 = (sockaddr_in6*)sa;
                        _in._in6addr = sin6->sin6_addr;
                        _port = ntohs(sin6->sin6_port);
                }
        }

        SOCKADDR_STORAGE get_sockaddr_storage(socklen_t *len = NULL) const
        {
                SOCKADDR_STORAGE sa;
                const byte family = get_family();
                if (family == AF_INET) {
                        sockaddr_in *sin = (sockaddr_in*)&sa;
                        if (len) *len = sizeof(sockaddr_in);
                        memset(sin, 0, sizeof(sockaddr_in));
                        sin->sin_family = family;
                        sin->sin_port = htons(_port);
                        sin->sin_addr.s_addr = _sin4;
                } else {
                        sockaddr_in6 *sin6 = (sockaddr_in6*)&sa;
                        memset(sin6, 0, sizeof(sockaddr_in6));
                        if (len) *len = sizeof(sockaddr_in6);
                        sin6->sin6_family = family;
                        sin6->sin6_addr = _in._in6addr;
                        sin6->sin6_port = htons(_port);
                }
                return sa;
        }

        cstr fmt(str s, size_t len) const
        {
                memset(s, 0, len);
                const byte family = get_family();
                str i;
                if (family == AF_INET) {
                        inet_ntop(family, (uint32*)&_sin4, s, len);
                        i = s;
                        while (*++i) {}
                } else {
                        i = s;
                        *i++ = '[';
                        inet_ntop(family, (in6_addr*)&_in._in6addr, i, len-1);
                        while (*++i) {}
                        *i++ = ']';
                }
                snprintf(i, len - (i-s), ":%u", _port);
                return s;
        }
};

struct PACKED_ATTRIBUTE RST_Info {
        PackedSockAddr addr;
        uint32 connid;
        uint32 timestamp;
        uint16 ack_nr;
};

// these packet sizes are including the uTP header wich
// is either 20 or 23 bytes depending on version
#define PACKET_SIZE_EMPTY_BUCKET 0
#define PACKET_SIZE_EMPTY 23
#define PACKET_SIZE_SMALL_BUCKET 1
#define PACKET_SIZE_SMALL 373
#define PACKET_SIZE_MID_BUCKET 2
#define PACKET_SIZE_MID 723
#define PACKET_SIZE_BIG_BUCKET 3
#define PACKET_SIZE_BIG 1400
#define PACKET_SIZE_HUGE_BUCKET 4

struct PACKED_ATTRIBUTE PacketFormat {
        // connection ID
        uint32_big connid;
        uint32_big tv_sec;
        uint32_big tv_usec;
        uint32_big reply_micro;
        // receive window size in PACKET_SIZE chunks
        byte windowsize;
        // Type of the first extension header
        byte ext;
        // Flags
        byte flags;
        // Sequence number
        uint16_big seq_nr;
        // Acknowledgment number
        uint16_big ack_nr;
};

struct PACKED_ATTRIBUTE PacketFormatAck {
        PacketFormat pf;
        byte ext_next;
        byte ext_len;
        byte acks[4];
};

struct PACKED_ATTRIBUTE PacketFormatExtensions {
        PacketFormat pf;
        byte ext_next;
        byte ext_len;
        byte extensions[8];
};

struct PACKED_ATTRIBUTE PacketFormatV1 {
        // packet_type (4 high bits)
        // protocol version (4 low bits)
        byte ver_type;
        byte version() const { return ver_type & 0xf; }
        byte type() const { return ver_type >> 4; }
        void set_version(byte v) { ver_type = (ver_type & 0xf0) | (v & 0xf); }
        void set_type(byte t) { ver_type = (ver_type & 0xf) | (t << 4); }

        // Type of the first extension header
        byte ext;
        // connection ID
        uint16_big connid;
        uint32_big tv_usec;
        uint32_big reply_micro;
        // receive window size in bytes
        uint32_big windowsize;
        // Sequence number
        uint16_big seq_nr;
        // Acknowledgment number
        uint16_big ack_nr;
};

struct PACKED_ATTRIBUTE PacketFormatAckV1 {
        PacketFormatV1 pf;
        byte ext_next;
        byte ext_len;
        byte acks[4];
};

struct PACKED_ATTRIBUTE PacketFormatExtensionsV1 {
        PacketFormatV1 pf;
        byte ext_next;
        byte ext_len;
        byte extensions[8];
};

#pragma pack(pop)

enum {
        ST_DATA = 0,            // Data packet.
        ST_FIN = 1,                     // Finalize the connection. This is the last packet.
        ST_STATE = 2,           // State packet. Used to transmit an ACK with no data.
        ST_RESET = 3,           // Terminate connection forcefully.
        ST_SYN = 4,                     // Connect SYN
        ST_NUM_STATES,          // used for bounds checking
};

static const cstr flagnames[] = {
        "ST_DATA","ST_FIN","ST_STATE","ST_RESET","ST_SYN"
};

enum CONN_STATE {
        CS_IDLE = 0,
        CS_SYN_SENT = 1,
        CS_CONNECTED = 2,
        CS_CONNECTED_FULL = 3,
        CS_GOT_FIN = 4,
        CS_DESTROY_DELAY = 5,
        CS_FIN_SENT = 6,
        CS_RESET = 7,
        CS_DESTROY = 8,
};

static const cstr statenames[] = {
        "IDLE","SYN_SENT","CONNECTED","CONNECTED_FULL","GOT_FIN","DESTROY_DELAY","FIN_SENT","RESET","DESTROY"
};

struct OutgoingPacket {
        size_t length;
        size_t payload;
        uint64 time_sent; // microseconds
        uint transmissions:31;
        bool need_resend:1;
        byte data[1];
};

void no_read(void *socket, const byte *bytes, size_t count) {}
void no_write(void *socket, byte *bytes, size_t count) {}
size_t no_rb_size(void *socket) { return 0; }
void no_state(void *socket, int state) {}
void no_error(void *socket, int errcode) {}
void no_overhead(void *socket, bool send, size_t count, int type) {}

UTPFunctionTable zero_funcs = {
        &no_read,
        &no_write,
        &no_rb_size,
        &no_state,
        &no_error,
        &no_overhead,
};

struct SizableCircularBuffer {
        // This is the mask. Since it's always a power of 2, adding 1 to this value will return the size.
        size_t mask;
        // This is the elements that the circular buffer points to
        void **elements;

        void *get(size_t i) { assert(elements); return elements ? elements[i & mask] : NULL; }
        void put(size_t i, void *data) { assert(elements); elements[i&mask] = data; }

        void grow(size_t item, size_t index);
        void ensure_size(size_t item, size_t index) { if (index > mask) grow(item, index); }
        size_t size() { return mask + 1; }
};

static struct UTPGlobalStats _global_stats;

// Item contains the element we want to make space for
// index is the index in the list.
void SizableCircularBuffer::grow(size_t item, size_t index)
{
        // Figure out the new size.
        size_t size = mask + 1;
        do size *= 2; while (index >= size);

        // Allocate the new buffer
        void **buf = (void**)calloc(size, sizeof(void*));

        size--;

        // Copy elements from the old buffer to the new buffer
        for (size_t i = 0; i <= mask; i++) {
                buf[(item - index + i) & size] = get(item - index + i);
        }

        // Swap to the newly allocated buffer
        mask = size;
        free(elements);
        elements = buf;
}

// compare if lhs is less than rhs, taking wrapping
// into account. if lhs is close to UINT_MAX and rhs
// is close to 0, lhs is assumed to have wrapped and
// considered smaller
bool wrapping_compare_less(uint32 lhs, uint32 rhs)
{
        // distance walking from lhs to rhs, downwards
        const uint32 dist_down = lhs - rhs;
        // distance walking from lhs to rhs, upwards
        const uint32 dist_up = rhs - lhs;

        // if the distance walking up is shorter, lhs
        // is less than rhs. If the distance walking down
        // is shorter, then rhs is less than lhs
        return dist_up < dist_down;
}

struct DelayHist {
        uint32 delay_base;

        // this is the history of delay samples,
        // normalized by using the delay_base. These
        // values are always greater than 0 and measures
        // the queuing delay in microseconds
        uint32 cur_delay_hist[CUR_DELAY_SIZE];
        size_t cur_delay_idx;

        // this is the history of delay_base. It's
        // a number that doesn't have an absolute meaning
        // only relative. It doesn't make sense to initialize
        // it to anything other than values relative to
        // what's been seen in the real world.
        uint32 delay_base_hist[DELAY_BASE_HISTORY];
        size_t delay_base_idx;
        // the time when we last stepped the delay_base_idx
        uint32 delay_base_time;

        bool delay_base_initialized;

        void clear()
        {
                delay_base_initialized = false;
                delay_base = 0;
                cur_delay_idx = 0;
                delay_base_idx = 0;
                delay_base_time = g_current_ms;
                for (size_t i = 0; i < CUR_DELAY_SIZE; i++) {
                        cur_delay_hist[i] = 0;
                }
                for (size_t i = 0; i < DELAY_BASE_HISTORY; i++) {
                        delay_base_hist[i] = 0;
                }
        }

        void shift(const uint32 offset)
        {
                // the offset should never be "negative"
                // assert(offset < 0x10000000);

                // increase all of our base delays by this amount
                // this is used to take clock skew into account
                // by observing the other side's changes in its base_delay
                for (size_t i = 0; i < DELAY_BASE_HISTORY; i++) {
                        delay_base_hist[i] += offset;
                }
                delay_base += offset;
        }

        void add_sample(const uint32 sample)
        {
                // The two clocks (in the two peers) are assumed not to
                // progress at the exact same rate. They are assumed to be
                // drifting, which causes the delay samples to contain
                // a systematic error, either they are under-
                // estimated or over-estimated. This is why we update the
                // delay_base every two minutes, to adjust for this.

                // This means the values will keep drifting and eventually wrap.
                // We can cross the wrapping boundry in two directions, either
                // going up, crossing the highest value, or going down, crossing 0.

                // if the delay_base is close to the max value and sample actually
                // wrapped on the other end we would see something like this:
                // delay_base = 0xffffff00, sample = 0x00000400
                // sample - delay_base = 0x500 which is the correct difference

                // if the delay_base is instead close to 0, and we got an even lower
                // sample (that will eventually update the delay_base), we may see
                // something like this:
                // delay_base = 0x00000400, sample = 0xffffff00
                // sample - delay_base = 0xfffffb00
                // this needs to be interpreted as a negative number and the actual
                // recorded delay should be 0.

                // It is important that all arithmetic that assume wrapping
                // is done with unsigned intergers. Signed integers are not guaranteed
                // to wrap the way unsigned integers do. At least GCC takes advantage
                // of this relaxed rule and won't necessarily wrap signed ints.

                // remove the clock offset and propagation delay.
                // delay base is min of the sample and the current
                // delay base. This min-operation is subject to wrapping
                // and care needs to be taken to correctly choose the
                // true minimum.

                // specifically the problem case is when delay_base is very small
                // and sample is very large (because it wrapped past zero), sample
                // needs to be considered the smaller

                if (!delay_base_initialized) {
                        // delay_base being 0 suggests that we haven't initialized
                        // it or its history with any real measurements yet. Initialize
                        // everything with this sample.
                        for (size_t i = 0; i < DELAY_BASE_HISTORY; i++) {
                                // if we don't have a value, set it to the current sample
                                delay_base_hist[i] = sample;
                                continue;
                        }
                        delay_base = sample;
                        delay_base_initialized = true;
                }

                if (wrapping_compare_less(sample, delay_base_hist[delay_base_idx])) {
                        // sample is smaller than the current delay_base_hist entry
                        // update it
                        delay_base_hist[delay_base_idx] = sample;
                }

                // is sample lower than delay_base? If so, update delay_base
                if (wrapping_compare_less(sample, delay_base)) {
                        // sample is smaller than the current delay_base
                        // update it
                        delay_base = sample;
                }
                
                // this operation may wrap, and is supposed to
                const uint32 delay = sample - delay_base;
                // sanity check. If this is triggered, something fishy is going on
                // it means the measured sample was greater than 32 seconds!
//              assert(delay < 0x2000000);

                cur_delay_hist[cur_delay_idx] = delay;
                cur_delay_idx = (cur_delay_idx + 1) % CUR_DELAY_SIZE;

                // once every minute
                if (g_current_ms - delay_base_time > 60 * 1000) {
                        delay_base_time = g_current_ms;
                        delay_base_idx = (delay_base_idx + 1) % DELAY_BASE_HISTORY;
                        // clear up the new delay base history spot by initializing
                        // it to the current sample, then update it 
                        delay_base_hist[delay_base_idx] = sample;
                        delay_base = delay_base_hist[0];
                        // Assign the lowest delay in the last 2 minutes to delay_base
                        for (size_t i = 0; i < DELAY_BASE_HISTORY; i++) {
                                if (wrapping_compare_less(delay_base_hist[i], delay_base))
                                        delay_base = delay_base_hist[i];
                        }
                }
        }

        uint32 get_value()
        {
                uint32 value = UINT_MAX;
                for (size_t i = 0; i < CUR_DELAY_SIZE; i++) {
                        value = min<uint32>(cur_delay_hist[i], value);
                }
                // value could be UINT_MAX if we have no samples yet...
                return value;
        }
};

struct UTPSocket {
        PackedSockAddr addr;

        size_t idx;

        uint16 reorder_count;
        byte duplicate_ack;

        // the number of bytes we've received but not acked yet
        size_t bytes_since_ack;

        // the number of packets in the send queue. Packets that haven't
        // yet been sent count as well as packets marked as needing resend
        // the oldest un-acked packet in the send queue is seq_nr - cur_window_packets
        uint16 cur_window_packets;

        // how much of the window is used, number of bytes in-flight
        // packets that have not yet been sent do not count, packets
        // that are marked as needing to be re-sent (due to a timeout)
        // don't count either
        size_t cur_window;
        // maximum window size, in bytes
        size_t max_window;
        // SO_SNDBUF setting, in bytes
        size_t opt_sndbuf;
        // SO_RCVBUF setting, in bytes
        size_t opt_rcvbuf;

        // Is a FIN packet in the reassembly buffer?
        bool got_fin:1;
        // Timeout procedure
        bool fast_timeout:1;

        // max receive window for other end, in bytes
        size_t max_window_user;
        // 0 = original uTP header, 1 = second revision
        byte version;
        CONN_STATE state;
        // TickCount when we last decayed window (wraps)
        int32 last_rwin_decay;

        // the sequence number of the FIN packet. This field is only set
        // when we have received a FIN, and the flag field has the FIN flag set.
        // it is used to know when it is safe to destroy the socket, we must have
        // received all packets up to this sequence number first.
        uint16 eof_pkt;

        // All sequence numbers up to including this have been properly received
        // by us
        uint16 ack_nr;
        // This is the sequence number for the next packet to be sent.
        uint16 seq_nr;

        uint16 timeout_seq_nr;

        // This is the sequence number of the next packet we're allowed to
        // do a fast resend with. This makes sure we only do a fast-resend
        // once per packet. We can resend the packet with this sequence number
        // or any later packet (with a higher sequence number).
        uint16 fast_resend_seq_nr;

        uint32 reply_micro;

        // the time when we need to send another ack. If there's
        // nothing to ack, this is a very large number
        uint32 ack_time;

        uint32 last_got_packet;
        uint32 last_sent_packet;
        uint32 last_measured_delay;
        uint32 last_maxed_out_window;

        // the last time we added send quota to the connection
        // when adding send quota, this is subtracted from the
        // current time multiplied by max_window / rtt
        // which is the current allowed send rate.
        int32 last_send_quota;

        // the number of bytes we are allowed to send on
        // this connection. If this is more than one packet
        // size when we run out of data to send, it is clamped
        // to the packet size
        // this value is multiplied by 100 in order to get
        // higher accuracy when dealing with low rates
        int32 send_quota;

        SendToProc *send_to_proc;
        void *send_to_userdata;
        UTPFunctionTable func;
        void *userdata;

        // Round trip time
        uint rtt;
        // Round trip time variance
        uint rtt_var;
        // Round trip timeout
        uint rto;
        DelayHist rtt_hist;
        uint retransmit_timeout;
        // The RTO timer will timeout here.
        uint rto_timeout;
        // When the window size is set to zero, start this timer. It will send a new packet every 30secs.
        uint32 zerowindow_time;

        uint32 conn_seed;
        // Connection ID for packets I receive
        uint32 conn_id_recv;
        // Connection ID for packets I send
        uint32 conn_id_send;
        // Last rcv window we advertised, in bytes
        size_t last_rcv_win;

        DelayHist our_hist;
        DelayHist their_hist;

        // extension bytes from SYN packet
        byte extensions[8];

        SizableCircularBuffer inbuf, outbuf;

#ifdef _DEBUG
        // Public stats, returned by UTP_GetStats().  See utp.h
        UTPStats _stats;
#endif // _DEBUG

        // Calculates the current receive window
        size_t get_rcv_window() const
        {
                // If we don't have a connection (such as during connection
                // establishment, always act as if we have an empty buffer).
                if (!userdata) return opt_rcvbuf;

                // Trim window down according to what's already in buffer.
                const size_t numbuf = func.get_rb_size(userdata);
                assert((int)numbuf >= 0);
                return opt_rcvbuf > numbuf ? opt_rcvbuf - numbuf : 0;
        }

        // Test if we're ready to decay max_window
        // XXX this breaks when spaced by > INT_MAX/2, which is 49
        // days; the failure mode in that case is we do an extra decay
        // or fail to do one when we really shouldn't.
        bool can_decay_win(int32 msec) const
        {
                return msec - last_rwin_decay >= MAX_WINDOW_DECAY;
        }

        // If we can, decay max window, returns true if we actually did so
        void maybe_decay_win()
        {
                if (can_decay_win(g_current_ms)) {
                        // TCP uses 0.5
                        max_window = (size_t)(max_window * .5);
                        last_rwin_decay = g_current_ms;
                        if (max_window < MIN_WINDOW_SIZE)
                                max_window = MIN_WINDOW_SIZE;
                }
        }

        size_t get_header_size() const
        {
                return (version ? sizeof(PacketFormatV1) : sizeof(PacketFormat));
        }

        size_t get_header_extensions_size() const
        {
                return (version ? sizeof(PacketFormatExtensionsV1) : sizeof(PacketFormatExtensions));
        }

        void sent_ack()
        {
                ack_time = g_current_ms + 0x70000000;
                bytes_since_ack = 0;
        }

        size_t get_udp_mtu() const
        {
                socklen_t len;
                SOCKADDR_STORAGE sa = addr.get_sockaddr_storage(&len);
                return UTP_GetUDPMTU((const struct sockaddr *)&sa, len);
        }

        size_t get_udp_overhead() const
        {
                socklen_t len;
                SOCKADDR_STORAGE sa = addr.get_sockaddr_storage(&len);
                return UTP_GetUDPOverhead((const struct sockaddr *)&sa, len);
        }

        uint64 get_global_utp_bytes_sent() const
        {
                socklen_t len;
                SOCKADDR_STORAGE sa = addr.get_sockaddr_storage(&len);
                return UTP_GetGlobalUTPBytesSent((const struct sockaddr *)&sa, len);
        }

        size_t get_overhead() const
        {
                return get_udp_overhead() + get_header_size();
        }

        void send_data(PacketFormat* b, size_t length, bandwidth_type_t type);

        void send_ack(bool synack = false);

        void send_keep_alive();

        static void send_rst(SendToProc *send_to_proc, void *send_to_userdata,
                                                 const PackedSockAddr &addr, uint32 conn_id_send,
                                                 uint16 ack_nr, uint16 seq_nr, byte version);

        void send_packet(OutgoingPacket *pkt);

        bool is_writable(size_t to_write);

        bool flush_packets();

        void write_outgoing_packet(size_t payload, uint flags);

        void update_send_quota();

#ifdef _DEBUG
        void check_invariant();
#endif

        void check_timeouts();

        int ack_packet(uint16 seq);

        size_t selective_ack_bytes(uint base, const byte* mask, byte len, int64& min_rtt);

        void selective_ack(uint base, const byte *mask, byte len);

        void apply_ledbat_ccontrol(size_t bytes_acked, uint32 actual_delay, int64 min_rtt);

        size_t get_packet_size();
};

Array<RST_Info> g_rst_info;
Array<UTPSocket*> g_utp_sockets;

static void UTP_RegisterSentPacket(size_t length) {
        if (length <= PACKET_SIZE_MID) {
                if (length <= PACKET_SIZE_EMPTY) {
                        _global_stats._nraw_send[PACKET_SIZE_EMPTY_BUCKET]++;
                } else if (length <= PACKET_SIZE_SMALL) {
                        _global_stats._nraw_send[PACKET_SIZE_SMALL_BUCKET]++;
                } else
                        _global_stats._nraw_send[PACKET_SIZE_MID_BUCKET]++;
        } else {
                if (length <= PACKET_SIZE_BIG) {
                        _global_stats._nraw_send[PACKET_SIZE_BIG_BUCKET]++;
                } else
                        _global_stats._nraw_send[PACKET_SIZE_HUGE_BUCKET]++;
        }
}

void send_to_addr(SendToProc *send_to_proc, void *send_to_userdata, const byte *p, size_t len, const PackedSockAddr &addr)
{
        socklen_t tolen;
        SOCKADDR_STORAGE to = addr.get_sockaddr_storage(&tolen);
        UTP_RegisterSentPacket(len);
        send_to_proc(send_to_userdata, p, len, (const struct sockaddr *)&to, tolen);
}

void UTPSocket::send_data(PacketFormat* b, size_t length, bandwidth_type_t type)
{
        // time stamp this packet with local time, the stamp goes into
        // the header of every packet at the 8th byte for 8 bytes :
        // two integers, check packet.h for more
        uint64 time = UTP_GetMicroseconds();

        PacketFormatV1* b1 = (PacketFormatV1*)b;
        if (version == 0) {
                b->tv_sec = (uint32)(time / 1000000);
                b->tv_usec = time % 1000000;
                b->reply_micro = reply_micro;
        } else {
                b1->tv_usec = (uint32)time;
                b1->reply_micro = reply_micro;
        }

        last_sent_packet = g_current_ms;

#ifdef _DEBUG
        _stats._nbytes_xmit += length;
        ++_stats._nxmit;
#endif
        if (userdata) {
                size_t n;
                if (type == payload_bandwidth) {
                        // if this packet carries payload, just
                        // count the header as overhead
                        type = header_overhead;
                        n = get_overhead();
                } else {
                        n = length + get_udp_overhead();
                }
                func.on_overhead(userdata, true, n, type);
        }
#if g_log_utp_verbose
        int flags = version == 0 ? b->flags : b1->type();
        uint16 seq_nr = version == 0 ? b->seq_nr : b1->seq_nr;
        uint16 ack_nr = version == 0 ? b->ack_nr : b1->ack_nr;
        LOG_UTPV("0x%08x: send %s len:%u id:%u timestamp:"I64u" reply_micro:%u flags:%s seq_nr:%u ack_nr:%u",
                         this, addrfmt(addr, addrbuf), (uint)length, conn_id_send, time, reply_micro, flagnames[flags],
                         seq_nr, ack_nr);
#endif
        send_to_addr(send_to_proc, send_to_userdata, (const byte*)b, length, addr);
}

void UTPSocket::send_ack(bool synack)
{
        PacketFormatExtensions pfe;
        zeromem(&pfe);
        PacketFormatExtensionsV1& pfe1 = (PacketFormatExtensionsV1&)pfe;
        PacketFormatAck& pfa = (PacketFormatAck&)pfe1;
        PacketFormatAckV1& pfa1 = (PacketFormatAckV1&)pfe1;

        size_t len;
        last_rcv_win = get_rcv_window();
        if (version == 0) {
                pfa.pf.connid = conn_id_send;
                pfa.pf.ack_nr = (uint16)ack_nr;
                pfa.pf.seq_nr = (uint16)seq_nr;
                pfa.pf.flags = ST_STATE;
                pfa.pf.ext = 0;
                pfa.pf.windowsize = (byte)DIV_ROUND_UP(last_rcv_win, PACKET_SIZE);
                len = sizeof(PacketFormat);
        } else {
                pfa1.pf.set_version(1);
                pfa1.pf.set_type(ST_STATE);
                pfa1.pf.ext = 0;
                pfa1.pf.connid = conn_id_send;
                pfa1.pf.ack_nr = ack_nr;
                pfa1.pf.seq_nr = seq_nr;
                pfa1.pf.windowsize = (uint32)last_rcv_win;
                len = sizeof(PacketFormatV1);
        }

        // we never need to send EACK for connections
        // that are shutting down
        if (reorder_count != 0 && state < CS_GOT_FIN) {
                // if reorder count > 0, send an EACK.
                // reorder count should always be 0
                // for synacks, so this should not be
                // as synack
                assert(!synack);
                if (version == 0) {
                        pfa.pf.ext = 1;
                        pfa.ext_next = 0;
                        pfa.ext_len = 4;
                } else {
                        pfa1.pf.ext = 1;
                        pfa1.ext_next = 0;
                        pfa1.ext_len = 4;
                }
                uint m = 0;

                // reorder count should only be non-zero
                // if the packet ack_nr + 1 has not yet
                // been received
                assert(inbuf.get(ack_nr + 1) == NULL);
                size_t window = min<size_t>(14+16, inbuf.size());
                // Generate bit mask of segments received.
                for (size_t i = 0; i < window; i++) {
                        if (inbuf.get(ack_nr + i + 2) != NULL) {
                                m |= 1 << i;
                                LOG_UTPV("0x%08x: EACK packet [%u]", this, ack_nr + i + 2);
                        }
                }
                if (version == 0) {
                        pfa.acks[0] = (byte)m;
                        pfa.acks[1] = (byte)(m >> 8);
                        pfa.acks[2] = (byte)(m >> 16);
                        pfa.acks[3] = (byte)(m >> 24);
                } else {
                        pfa1.acks[0] = (byte)m;
                        pfa1.acks[1] = (byte)(m >> 8);
                        pfa1.acks[2] = (byte)(m >> 16);
                        pfa1.acks[3] = (byte)(m >> 24);
                }
                len += 4 + 2;
                LOG_UTPV("0x%08x: Sending EACK %u [%u] bits:[%032b]", this, ack_nr, conn_id_send, m);
        } else if (synack) {
                // we only send "extensions" in response to SYN
                // and the reorder count is 0 in that state

                LOG_UTPV("0x%08x: Sending ACK %u [%u] with extension bits", this, ack_nr, conn_id_send);
                if (version == 0) {
                        pfe.pf.ext = 2;
                        pfe.ext_next = 0;
                        pfe.ext_len = 8;
                        memset(pfe.extensions, 0, 8);
                } else {
                        pfe1.pf.ext = 2;
                        pfe1.ext_next = 0;
                        pfe1.ext_len = 8;
                        memset(pfe1.extensions, 0, 8);
                }
                len += 8 + 2;
        } else {
                LOG_UTPV("0x%08x: Sending ACK %u [%u]", this, ack_nr, conn_id_send);
        }

        sent_ack();
        send_data((PacketFormat*)&pfe, len, ack_overhead);
}

void UTPSocket::send_keep_alive()
{
        ack_nr--;
        LOG_UTPV("0x%08x: Sending KeepAlive ACK %u [%u]", this, ack_nr, conn_id_send);
        send_ack();
        ack_nr++;
}

void UTPSocket::send_rst(SendToProc *send_to_proc, void *send_to_userdata,
                                                 const PackedSockAddr &addr, uint32 conn_id_send, uint16 ack_nr, uint16 seq_nr, byte version)
{
        PacketFormat pf;
        zeromem(&pf);
        PacketFormatV1& pf1 = (PacketFormatV1&)pf;

        size_t len;
        if (version == 0) {
                pf.connid = conn_id_send;
                pf.ack_nr = ack_nr;
                pf.seq_nr = seq_nr;
                pf.flags = ST_RESET;
                pf.ext = 0;
                pf.windowsize = 0;
                len = sizeof(PacketFormat);
        } else {
                pf1.set_version(1);
                pf1.set_type(ST_RESET);
                pf1.ext = 0;
                pf1.connid = conn_id_send;
                pf1.ack_nr = ack_nr;
                pf1.seq_nr = seq_nr;
                pf1.windowsize = 0;
                len = sizeof(PacketFormatV1);
        }

        LOG_UTPV("%s: Sending RST id:%u seq_nr:%u ack_nr:%u", addrfmt(addr, addrbuf), conn_id_send, seq_nr, ack_nr);
        LOG_UTPV("send %s len:%u id:%u", addrfmt(addr, addrbuf), (uint)len, conn_id_send);
        send_to_addr(send_to_proc, send_to_userdata, (const byte*)&pf1, len, addr);
}

void UTPSocket::send_packet(OutgoingPacket *pkt)
{
        // only count against the quota the first time we
        // send the packet. Don't enforce quota when closing
        // a socket. Only enforce the quota when we're sending
        // at slow rates (max window < packet size)
        size_t max_send = min(max_window, opt_sndbuf, max_window_user);

        if (pkt->transmissions == 0 || pkt->need_resend) {
                cur_window += pkt->payload;
        }

        size_t packet_size = get_packet_size();
        if (pkt->transmissions == 0 && max_send < packet_size) {
                assert(state == CS_FIN_SENT ||
                           (int32)pkt->payload <= send_quota / 100);
                send_quota = send_quota - (int32)(pkt->payload * 100);
        }

        pkt->need_resend = false;

        PacketFormatV1* p1 = (PacketFormatV1*)pkt->data;
        PacketFormat* p = (PacketFormat*)pkt->data;
        if (version == 0) {
                p->ack_nr = ack_nr;
        } else {
                p1->ack_nr = ack_nr;
        }
        pkt->time_sent = UTP_GetMicroseconds();
        pkt->transmissions++;
        sent_ack();
        send_data((PacketFormat*)pkt->data, pkt->length,
                (state == CS_SYN_SENT) ? connect_overhead
                : (pkt->transmissions == 1) ? payload_bandwidth
                : retransmit_overhead);
}

bool UTPSocket::is_writable(size_t to_write)
{
        // return true if it's OK to stuff another packet into the
        // outgoing queue. Since we may be using packet pacing, we
        // might not actually send the packet right away to affect the
        // cur_window. The only thing that happens when we add another
        // packet is that cur_window_packets is increased.
        size_t max_send = min(max_window, opt_sndbuf, max_window_user);

        size_t packet_size = get_packet_size();

        if (cur_window + packet_size >= max_window)
                last_maxed_out_window = g_current_ms;

        // if we don't have enough quota, we can't write regardless
        if (USE_PACKET_PACING) {
                if (send_quota / 100 < (int32)to_write) return false;
        }

        // subtract one to save space for the FIN packet
        if (cur_window_packets >= OUTGOING_BUFFER_MAX_SIZE - 1) return false;

        // if sending another packet would not make the window exceed
        // the max_window, we can write
        if (cur_window + packet_size <= max_send) return true;

        // if the window size is less than a packet, and we have enough
        // quota to send a packet, we can write, even though it would
        // make the window exceed the max size
        // the last condition is needed to not put too many packets
        // in the send buffer. cur_window isn't updated until we flush
        // the send buffer, so we need to take the number of packets
        // into account
        if (USE_PACKET_PACING) {
                if (max_window < to_write &&
                        cur_window < max_window &&
                        cur_window_packets == 0) {
                        return true;
                }
        }

        return false;
}

bool UTPSocket::flush_packets()
{
        size_t packet_size = get_packet_size();

        // send packets that are waiting on the pacer to be sent
        // i has to be an unsigned 16 bit counter to wrap correctly
        // signed types are not guaranteed to wrap the way you expect
        for (uint16 i = seq_nr - cur_window_packets; i != seq_nr; ++i) {
                OutgoingPacket *pkt = (OutgoingPacket*)outbuf.get(i);
                if (pkt == 0 || (pkt->transmissions > 0 && pkt->need_resend == false)) continue;
                // have we run out of quota?
                if (!is_writable(pkt->payload)) {
                        return true;
                }

                // Nagle check
                // don't send the last packet if we have one packet in-flight
                // and the current packet is still smaller than packet_size.
                if (i != ((seq_nr - 1) & ACK_NR_MASK) ||
                        cur_window_packets == 1 ||
                        pkt->payload >= packet_size) {
                        send_packet(pkt);

                        // No need to send another ack if there is nothing to reorder.
                        if (reorder_count == 0) {
                                sent_ack();
                        }
                }
        }
        return false;
}

void UTPSocket::write_outgoing_packet(size_t payload, uint flags)
{
        // Setup initial timeout timer
        if (cur_window_packets == 0) {
                retransmit_timeout = rto;
                rto_timeout = g_current_ms + retransmit_timeout;
                assert(cur_window == 0);
        }

        size_t packet_size = get_packet_size();
        do {
                assert(cur_window_packets < OUTGOING_BUFFER_MAX_SIZE);
                assert(flags == ST_DATA || flags == ST_FIN);

                size_t added = 0;

                OutgoingPacket *pkt = NULL;
                
                if (cur_window_packets > 0) {
                        pkt = (OutgoingPacket*)outbuf.get(seq_nr - 1);
                }

                const size_t header_size = get_header_size();
                bool append = true;

                // if there's any room left in the last packet in the window
                // and it hasn't been sent yet, fill that frame first
                if (payload && pkt && !pkt->transmissions && pkt->payload < packet_size) {
                        // Use the previous unsent packet
                        added = min(payload + pkt->payload, max<size_t>(packet_size, pkt->payload)) - pkt->payload;
                        pkt = (OutgoingPacket*)realloc(pkt,
                                                                                   (sizeof(OutgoingPacket) - 1) +
                                                                                   header_size +
                                                                                   pkt->payload + added);
                        outbuf.put(seq_nr - 1, pkt);
                        append = false;
                        assert(!pkt->need_resend);
                } else {
                        // Create the packet to send.
                        added = payload;
                        pkt = (OutgoingPacket*)malloc((sizeof(OutgoingPacket) - 1) +
                                                                                  header_size +
                                                                                  added);
                        pkt->payload = 0;
                        pkt->transmissions = 0;
                        pkt->need_resend = false;
                }

                if (added) {
                        // Fill it with data from the upper layer.
                        func.on_write(userdata, pkt->data + header_size + pkt->payload, added);
                }
                pkt->payload += added;
                pkt->length = header_size + pkt->payload;

                last_rcv_win = get_rcv_window();

                PacketFormat* p = (PacketFormat*)pkt->data;
                PacketFormatV1* p1 = (PacketFormatV1*)pkt->data;
                if (version == 0) {
                        p->connid = conn_id_send;
                        p->ext = 0;
                        p->windowsize = (byte)DIV_ROUND_UP(last_rcv_win, PACKET_SIZE);
                        p->ack_nr = ack_nr;
                        p->flags = flags;
                } else {
                        p1->set_version(1);
                        p1->set_type(flags);
                        p1->ext = 0;
                        p1->connid = conn_id_send;
                        p1->windowsize = (uint32)last_rcv_win;
                        p1->ack_nr = ack_nr;
                }

                if (append) {
                        // Remember the message in the outgoing queue.
                        outbuf.ensure_size(seq_nr, cur_window_packets);
                        outbuf.put(seq_nr, pkt);
                        if (version == 0) p->seq_nr = seq_nr;
                        else p1->seq_nr = seq_nr;
                        seq_nr++;
                        cur_window_packets++;
                }

                payload -= added;

        } while (payload);

        flush_packets();
}

void UTPSocket::update_send_quota()
{
        int dt = g_current_ms - last_send_quota;
        if (dt == 0) return;
        last_send_quota = g_current_ms;
        size_t add = max_window * dt * 100 / (rtt_hist.delay_base?rtt_hist.delay_base:50);
        if (add > max_window * 100 && add > MAX_CWND_INCREASE_BYTES_PER_RTT * 100) add = max_window;
        send_quota += (int32)add;
//      LOG_UTPV("0x%08x: UTPSocket::update_send_quota dt:%d rtt:%u max_window:%u quota:%d",
//                       this, dt, rtt, (uint)max_window, send_quota / 100);
}

#ifdef _DEBUG
void UTPSocket::check_invariant()
{
        if (reorder_count > 0) {
                assert(inbuf.get(ack_nr + 1) == NULL);
        }

        size_t outstanding_bytes = 0;
        for (int i = 0; i < cur_window_packets; ++i) {
                OutgoingPacket *pkt = (OutgoingPacket*)outbuf.get(seq_nr - i - 1);
                if (pkt == 0 || pkt->transmissions == 0 || pkt->need_resend) continue;
                outstanding_bytes += pkt->payload;
        }
        assert(outstanding_bytes == cur_window);
}
#endif

void UTPSocket::check_timeouts()
{
#ifdef _DEBUG
        check_invariant();
#endif

        // this invariant should always be true
        assert(cur_window_packets == 0 || outbuf.get(seq_nr - cur_window_packets));

        LOG_UTPV("0x%08x: CheckTimeouts timeout:%d max_window:%u cur_window:%u quota:%d "
                         "state:%s cur_window_packets:%u bytes_since_ack:%u ack_time:%d",
                         this, (int)(rto_timeout - g_current_ms), (uint)max_window, (uint)cur_window,
                         send_quota / 100, statenames[state], cur_window_packets,
                         (uint)bytes_since_ack, (int)(g_current_ms - ack_time));

        update_send_quota();
        flush_packets();


        if (USE_PACKET_PACING) {
                // In case the new send quota made it possible to send another packet
                // Mark the socket as writable. If we don't use pacing, the send
                // quota does not affect if the socket is writeable
                // if we don't use packet pacing, the writable event is triggered
                // whenever the cur_window falls below the max_window, so we don't
                // need this check then
                if (state == CS_CONNECTED_FULL && is_writable(get_packet_size())) {
                        state = CS_CONNECTED;
                        LOG_UTPV("0x%08x: Socket writable. max_window:%u cur_window:%u quota:%d packet_size:%u",
                                         this, (uint)max_window, (uint)cur_window, send_quota / 100, (uint)get_packet_size());
                        func.on_state(userdata, UTP_STATE_WRITABLE);
                }
        }

        switch (state) {
        case CS_SYN_SENT:
        case CS_CONNECTED_FULL:
        case CS_CONNECTED:
        case CS_FIN_SENT: {

                // Reset max window...
                if ((int)(g_current_ms - zerowindow_time) >= 0 && max_window_user == 0) {
                        max_window_user = PACKET_SIZE;
                }

                if ((int)(g_current_ms - rto_timeout) >= 0 &&
                        (!(USE_PACKET_PACING) || cur_window_packets > 0) &&
                        rto_timeout > 0) {

                        /*
                        OutgoingPacket *pkt = (OutgoingPacket*)outbuf.get(seq_nr - cur_window_packets);
                        
                        // If there were a lot of retransmissions, force recomputation of round trip time
                        if (pkt->transmissions >= 4)
                                rtt = 0;
                        */

                        // Increase RTO
                        const uint new_timeout = retransmit_timeout * 2;
                        if (new_timeout >= 30000 || (state == CS_SYN_SENT && new_timeout > 6000)) {
                                // more than 30 seconds with no reply. kill it.
                                // if we haven't even connected yet, give up sooner. 6 seconds
                                // means 2 tries at the following timeouts: 3, 6 seconds
                                if (state == CS_FIN_SENT)
                                        state = CS_DESTROY;
                                else
                                        state = CS_RESET;
                                func.on_error(userdata, ETIMEDOUT);
                                goto getout;
                        }

                        retransmit_timeout = new_timeout;
                        rto_timeout = g_current_ms + new_timeout;

                        // On Timeout
                        duplicate_ack = 0;

                        // rate = min_rate
                        max_window = get_packet_size();
                        send_quota = max<int32>((int32)max_window * 100, send_quota);

                        // every packet should be considered lost
                        for (int i = 0; i < cur_window_packets; ++i) {
                                OutgoingPacket *pkt = (OutgoingPacket*)outbuf.get(seq_nr - i - 1);
                                if (pkt == 0 || pkt->transmissions == 0 || pkt->need_resend) continue;
                                pkt->need_resend = true;
                                assert(cur_window >= pkt->payload);
                                cur_window -= pkt->payload;
                        }

                        // used in parse_log.py
                        LOG_UTP("0x%08x: Packet timeout. Resend. seq_nr:%u. timeout:%u max_window:%u",
                                        this, seq_nr - cur_window_packets, retransmit_timeout, (uint)max_window);

                        fast_timeout = true;
                        timeout_seq_nr = seq_nr;

                        if (cur_window_packets > 0) {
                                OutgoingPacket *pkt = (OutgoingPacket*)outbuf.get(seq_nr - cur_window_packets);
                                assert(pkt);
                                send_quota = max<int32>((int32)pkt->length * 100, send_quota);

                                // Re-send the packet.
                                send_packet(pkt);
                        }
                }

                // Mark the socket as writable
                if (state == CS_CONNECTED_FULL && is_writable(get_packet_size())) {
                        state = CS_CONNECTED;
                        LOG_UTPV("0x%08x: Socket writable. max_window:%u cur_window:%u quota:%d packet_size:%u",
                                         this, (uint)max_window, (uint)cur_window, send_quota / 100, (uint)get_packet_size());
                        func.on_state(userdata, UTP_STATE_WRITABLE);
                }

                if (state >= CS_CONNECTED && state <= CS_FIN_SENT) {
                        // Send acknowledgment packets periodically, or when the threshold is reached
                        if (bytes_since_ack > DELAYED_ACK_BYTE_THRESHOLD ||
                                (int)(g_current_ms - ack_time) >= 0) {
                                send_ack();
                        }

                        if ((int)(g_current_ms - last_sent_packet) >= KEEPALIVE_INTERVAL) {
                                send_keep_alive();
                        }
                }

                break;
        }

        // Close?
        case CS_GOT_FIN:
        case CS_DESTROY_DELAY:
                if ((int)(g_current_ms - rto_timeout) >= 0) {
                        state = (state == CS_DESTROY_DELAY) ? CS_DESTROY : CS_RESET;
                        if (cur_window_packets > 0 && userdata) {
                                func.on_error(userdata, ECONNRESET);
                        }
                }
                break;
        // prevent warning
        case CS_IDLE:
        case CS_RESET:
        case CS_DESTROY:
                break;
        }

        getout:

        // make sure we don't accumulate quota when we don't have
        // anything to send
        int32 limit = max<int32>((int32)max_window / 2, 5 * (int32)get_packet_size()) * 100;
        if (send_quota > limit) send_quota = limit;
}

// returns:
// 0: the packet was acked.
// 1: it means that the packet had already been acked
// 2: the packet has not been sent yet
int UTPSocket::ack_packet(uint16 seq)
{
        OutgoingPacket *pkt = (OutgoingPacket*)outbuf.get(seq);

        // the packet has already been acked (or not sent)
        if (pkt == NULL) {
                LOG_UTPV("0x%08x: got ack for:%u (already acked, or never sent)", this, seq);
                return 1;
        }

        // can't ack packets that haven't been sent yet!
        if (pkt->transmissions == 0) {
                LOG_UTPV("0x%08x: got ack for:%u (never sent, pkt_size:%u need_resend:%u)",
                                 this, seq, (uint)pkt->payload, pkt->need_resend);
                return 2;
        }

        LOG_UTPV("0x%08x: got ack for:%u (pkt_size:%u need_resend:%u)",
                         this, seq, (uint)pkt->payload, pkt->need_resend);

        outbuf.put(seq, NULL);

        // if we never re-sent the packet, update the RTT estimate
        if (pkt->transmissions == 1) {
                // Estimate the round trip time.
                const uint32 ertt = (uint32)((UTP_GetMicroseconds() - pkt->time_sent) / 1000);
                if (rtt == 0) {
                        // First round trip time sample
                        rtt = ertt;
                        rtt_var = ertt / 2;
                        // sanity check. rtt should never be more than 6 seconds
//                      assert(rtt < 6000);
                } else {
                        // Compute new round trip times
                        const int delta = (int)rtt - ertt;
                        rtt_var = rtt_var + (int)(abs(delta) - rtt_var) / 4;
                        rtt = rtt - rtt/8 + ertt/8;
                        // sanity check. rtt should never be more than 6 seconds
//                      assert(rtt < 6000);
                        rtt_hist.add_sample(ertt);
                }
                rto = max<uint>(rtt + rtt_var * 4, 500);
                LOG_UTPV("0x%08x: rtt:%u avg:%u var:%u rto:%u",
                                 this, ertt, rtt, rtt_var, rto);
        }
        retransmit_timeout = rto;
        rto_timeout = g_current_ms + rto;
        // if need_resend is set, this packet has already
        // been considered timed-out, and is not included in
        // the cur_window anymore
        if (!pkt->need_resend) {
                assert(cur_window >= pkt->payload);
                cur_window -= pkt->payload;
        }
        free(pkt);
        return 0;
}

// count the number of bytes that were acked by the EACK header
size_t UTPSocket::selective_ack_bytes(uint base, const byte* mask, byte len, int64& min_rtt)
{
        if (cur_window_packets == 0) return 0;

        size_t acked_bytes = 0;
        int bits = len * 8;

        do {
                uint v = base + bits;

                // ignore bits that haven't been sent yet
                // see comment in UTPSocket::selective_ack
                if (((seq_nr - v - 1) & ACK_NR_MASK) >= (uint16)(cur_window_packets - 1))
                        continue;

                // ignore bits that represents packets we haven't sent yet
                // or packets that have already been acked
                OutgoingPacket *pkt = (OutgoingPacket*)outbuf.get(v);
                if (!pkt || pkt->transmissions == 0)
                        continue;

                // Count the number of segments that were successfully received past it.
                if (bits >= 0 && mask[bits>>3] & (1 << (bits & 7))) {
                        assert((int)(pkt->payload) >= 0);
                        acked_bytes += pkt->payload;
                        min_rtt = min<int64>(min_rtt, UTP_GetMicroseconds() - pkt->time_sent);
                        continue;
                }
        } while (--bits >= -1);
        return acked_bytes;
}

void UTPSocket::selective_ack(uint base, const byte *mask, byte len)
{
        if (cur_window_packets == 0) return;

        // the range is inclusive [0, 31] bits
        int bits = len * 8 - 1;

        int count = 0;

        // resends is a stack of sequence numbers we need to resend. Since we
        // iterate in reverse over the acked packets, at the end, the top packets
        // are the ones we want to resend
        int resends[32];
        int nr = 0;

        LOG_UTPV("0x%08x: Got EACK [%032b] base:%u", this, *(uint32*)mask, base);
        do {
                // we're iterating over the bits from higher sequence numbers
                // to lower (kind of in reverse order, wich might not be very
                // intuitive)
                uint v = base + bits;

                // ignore bits that haven't been sent yet
                // and bits that fall below the ACKed sequence number
                // this can happen if an EACK message gets
                // reordered and arrives after a packet that ACKs up past
                // the base for thie EACK message

                // this is essentially the same as:
                // if v >= seq_nr || v <= seq_nr - cur_window_packets
                // but it takes wrapping into account

                // if v == seq_nr the -1 will make it wrap. if v > seq_nr
                // it will also wrap (since it will fall further below 0)
                // and be > cur_window_packets.
                // if v == seq_nr - cur_window_packets, the result will be
                // seq_nr - (seq_nr - cur_window_packets) - 1
                // == seq_nr - seq_nr + cur_window_packets - 1
                // == cur_window_packets - 1 which will be caught by the
                // test. If v < seq_nr - cur_window_packets the result will grow
                // fall furhter outside of the cur_window_packets range.

                // sequence number space:
                //
                //     rejected <   accepted   > rejected 
                // <============+--------------+============>
                //              ^              ^
                //              |              |
                //        (seq_nr-wnd)         seq_nr

                if (((seq_nr - v - 1) & ACK_NR_MASK) >= (uint16)(cur_window_packets - 1))
                        continue;

                // this counts as a duplicate ack, even though we might have
                // received an ack for this packet previously (in another EACK
                // message for instance)
                bool bit_set = bits >= 0 && mask[bits>>3] & (1 << (bits & 7));

                // if this packet is acked, it counts towards the duplicate ack counter
                if (bit_set) count++;

                // ignore bits that represents packets we haven't sent yet
                // or packets that have already been acked
                OutgoingPacket *pkt = (OutgoingPacket*)outbuf.get(v);
                if (!pkt || pkt->transmissions == 0) {
                        LOG_UTPV("0x%08x: skipping %u. pkt:%08x transmissions:%u %s",
                                         this, v, pkt, pkt?pkt->transmissions:0, pkt?"(not sent yet?)":"(already acked?)");
                        continue;
                }

                // Count the number of segments that were successfully received past it.
                if (bit_set) {
                        // the selective ack should never ACK the packet we're waiting for to decrement cur_window_packets
                        assert((v & outbuf.mask) != ((seq_nr - cur_window_packets) & outbuf.mask));
                        ack_packet(v);
                        continue;
                }

                // Resend segments
                // if count is less than our re-send limit, we haven't seen enough
                // acked packets in front of this one to warrant a re-send.
                // if count == 0, we're still going through the tail of zeroes
                if (((v - fast_resend_seq_nr) & ACK_NR_MASK) <= OUTGOING_BUFFER_MAX_SIZE &&
                        count >= DUPLICATE_ACKS_BEFORE_RESEND &&
                        duplicate_ack < DUPLICATE_ACKS_BEFORE_RESEND) {
                        resends[nr++] = v;
                        LOG_UTPV("0x%08x: no ack for %u", this, v);
                } else {
                        LOG_UTPV("0x%08x: not resending %u count:%d dup_ack:%u fast_resend_seq_nr:%u",
                                         this, v, count, duplicate_ack, fast_resend_seq_nr);
                }
        } while (--bits >= -1);

        if (((base - 1 - fast_resend_seq_nr) & ACK_NR_MASK) < 256 &&
                count >= DUPLICATE_ACKS_BEFORE_RESEND &&
                duplicate_ack < DUPLICATE_ACKS_BEFORE_RESEND) {
                // if we get enough duplicate acks to start
                // resending, the first packet we should resend
                // is base-1
                resends[nr++] = base - 1;
        } else {
                LOG_UTPV("0x%08x: not resending %u count:%d dup_ack:%u fast_resend_seq_nr:%u",
                                 this, base - 1, count, duplicate_ack, fast_resend_seq_nr);
        }

        bool back_off = false;
        int i = 0;
        while (nr > 0) {
                uint v = resends[--nr];
                // don't consider the tail of 0:es to be lost packets
                // only unacked packets with acked packets after should
                // be considered lost
                OutgoingPacket *pkt = (OutgoingPacket*)outbuf.get(v);

                // this may be an old (re-ordered) packet, and some of the
                // packets in here may have been acked already. In which
                // case they will not be in the send queue anymore
                if (!pkt) continue;

                // used in parse_log.py
                LOG_UTP("0x%08x: Packet %u lost. Resending", this, v);

                // On Loss
                back_off = true;
#ifdef _DEBUG
                ++_stats._rexmit;
#endif
                send_packet(pkt);
                fast_resend_seq_nr = v + 1;

                // Re-send max 4 packets.
                if (++i >= 4) break;
        }

        if (back_off)
                maybe_decay_win();

        duplicate_ack = count;
}

void UTPSocket::apply_ledbat_ccontrol(size_t bytes_acked, uint32 actual_delay, int64 min_rtt)
{
        // the delay can never be greater than the rtt. The min_rtt
        // variable is the RTT in microseconds
        
        assert(min_rtt >= 0);
        int32 our_delay = min<uint32>(our_hist.get_value(), uint32(min_rtt));
        assert(our_delay != INT_MAX);
        assert(our_delay >= 0);

        SOCKADDR_STORAGE sa = addr.get_sockaddr_storage();
        UTP_DelaySample((sockaddr*)&sa, our_delay / 1000);

        // This test the connection under heavy load from foreground
        // traffic. Pretend that our delays are very high to force the
        // connection to use sub-packet size window sizes
        //our_delay *= 4;

        // target is microseconds
        int target = CCONTROL_TARGET;
        if (target <= 0) target = 100000;

        double off_target = target - our_delay;

        // this is the same as:
        //
        //    (min(off_target, target) / target) * (bytes_acked / max_window) * MAX_CWND_INCREASE_BYTES_PER_RTT
        //
        // so, it's scaling the max increase by the fraction of the window this ack represents, and the fraction
        // of the target delay the current delay represents.
        // The min() around off_target protects against crazy values of our_delay, which may happen when th
        // timestamps wraps, or by just having a malicious peer sending garbage. This caps the increase
        // of the window size to MAX_CWND_INCREASE_BYTES_PER_RTT per rtt.
        // as for large negative numbers, this direction is already capped at the min packet size further down
        // the min around the bytes_acked protects against the case where the window size was recently
        // shrunk and the number of acked bytes exceeds that. This is considered no more than one full
        // window, in order to keep the gain within sane boundries.

        assert(bytes_acked > 0);
        double window_factor = (double)min(bytes_acked, max_window) / (double)max(max_window, bytes_acked);
        double delay_factor = off_target / target;
        double scaled_gain = MAX_CWND_INCREASE_BYTES_PER_RTT * window_factor * delay_factor;

        // since MAX_CWND_INCREASE_BYTES_PER_RTT is a cap on how much the window size (max_window)
        // may increase per RTT, we may not increase the window size more than that proportional
        // to the number of bytes that were acked, so that once one window has been acked (one rtt)
        // the increase limit is not exceeded
        // the +1. is to allow for floating point imprecision
        assert(scaled_gain <= 1. + MAX_CWND_INCREASE_BYTES_PER_RTT * (int)min(bytes_acked, max_window) / (double)max(max_window, bytes_acked));

        if (scaled_gain > 0 && g_current_ms - last_maxed_out_window > 300) {
                // if it was more than 300 milliseconds since we tried to send a packet
                // and stopped because we hit the max window, we're most likely rate
                // limited (which prevents us from ever hitting the window size)
                // if this is the case, we cannot let the max_window grow indefinitely
                scaled_gain = 0;
        }

        if (scaled_gain + max_window < MIN_WINDOW_SIZE) {
                max_window = MIN_WINDOW_SIZE;
        } else {
                max_window = (size_t)(max_window + scaled_gain);
        }

        // make sure that the congestion window is below max
        // make sure that we don't shrink our window too small
        max_window = clamp<size_t>(max_window, MIN_WINDOW_SIZE, opt_sndbuf);

        // used in parse_log.py
        LOG_UTP("0x%08x: actual_delay:%u our_delay:%d their_delay:%u off_target:%d max_window:%u "
                        "delay_base:%u delay_sum:%d target_delay:%d acked_bytes:%u cur_window:%u "
                        "scaled_gain:%f rtt:%u rate:%u quota:%d wnduser:%u rto:%u timeout:%d get_microseconds:"I64u" "
                        "cur_window_packets:%u packet_size:%u their_delay_base:%u their_actual_delay:%u",
                        this, actual_delay, our_delay / 1000, their_hist.get_value() / 1000,
                        (int)off_target / 1000, (uint)(max_window),  our_hist.delay_base,
                        (our_delay + their_hist.get_value()) / 1000, target / 1000, (uint)bytes_acked,
                        (uint)(cur_window - bytes_acked), (float)(scaled_gain), rtt,
                        (uint)(max_window * 1000 / (rtt_hist.delay_base?rtt_hist.delay_base:50)),
                        send_quota / 100, (uint)max_window_user, rto, (int)(rto_timeout - g_current_ms),
                        UTP_GetMicroseconds(), cur_window_packets, (uint)get_packet_size(),
                        their_hist.delay_base, their_hist.delay_base + their_hist.get_value());
}

static void UTP_RegisterRecvPacket(UTPSocket *conn, size_t len)
{
#ifdef _DEBUG
        ++conn->_stats._nrecv;
        conn->_stats._nbytes_recv += len;
#endif

        if (len <= PACKET_SIZE_MID) {
                if (len <= PACKET_SIZE_EMPTY) {
                        _global_stats._nraw_recv[PACKET_SIZE_EMPTY_BUCKET]++;
                } else if (len <= PACKET_SIZE_SMALL) {
                        _global_stats._nraw_recv[PACKET_SIZE_SMALL_BUCKET]++;
                } else 
                        _global_stats._nraw_recv[PACKET_SIZE_MID_BUCKET]++;
        } else {
                if (len <= PACKET_SIZE_BIG) {
                        _global_stats._nraw_recv[PACKET_SIZE_BIG_BUCKET]++;
                } else 
                        _global_stats._nraw_recv[PACKET_SIZE_HUGE_BUCKET]++;
        }
}

// returns the max number of bytes of payload the uTP
// connection is allowed to send
size_t UTPSocket::get_packet_size()
{
        int header_size = version == 1
                ? sizeof(PacketFormatV1)
                : sizeof(PacketFormat);

        size_t mtu = get_udp_mtu();

        if (DYNAMIC_PACKET_SIZE_ENABLED) {
                SOCKADDR_STORAGE sa = addr.get_sockaddr_storage();
                size_t max_packet_size = UTP_GetPacketSize((sockaddr*)&sa);
                return min(mtu - header_size, max_packet_size);
        }
        else
        {
                return mtu - header_size;
        }
}

// Process an incoming packet
// syn is true if this is the first packet received. It will cut off parsing
// as soon as the header is done
size_t UTP_ProcessIncoming(UTPSocket *conn, const byte *packet, size_t len, bool syn = false)
{
        UTP_RegisterRecvPacket(conn, len);

        g_current_ms = UTP_GetMilliseconds();

        conn->update_send_quota();

        const PacketFormat *pf = (PacketFormat*)packet;
        const PacketFormatV1 *pf1 = (PacketFormatV1*)packet;
        const byte *packet_end = packet + len;

        uint16 pk_seq_nr;
        uint16 pk_ack_nr;
        uint8 pk_flags;
        if (conn->version == 0) {
                pk_seq_nr = pf->seq_nr;
                pk_ack_nr = pf->ack_nr;
                pk_flags = pf->flags;
        } else {
                pk_seq_nr = pf1->seq_nr;
                pk_ack_nr = pf1->ack_nr;
                pk_flags = pf1->type();
        }

        if (pk_flags >= ST_NUM_STATES) return 0;

        LOG_UTPV("0x%08x: Got %s. seq_nr:%u ack_nr:%u state:%s version:%u timestamp:"I64u" reply_micro:%u",
                         conn, flagnames[pk_flags], pk_seq_nr, pk_ack_nr, statenames[conn->state], conn->version,
                         conn->version == 0?(uint64)(pf->tv_sec) * 1000000 + pf->tv_usec:uint64(pf1->tv_usec),
                         conn->version == 0?(uint32)(pf->reply_micro):(uint32)(pf1->reply_micro));

        // mark receipt time
        uint64 time = UTP_GetMicroseconds();

        // RSTs are handled earlier, since the connid matches the send id not the recv id
        assert(pk_flags != ST_RESET);

        // TODO: maybe send a ST_RESET if we're in CS_RESET?

        const byte *selack_ptr = NULL;

        // Unpack UTP packet options
        // Data pointer
        const byte *data = (const byte*)pf + conn->get_header_size();
        if (conn->get_header_size() > len) {
                LOG_UTPV("0x%08x: Invalid packet size (less than header size)", conn);
                return 0;
        }
        // Skip the extension headers
        uint extension = conn->version == 0 ? pf->ext : pf1->ext;
        if (extension != 0) {
                do {
                        // Verify that the packet is valid.
                        data += 2;

                        if ((int)(packet_end - data) < 0 || (int)(packet_end - data) < data[-1]) {
                                LOG_UTPV("0x%08x: Invalid len of extensions", conn);
                                return 0;
                        }

                        switch(extension) {
                        case 1: // Selective Acknowledgment
                                selack_ptr = data;
                                break;
                        case 2: // extension bits
                                if (data[-1] != 8) {
                                        LOG_UTPV("0x%08x: Invalid len of extension bits header", conn);
                                        return 0;
                                }
                                memcpy(conn->extensions, data, 8);
                                LOG_UTPV("0x%08x: got extension bits:%02x%02x%02x%02x%02x%02x%02x%02x", conn,
                                        conn->extensions[0], conn->extensions[1], conn->extensions[2], conn->extensions[3],
                                        conn->extensions[4], conn->extensions[5], conn->extensions[6], conn->extensions[7]);
                        }
                        extension = data[-2];
                        data += data[-1];
                } while (extension);
        }

        if (conn->state == CS_SYN_SENT) {
                // if this is a syn-ack, initialize our ack_nr
                // to match the sequence number we got from
                // the other end
                conn->ack_nr = (pk_seq_nr - 1) & SEQ_NR_MASK;
        }

        g_current_ms = UTP_GetMilliseconds();
        conn->last_got_packet = g_current_ms;

        if (syn) {
                return 0;
        }

        // seqnr is the number of packets past the expected
        // packet this is. ack_nr is the last acked, seq_nr is the
        // current. Subtracring 1 makes 0 mean "this is the next
        // expected packet".
        const uint seqnr = (pk_seq_nr - conn->ack_nr - 1) & SEQ_NR_MASK;

        // Getting an invalid sequence number?
        if (seqnr >= REORDER_BUFFER_MAX_SIZE) {
                if (seqnr >= (SEQ_NR_MASK + 1) - REORDER_BUFFER_MAX_SIZE && pk_flags != ST_STATE) {
                        conn->ack_time = g_current_ms + min<uint>(conn->ack_time - g_current_ms, DELAYED_ACK_TIME_THRESHOLD);
                }
                LOG_UTPV("    Got old Packet/Ack (%u/%u)=%u!", pk_seq_nr, conn->ack_nr, seqnr);
                return 0;
        }

        // Process acknowledgment
        // acks is the number of packets that was acked
        int acks = (pk_ack_nr - (conn->seq_nr - 1 - conn->cur_window_packets)) & ACK_NR_MASK;

        // this happens when we receive an old ack nr
        if (acks > conn->cur_window_packets) acks = 0;

        // if we get the same ack_nr as in the last packet
        // increase the duplicate_ack counter, otherwise reset
        // it to 0
        if (conn->cur_window_packets > 0) {
                if (pk_ack_nr == ((conn->seq_nr - conn->cur_window_packets - 1) & ACK_NR_MASK) &&
                        conn->cur_window_packets > 0) {
                        //++conn->duplicate_ack;
                } else {
                        conn->duplicate_ack = 0;
                }

                // TODO: if duplicate_ack == DUPLICATE_ACK_BEFORE_RESEND
                // and fast_resend_seq_nr <= ack_nr + 1
                //    resend ack_nr + 1
        }

        // figure out how many bytes were acked
        size_t acked_bytes = 0;

        // the minimum rtt of all acks
        // this is the upper limit on the delay we get back
        // from the other peer. Our delay cannot exceed
        // the rtt of the packet. If it does, clamp it.
        // this is done in apply_ledbat_ccontrol()
        int64 min_rtt = INT64_MAX;

        for (int i = 0; i < acks; ++i) {
                int seq = conn->seq_nr - conn->cur_window_packets + i;
                OutgoingPacket *pkt = (OutgoingPacket*)conn->outbuf.get(seq);
                if (pkt == 0 || pkt->transmissions == 0) continue;
                assert((int)(pkt->payload) >= 0);
                acked_bytes += pkt->payload;
                min_rtt = min<int64>(min_rtt, UTP_GetMicroseconds() - pkt->time_sent);
        }
        
        // count bytes acked by EACK
        if (selack_ptr != NULL) {
                acked_bytes += conn->selective_ack_bytes((pk_ack_nr + 2) & ACK_NR_MASK,
                                                                                                 selack_ptr, selack_ptr[-1], min_rtt);
        }

        LOG_UTPV("0x%08x: acks:%d acked_bytes:%u seq_nr:%d cur_window:%u cur_window_packets:%u relative_seqnr:%u max_window:%u min_rtt:%u rtt:%u",
                         conn, acks, (uint)acked_bytes, conn->seq_nr, (uint)conn->cur_window, conn->cur_window_packets,
                         seqnr, (uint)conn->max_window, (uint)(min_rtt / 1000), conn->rtt);

        uint64 p;

        if (conn->version == 0) {
                p = uint64(pf->tv_sec) * 1000000 + pf->tv_usec;
        } else {
                p = pf1->tv_usec;
        }

        conn->last_measured_delay = g_current_ms;

        // get delay in both directions
        // record the delay to report back
        const uint32 their_delay = (uint32)(p == 0 ? 0 : time - p);
        conn->reply_micro = their_delay;
        uint32 prev_delay_base = conn->their_hist.delay_base;
        if (their_delay != 0) conn->their_hist.add_sample(their_delay);

        // if their new delay base is less than their previous one
        // we should shift our delay base in the other direction in order
        // to take the clock skew into account
        if (prev_delay_base != 0 &&
                wrapping_compare_less(conn->their_hist.delay_base, prev_delay_base)) {
                // never adjust more than 10 milliseconds
                if (prev_delay_base - conn->their_hist.delay_base <= 10000) {
                        conn->our_hist.shift(prev_delay_base - conn->their_hist.delay_base);
                }
        }

        const uint32 actual_delay = conn->version==0
                ?(pf->reply_micro==INT_MAX?0:uint32(pf->reply_micro))
                :(uint32(pf1->reply_micro)==INT_MAX?0:uint32(pf1->reply_micro));

        // if the actual delay is 0, it means the other end
        // hasn't received a sample from us yet, and doesn't
        // know what it is. We can't update out history unless
        // we have a true measured sample
        prev_delay_base = conn->our_hist.delay_base;
        if (actual_delay != 0) conn->our_hist.add_sample(actual_delay);

        // if our new delay base is less than our previous one
        // we should shift the other end's delay base in the other
        // direction in order to take the clock skew into account
        // This is commented out because it creates bad interactions
        // with our adjustment in the other direction. We don't really
        // need our estimates of the other peer to be very accurate
        // anyway. The problem with shifting here is that we're more
        // likely shift it back later because of a low latency. This
        // second shift back would cause us to shift our delay base
        // which then get's into a death spiral of shifting delay bases
/*      if (prev_delay_base != 0 &&
                wrapping_compare_less(conn->our_hist.delay_base, prev_delay_base)) {
                // never adjust more than 10 milliseconds
                if (prev_delay_base - conn->our_hist.delay_base <= 10000) {
                        conn->their_hist.Shift(prev_delay_base - conn->our_hist.delay_base);
                }
        }
*/

        // if the delay estimate exceeds the RTT, adjust the base_delay to
        // compensate
        if (conn->our_hist.get_value() > uint32(min_rtt)) {
                conn->our_hist.shift(conn->our_hist.get_value() - min_rtt);
        }

        // only apply the congestion controller on acks
        // if we don't have a delay measurement, there's
        // no point in invoking the congestion control
        if (actual_delay != 0 && acked_bytes >= 1)
                conn->apply_ledbat_ccontrol(acked_bytes, actual_delay, min_rtt);

        // sanity check, the other end should never ack packets
        // past the point we've sent
        if (acks <= conn->cur_window_packets) {
                conn->max_window_user = conn->version == 0
                        ? pf->windowsize * PACKET_SIZE : pf1->windowsize;

                // If max user window is set to 0, then we startup a timer
                // That will reset it to 1 after 15 seconds.
                if (conn->max_window_user == 0)
                        // Reset max_window_user to 1 every 15 seconds.
                        conn->zerowindow_time = g_current_ms + 15000;

                // Respond to connect message
                // Switch to CONNECTED state.
                if (conn->state == CS_SYN_SENT) {
                        conn->state = CS_CONNECTED;
                        conn->func.on_state(conn->userdata, UTP_STATE_CONNECT);

                // We've sent a fin, and everything was ACKed (including the FIN),
                // it's safe to destroy the socket. cur_window_packets == acks
                // means that this packet acked all the remaining packets that
                // were in-flight.
                } else if (conn->state == CS_FIN_SENT && conn->cur_window_packets == acks) {
                        conn->state = CS_DESTROY;
                }

                // Update fast resend counter
                if (wrapping_compare_less(conn->fast_resend_seq_nr, (pk_ack_nr + 1) & ACK_NR_MASK))
                        conn->fast_resend_seq_nr = pk_ack_nr + 1;

                LOG_UTPV("0x%08x: fast_resend_seq_nr:%u", conn, conn->fast_resend_seq_nr);

                for (int i = 0; i < acks; ++i) {
                        int ack_status = conn->ack_packet(conn->seq_nr - conn->cur_window_packets);
                        // if ack_status is 0, the packet was acked.
                        // if acl_stauts is 1, it means that the packet had already been acked
                        // if it's 2, the packet has not been sent yet
                        // We need to break this loop in the latter case. This could potentially
                        // happen if we get an ack_nr that does not exceed what we have stuffed
                        // into the outgoing buffer, but does exceed what we have sent
                        if (ack_status == 2) {
#ifdef _DEBUG
                                OutgoingPacket* pkt = (OutgoingPacket*)conn->outbuf.get(conn->seq_nr - conn->cur_window_packets);
                                assert(pkt->transmissions == 0);
#endif
                                break;
                        }
                        conn->cur_window_packets--;
                }
#ifdef _DEBUG
                if (conn->cur_window_packets == 0) assert(conn->cur_window == 0);
#endif

                // packets in front of this may have been acked by a
                // selective ack (EACK). Keep decreasing the window packet size
                // until we hit a packet that is still waiting to be acked
                // in the send queue
                // this is especially likely to happen when the other end
                // has the EACK send bug older versions of uTP had
                while (conn->cur_window_packets > 0 && !conn->outbuf.get(conn->seq_nr - conn->cur_window_packets))
                        conn->cur_window_packets--;

#ifdef _DEBUG
                if (conn->cur_window_packets == 0) assert(conn->cur_window == 0);
#endif

                // this invariant should always be true
                assert(conn->cur_window_packets == 0 || conn->outbuf.get(conn->seq_nr - conn->cur_window_packets));

                // flush Nagle
                if (conn->cur_window_packets == 1) {
                        OutgoingPacket *pkt = (OutgoingPacket*)conn->outbuf.get(conn->seq_nr - 1);
                        // do we still have quota?
                        if (pkt->transmissions == 0 &&
                                (!(USE_PACKET_PACING) || conn->send_quota / 100 >= (int32)pkt->length)) {
                                conn->send_packet(pkt);

                                // No need to send another ack if there is nothing to reorder.
                                if (conn->reorder_count == 0) {
                                        conn->sent_ack();
                                }
                        }
                }

                // Fast timeout-retry
                if (conn->fast_timeout) {
                        LOG_UTPV("Fast timeout %u,%u,%u?", (uint)conn->cur_window, conn->seq_nr - conn->timeout_seq_nr, conn->timeout_seq_nr);
                        // if the fast_resend_seq_nr is not pointing to the oldest outstanding packet, it suggests that we've already
                        // resent the packet that timed out, and we should leave the fast-timeout mode.
                        if (((conn->seq_nr - conn->cur_window_packets) & ACK_NR_MASK) != conn->fast_resend_seq_nr) {
                                conn->fast_timeout = false;
                        } else {
                                // resend the oldest packet and increment fast_resend_seq_nr
                                // to not allow another fast resend on it again
                                OutgoingPacket *pkt = (OutgoingPacket*)conn->outbuf.get(conn->seq_nr - conn->cur_window_packets);
                                if (pkt && pkt->transmissions > 0) {
                                        LOG_UTPV("0x%08x: Packet %u fast timeout-retry.", conn, conn->seq_nr - conn->cur_window_packets);
#ifdef _DEBUG
                                        ++conn->_stats._fastrexmit;
#endif
                                        conn->fast_resend_seq_nr++;
                                        conn->send_packet(pkt);
                                }
                        }
                }
        }

        // Process selective acknowledgent
        if (selack_ptr != NULL) {
                conn->selective_ack(pk_ack_nr + 2, selack_ptr, selack_ptr[-1]);
        }

        // this invariant should always be true
        assert(conn->cur_window_packets == 0 || conn->outbuf.get(conn->seq_nr - conn->cur_window_packets));

        LOG_UTPV("0x%08x: acks:%d acked_bytes:%u seq_nr:%u cur_window:%u cur_window_packets:%u quota:%d",
                         conn, acks, (uint)acked_bytes, conn->seq_nr, (uint)conn->cur_window, conn->cur_window_packets,
                         conn->send_quota / 100);

        // In case the ack dropped the current window below
        // the max_window size, Mark the socket as writable
        if (conn->state == CS_CONNECTED_FULL && conn->is_writable(conn->get_packet_size())) {
                conn->state = CS_CONNECTED;
                LOG_UTPV("0x%08x: Socket writable. max_window:%u cur_window:%u quota:%d packet_size:%u",
                                 conn, (uint)conn->max_window, (uint)conn->cur_window, conn->send_quota / 100, (uint)conn->get_packet_size());
                conn->func.on_state(conn->userdata, UTP_STATE_WRITABLE);
        }

        if (pk_flags == ST_STATE) {
                // This is a state packet only.
                return 0;
        }

        // The connection is not in a state that can accept data?
        if (conn->state != CS_CONNECTED &&
                conn->state != CS_CONNECTED_FULL &&
                conn->state != CS_FIN_SENT) {
                return 0;
        }

        // Is this a finalize packet?
        if (pk_flags == ST_FIN && !conn->got_fin) {
                LOG_UTPV("Got FIN eof_pkt:%u", pk_seq_nr);
                conn->got_fin = true;
                conn->eof_pkt = pk_seq_nr;
                // at this point, it is possible for the
                // other end to have sent packets with
                // sequence numbers higher than seq_nr.
                // if this is the case, our reorder_count
                // is out of sync. This case is dealt with
                // when we re-order and hit the eof_pkt.
                // we'll just ignore any packets with
                // sequence numbers past this
        }

        // Getting an in-order packet?
        if (seqnr == 0) {
                size_t count = packet_end - data;
                if (count > 0 && conn->state != CS_FIN_SENT) {
                        LOG_UTPV("0x%08x: Got Data len:%u (rb:%u)", conn, (uint)count, (uint)conn->func.get_rb_size(conn->userdata));
                        // Post bytes to the upper layer
                        conn->func.on_read(conn->userdata, data, count);
                }
                conn->ack_nr++;
                conn->bytes_since_ack += count;

                // Check if the next packet has been received too, but waiting
                // in the reorder buffer.
                for (;;) {

                        if (conn->got_fin && conn->eof_pkt == conn->ack_nr) {
                                if (conn->state != CS_FIN_SENT) {
                                        conn->state = CS_GOT_FIN;
                                        conn->rto_timeout = g_current_ms + min<uint>(conn->rto * 3, 60);

                                        LOG_UTPV("0x%08x: Posting EOF", conn);
                                        conn->func.on_state(conn->userdata, UTP_STATE_EOF);
                                }

                                // if the other end wants to close, ack immediately
                                conn->send_ack();

                                // reorder_count is not necessarily 0 at this point.
                                // even though it is most of the time, the other end
                                // may have sent packets with higher sequence numbers
                                // than what later end up being eof_pkt
                                // since we have received all packets up to eof_pkt
                                // just ignore the ones after it.
                                conn->reorder_count = 0;
                        }

                        // Quick get-out in case there is nothing to reorder
                        if (conn->reorder_count == 0)
                                break;

                        // Check if there are additional buffers in the reorder buffers
                        // that need delivery.
                        byte *p = (byte*)conn->inbuf.get(conn->ack_nr+1);
                        if (p == NULL)
                                break;
                        conn->inbuf.put(conn->ack_nr+1, NULL);
                        count = *(uint*)p;
                        if (count > 0 && conn->state != CS_FIN_SENT) {
                                // Pass the bytes to the upper layer
                                conn->func.on_read(conn->userdata, p + sizeof(uint), count);
                        }
                        conn->ack_nr++;
                        conn->bytes_since_ack += count;

                        // Free the element from the reorder buffer
                        free(p);
                        assert(conn->reorder_count > 0);
                        conn->reorder_count--;
                }

                // start the delayed ACK timer
                conn->ack_time = g_current_ms + min<uint>(conn->ack_time - g_current_ms, DELAYED_ACK_TIME_THRESHOLD);
        } else {
                // Getting an out of order packet.
                // The packet needs to be remembered and rearranged later.

                // if we have received a FIN packet, and the EOF-sequence number
                // is lower than the sequence number of the packet we just received
                // something is wrong.
                if (conn->got_fin && pk_seq_nr > conn->eof_pkt) {
                        LOG_UTPV("0x%08x: Got an invalid packet sequence number, past EOF "
                                "reorder_count:%u len:%u (rb:%u)",
                                conn, conn->reorder_count, (uint)(packet_end - data), (uint)conn->func.get_rb_size(conn->userdata));
                        return 0;
                }

                // if the sequence number is entirely off the expected
                // one, just drop it. We can't allocate buffer space in
                // the inbuf entirely based on untrusted input
                if (seqnr > 0x3ff) {
                        LOG_UTPV("0x%08x: Got an invalid packet sequence number, too far off "
                                "reorder_count:%u len:%u (rb:%u)",
                                conn, conn->reorder_count, (uint)(packet_end - data), (uint)conn->func.get_rb_size(conn->userdata));
                        return 0;
                }

                // we need to grow the circle buffer before we
                // check if the packet is already in here, so that
                // we don't end up looking at an older packet (since
                // the indices wraps around).
                conn->inbuf.ensure_size(pk_seq_nr + 1, seqnr + 1);

                // Has this packet already been received? (i.e. a duplicate)
                // If that is the case, just discard it.
                if (conn->inbuf.get(pk_seq_nr) != NULL) {
#ifdef _DEBUG
                        ++conn->_stats._nduprecv;
#endif
                        return 0;
                }

                // Allocate memory to fit the packet that needs to re-ordered
                byte *mem = (byte*)malloc((packet_end - data) + sizeof(uint));
                *(uint*)mem = (uint)(packet_end - data);
                memcpy(mem + sizeof(uint), data, packet_end - data);

                // Insert into reorder buffer and increment the count
                // of # of packets to be reordered.
                // we add one to seqnr in order to leave the last
                // entry empty, that way the assert in send_ack
                // is valid. we have to add one to seqnr too, in order
                // to make the circular buffer grow around the correct
                // point (which is conn->ack_nr + 1).
                assert(conn->inbuf.get(pk_seq_nr) == NULL);
                assert((pk_seq_nr & conn->inbuf.mask) != ((conn->ack_nr+1) & conn->inbuf.mask));
                conn->inbuf.put(pk_seq_nr, mem);
                conn->reorder_count++;

                LOG_UTPV("0x%08x: Got out of order data reorder_count:%u len:%u (rb:%u)",
                        conn, conn->reorder_count, (uint)(packet_end - data), (uint)conn->func.get_rb_size(conn->userdata));

                // Setup so the partial ACK message will get sent immediately.
                conn->ack_time = g_current_ms + min<uint>(conn->ack_time - g_current_ms, 1);
        }

        // If ack_time or ack_bytes indicate that we need to send and ack, send one
        // here instead of waiting for the timer to trigger
        LOG_UTPV("bytes_since_ack:%u ack_time:%d",
                         (uint)conn->bytes_since_ack, (int)(g_current_ms - conn->ack_time));
        if (conn->state == CS_CONNECTED || conn->state == CS_CONNECTED_FULL) {
                if (conn->bytes_since_ack > DELAYED_ACK_BYTE_THRESHOLD ||
                        (int)(g_current_ms - conn->ack_time) >= 0) {
                        conn->send_ack();
                }
        }
        return (size_t)(packet_end - data);
}

inline bool UTP_IsV1(PacketFormatV1 const* pf)
{
        return pf->version() == 1 && pf->type() < ST_NUM_STATES && pf->ext < 3;
}

void UTP_Free(UTPSocket *conn)
{
        LOG_UTPV("0x%08x: Killing socket", conn);

        conn->func.on_state(conn->userdata, UTP_STATE_DESTROYING);
        UTP_SetCallbacks(conn, NULL, NULL);

        assert(conn->idx < g_utp_sockets.GetCount());
        assert(g_utp_sockets[conn->idx] == conn);

        // Unlink object from the global list
        assert(g_utp_sockets.GetCount() > 0);

        UTPSocket *last = g_utp_sockets[g_utp_sockets.GetCount() - 1];

        assert(last->idx < g_utp_sockets.GetCount());
        assert(g_utp_sockets[last->idx] == last);

        last->idx = conn->idx;
        
        g_utp_sockets[conn->idx] = last;

        // Decrease the count
        g_utp_sockets.SetCount(g_utp_sockets.GetCount() - 1);

        // Free all memory occupied by the socket object.
        for (size_t i = 0; i <= conn->inbuf.mask; i++) {
                free(conn->inbuf.elements[i]);
        }
        for (size_t i = 0; i <= conn->outbuf.mask; i++) {
                free(conn->outbuf.elements[i]);
        }
        free(conn->inbuf.elements);
        free(conn->outbuf.elements);

        // Finally free the socket object
        free(conn);
}


// Public functions:
///////////////////////////////////////////////////////////////////////////////

// Create a UTP socket
UTPSocket *UTP_Create(SendToProc *send_to_proc, void *send_to_userdata, const struct sockaddr *addr, socklen_t addrlen)
{
        UTPSocket *conn = (UTPSocket*)calloc(1, sizeof(UTPSocket));

        g_current_ms = UTP_GetMilliseconds();

        UTP_SetCallbacks(conn, NULL, NULL);
        conn->our_hist.clear();
        conn->their_hist.clear();
        conn->rto = 3000;
        conn->rtt_var = 800;
        conn->seq_nr = 1;
        conn->ack_nr = 0;
        conn->max_window_user = 255 * PACKET_SIZE;
        conn->addr = PackedSockAddr((const SOCKADDR_STORAGE*)addr, addrlen);
        conn->send_to_proc = send_to_proc;
        conn->send_to_userdata = send_to_userdata;
        conn->ack_time = g_current_ms + 0x70000000;
        conn->last_got_packet = g_current_ms;
        conn->last_sent_packet = g_current_ms;
        conn->last_measured_delay = g_current_ms + 0x70000000;
        conn->last_rwin_decay = int32(g_current_ms) - MAX_WINDOW_DECAY;
        conn->last_send_quota = g_current_ms;
        conn->send_quota = PACKET_SIZE * 100;
        conn->cur_window_packets = 0;
        conn->fast_resend_seq_nr = conn->seq_nr;

        // default to version 1
        UTP_SetSockopt(conn, SO_UTPVERSION, 1);

        // we need to fit one packet in the window
        // when we start the connection
        conn->max_window = conn->get_packet_size();
        conn->state = CS_IDLE;

        conn->outbuf.mask = 15;
        conn->inbuf.mask = 15;

        conn->outbuf.elements = (void**)calloc(16, sizeof(void*));
        conn->inbuf.elements = (void**)calloc(16, sizeof(void*));

        conn->idx = g_utp_sockets.Append(conn);

        LOG_UTPV("0x%08x: UTP_Create", conn);

        return conn;
}

void UTP_SetCallbacks(UTPSocket *conn, UTPFunctionTable *funcs, void *userdata)
{
        assert(conn);

        if (funcs == NULL) {
                funcs = &zero_funcs;
        }
        conn->func = *funcs;
        conn->userdata = userdata;
}

bool UTP_SetSockopt(UTPSocket* conn, int opt, int val)
{
        assert(conn);

        switch (opt) {
        case SO_SNDBUF:
                assert(val >= 1);
                conn->opt_sndbuf = val;
                return true;
        case SO_RCVBUF:
                conn->opt_rcvbuf = val;
                return true;
        case SO_UTPVERSION:
                assert(conn->state == CS_IDLE);
                if (conn->state != CS_IDLE) {
                        // too late
                        return false;
                }
                if (conn->version == 1 && val == 0) {
                        conn->reply_micro = INT_MAX;
                        conn->opt_rcvbuf = 200 * 1024;
                        conn->opt_sndbuf = OUTGOING_BUFFER_MAX_SIZE * PACKET_SIZE;
                } else if (conn->version == 0 && val == 1) {
                        conn->reply_micro = 0;
                        conn->opt_rcvbuf = 3 * 1024 * 1024 + 512 * 1024;
                        conn->opt_sndbuf = conn->opt_rcvbuf;
                }
                conn->version = val;
                return true;
        }

        return false;
}

// Try to connect to a specified host.
// 'initial' is the number of data bytes to send in the connect packet.
void UTP_Connect(UTPSocket *conn)
{
        assert(conn);

        assert(conn->state == CS_IDLE);
        assert(conn->cur_window_packets == 0);
        assert(conn->outbuf.get(conn->seq_nr) == NULL);
        assert(sizeof(PacketFormatV1) == 20);

        conn->state = CS_SYN_SENT;

        g_current_ms = UTP_GetMilliseconds();

        // Create and send a connect message
        uint32 conn_seed = UTP_Random();

        // we identify newer versions by setting the
        // first two bytes to 0x0001
        if (conn->version > 0) {
                conn_seed &= 0xffff;
        }

        // used in parse_log.py
        LOG_UTP("0x%08x: UTP_Connect conn_seed:%u packet_size:%u (B) "
                        "target_delay:%u (ms) delay_history:%u "
                        "delay_base_history:%u (minutes)",
                        conn, conn_seed, PACKET_SIZE, CCONTROL_TARGET / 1000,
                        CUR_DELAY_SIZE, DELAY_BASE_HISTORY);

        // Setup initial timeout timer.
        conn->retransmit_timeout = 3000;
        conn->rto_timeout = g_current_ms + conn->retransmit_timeout;
        conn->last_rcv_win = conn->get_rcv_window();

        conn->conn_seed = conn_seed;
        conn->conn_id_recv = conn_seed;
        conn->conn_id_send = conn_seed+1;
        // if you need compatibiltiy with 1.8.1, use this. it increases attackability though.
        //conn->seq_nr = 1;
        conn->seq_nr = UTP_Random();

        // Create the connect packet.
        const size_t header_ext_size = conn->get_header_extensions_size();

        OutgoingPacket *pkt = (OutgoingPacket*)malloc(sizeof(OutgoingPacket) - 1 + header_ext_size);

        PacketFormatExtensions* p = (PacketFormatExtensions*)pkt->data;
        PacketFormatExtensionsV1* p1 = (PacketFormatExtensionsV1*)pkt->data;

        memset(p, 0, header_ext_size);
        // SYN packets are special, and have the receive ID in the connid field,
        // instead of conn_id_send.
        if (conn->version == 0) {
                p->pf.connid = conn->conn_id_recv;
                p->pf.ext = 2;
                p->pf.windowsize = (byte)DIV_ROUND_UP(conn->last_rcv_win, PACKET_SIZE);
                p->pf.seq_nr = conn->seq_nr;
                p->pf.flags = ST_SYN;
                p->ext_next = 0;
                p->ext_len = 8;
                memset(p->extensions, 0, 8);
        } else {
                p1->pf.set_version(1);
                p1->pf.set_type(ST_SYN);
                p1->pf.ext = 2;
                p1->pf.connid = conn->conn_id_recv;
                p1->pf.windowsize = (uint32)conn->last_rcv_win;
                p1->pf.seq_nr = conn->seq_nr;
                p1->ext_next = 0;
                p1->ext_len = 8;
                memset(p1->extensions, 0, 8);
        }
        pkt->transmissions = 0;
        pkt->length = header_ext_size;
        pkt->payload = 0;

        //LOG_UTPV("0x%08x: Sending connect %s [%u].",
        //               conn, addrfmt(conn->addr, addrbuf), conn_seed);

        // Remember the message in the outgoing queue.
        conn->outbuf.ensure_size(conn->seq_nr, conn->cur_window_packets);
        conn->outbuf.put(conn->seq_nr, pkt);
        conn->seq_nr++;
        conn->cur_window_packets++;

        conn->send_packet(pkt);
}

bool UTP_IsIncomingUTP(UTPGotIncomingConnection *incoming_proc,
                                           SendToProc *send_to_proc, void *send_to_userdata,
                                           const byte *buffer, size_t len, const struct sockaddr *to, socklen_t tolen)
{
        const PackedSockAddr addr((const SOCKADDR_STORAGE*)to, tolen);

        if (len < sizeof(PacketFormat) && len < sizeof(PacketFormatV1)) {
                LOG_UTPV("recv %s len:%u too small", addrfmt(addr, addrbuf), (uint)len);
                return false;
        }

        const PacketFormat* p = (PacketFormat*)buffer;
        const PacketFormatV1* p1 = (PacketFormatV1*)buffer;

        const byte version = UTP_IsV1(p1);
        const uint32 id = (version == 0) ? p->connid : uint32(p1->connid);

        if (version == 0 && len < sizeof(PacketFormat)) {
                LOG_UTPV("recv %s len:%u version:%u too small", addrfmt(addr, addrbuf), (uint)len, version);
                return false;
        }

        if (version == 1 && len < sizeof(PacketFormatV1)) {
                LOG_UTPV("recv %s len:%u version:%u too small", addrfmt(addr, addrbuf), (uint)len, version);
                return false;
        }

        LOG_UTPV("recv %s len:%u id:%u", addrfmt(addr, addrbuf), (uint)len, id);

        const PacketFormat *pf = (PacketFormat*)p;
        const PacketFormatV1 *pf1 = (PacketFormatV1*)p;

        if (version == 0) {
                LOG_UTPV("recv id:%u seq_nr:%u ack_nr:%u", id, (uint)pf->seq_nr, (uint)pf->ack_nr);
        } else {
                LOG_UTPV("recv id:%u seq_nr:%u ack_nr:%u", id, (uint)pf1->seq_nr, (uint)pf1->ack_nr);
        }

        const byte flags = version == 0 ? pf->flags : pf1->type();

        for (size_t i = 0; i < g_utp_sockets.GetCount(); i++) {
                UTPSocket *conn = g_utp_sockets[i];
                //LOG_UTPV("Examining UTPSocket %s for %s and (seed:%u s:%u r:%u) for %u",
                //              addrfmt(conn->addr, addrbuf), addrfmt(addr, addrbuf2), conn->conn_seed, conn->conn_id_send, conn->conn_id_recv, id);
                if (conn->addr != addr)
                        continue;

                if (flags == ST_RESET && (conn->conn_id_send == id || conn->conn_id_recv == id)) {
                        LOG_UTPV("0x%08x: recv RST for existing connection", conn);
                        if (!conn->userdata || conn->state == CS_FIN_SENT) {
                                conn->state = CS_DESTROY;
                        } else {
                                conn->state = CS_RESET;
                        }
                        if (conn->userdata) {
                                conn->func.on_overhead(conn->userdata, false, len + conn->get_udp_overhead(),
                                                                           close_overhead);
                                const int err = conn->state == CS_SYN_SENT ?
                                        ECONNREFUSED :
                                        ECONNRESET;
                                conn->func.on_error(conn->userdata, err);
                        }
                        return true;
                } else if (flags != ST_SYN && conn->conn_id_recv == id) {
                        LOG_UTPV("0x%08x: recv processing", conn);
                        const size_t read = UTP_ProcessIncoming(conn, buffer, len);
                        if (conn->userdata) {
                                conn->func.on_overhead(conn->userdata, false,
                                        (len - read) + conn->get_udp_overhead(),
                                        header_overhead);
                        }
                        return true;
                }
        }

        if (flags == ST_RESET) {
                LOG_UTPV("recv RST for unknown connection");
                return true;
        }

        const uint32 seq_nr = version == 0 ? pf->seq_nr : pf1->seq_nr;
        if (flags != ST_SYN) {
                for (size_t i = 0; i < g_rst_info.GetCount(); i++) {
                        if (g_rst_info[i].connid != id)
                                continue;
                        if (g_rst_info[i].addr != addr)
                                continue;
                        if (seq_nr != g_rst_info[i].ack_nr)
                                continue;
                        g_rst_info[i].timestamp = UTP_GetMilliseconds();
                        LOG_UTPV("recv not sending RST to non-SYN (stored)");
                        return true;
                }
                if (g_rst_info.GetCount() > RST_INFO_LIMIT) {
                        LOG_UTPV("recv not sending RST to non-SYN (limit at %u stored)", (uint)g_rst_info.GetCount());
                        return true;
                }
                LOG_UTPV("recv send RST to non-SYN (%u stored)", (uint)g_rst_info.GetCount());
                RST_Info &r = g_rst_info.Append();
                r.addr = addr;
                r.connid = id;
                r.ack_nr = seq_nr;
                r.timestamp = UTP_GetMilliseconds();

                UTPSocket::send_rst(send_to_proc, send_to_userdata, addr, id, seq_nr, UTP_Random(), version);
                return true;
        }

        if (incoming_proc) {
                LOG_UTPV("Incoming connection from %s uTP version:%u", addrfmt(addr, addrbuf), version);

                // Create a new UTP socket to handle this new connection
                UTPSocket *conn = UTP_Create(send_to_proc, send_to_userdata, to, tolen);
                // Need to track this value to be able to detect duplicate CONNECTs
                conn->conn_seed = id;
                // This is value that identifies this connection for them.
                conn->conn_id_send = id;
                // This is value that identifies this connection for us.
                conn->conn_id_recv = id+1;
                conn->ack_nr = seq_nr;
                conn->seq_nr = UTP_Random();
                conn->fast_resend_seq_nr = conn->seq_nr;

                UTP_SetSockopt(conn, SO_UTPVERSION, version);
                conn->state = CS_CONNECTED;

                const size_t read = UTP_ProcessIncoming(conn, buffer, len, true);

                LOG_UTPV("0x%08x: recv send connect ACK", conn);
                conn->send_ack(true);

                incoming_proc(send_to_userdata, conn);

                // we report overhead after incoming_proc, because the callbacks are setup now
                if (conn->userdata) {
                        // SYN
                        conn->func.on_overhead(conn->userdata, false, (len - read) + conn->get_udp_overhead(),
                                                                   header_overhead);
                        // SYNACK
                        conn->func.on_overhead(conn->userdata, true, conn->get_overhead(),
                                                                   ack_overhead);
                }
        }

        return true;
}

bool UTP_HandleICMP(const byte* buffer, size_t len, const struct sockaddr *to, socklen_t tolen)
{
        const PackedSockAddr addr((const SOCKADDR_STORAGE*)to, tolen);

        // Want the whole packet so we have connection ID
        if (len < sizeof(PacketFormat)) {
                return false;
        }

        const PacketFormat* p = (PacketFormat*)buffer;
        const PacketFormatV1* p1 = (PacketFormatV1*)buffer;

        const byte version = UTP_IsV1(p1);
        const uint32 id = (version == 0) ? p->connid : uint32(p1->connid);

        for (size_t i = 0; i < g_utp_sockets.GetCount(); ++i) {
                UTPSocket *conn = g_utp_sockets[i];
                if (conn->addr == addr &&
                        conn->conn_id_recv == id) {
                        // Don't pass on errors for idle/closed connections
                        if (conn->state != CS_IDLE) {
                                if (!conn->userdata || conn->state == CS_FIN_SENT) {
                                        LOG_UTPV("0x%08x: icmp packet causing socket destruction", conn);
                                        conn->state = CS_DESTROY;
                                } else {
                                        conn->state = CS_RESET;
                                }
                                if (conn->userdata) {
                                        const int err = conn->state == CS_SYN_SENT ?
                                                ECONNREFUSED :
                                                ECONNRESET;
                                        LOG_UTPV("0x%08x: icmp packet causing error on socket:%d", conn, err);
                                        conn->func.on_error(conn->userdata, err);
                                }
                        }
                        return true;
                }
        }
        return false;
}

// Write bytes to the UTP socket.
// Returns true if the socket is still writable.
bool UTP_Write(UTPSocket *conn, size_t bytes)
{
        assert(conn);

#ifdef g_log_utp_verbose
        size_t param = bytes;
#endif

        if (conn->state != CS_CONNECTED) {
                LOG_UTPV("0x%08x: UTP_Write %u bytes = false (not CS_CONNECTED)", conn, (uint)bytes);
                return false;
        }

        g_current_ms = UTP_GetMilliseconds();

        conn->update_send_quota();

        // don't send unless it will all fit in the window
        size_t packet_size = conn->get_packet_size();
        size_t num_to_send = min<size_t>(bytes, packet_size);
        while (conn->is_writable(num_to_send)) {
                // Send an outgoing packet.
                // Also add it to the outgoing of packets that have been sent but not ACKed.

                if (num_to_send == 0) {
                        LOG_UTPV("0x%08x: UTP_Write %u bytes = true", conn, (uint)param);
                        return true;
                }
                bytes -= num_to_send;

                LOG_UTPV("0x%08x: Sending packet. seq_nr:%u ack_nr:%u wnd:%u/%u/%u rcv_win:%u size:%u quota:%d cur_window_packets:%u",
                                 conn, conn->seq_nr, conn->ack_nr,
                                 (uint)(conn->cur_window + num_to_send),
                                 (uint)conn->max_window, (uint)conn->max_window_user,
                                 (uint)conn->last_rcv_win, num_to_send, conn->send_quota / 100,
                                 conn->cur_window_packets);
                conn->write_outgoing_packet(num_to_send, ST_DATA);
                num_to_send = min<size_t>(bytes, packet_size);
        }

        // mark the socket as not being writable.
        conn->state = CS_CONNECTED_FULL;
        LOG_UTPV("0x%08x: UTP_Write %u bytes = false", conn, (uint)bytes);
        return false;
}

void UTP_RBDrained(UTPSocket *conn)
{
        assert(conn);

        const size_t rcvwin = conn->get_rcv_window();

        if (rcvwin > conn->last_rcv_win) {
                // If last window was 0 send ACK immediately, otherwise should set timer
                if (conn->last_rcv_win == 0) {
                        conn->send_ack();
                } else {
                        conn->ack_time = g_current_ms + min<uint>(conn->ack_time - g_current_ms, DELAYED_ACK_TIME_THRESHOLD);
                }
        }
}

void UTP_CheckTimeouts()
{
        g_current_ms = UTP_GetMilliseconds();

        for (size_t i = 0; i < g_rst_info.GetCount(); i++) {
                if ((int)(g_current_ms - g_rst_info[i].timestamp) >= RST_INFO_TIMEOUT) {
                        g_rst_info.MoveUpLast(i);
                        i--;
                }
        }
        if (g_rst_info.GetCount() != g_rst_info.GetAlloc()) {
                g_rst_info.Compact();
        }

        for (size_t i = 0; i != g_utp_sockets.GetCount(); i++) {
                UTPSocket *conn = g_utp_sockets[i];
                conn->check_timeouts();

                // Check if the object was deleted
                if (conn->state == CS_DESTROY) {
                        LOG_UTPV("0x%08x: Destroying", conn);
                        UTP_Free(conn);
                        i--;
                }
        }
}

size_t UTP_GetPacketSize(UTPSocket *socket)
{
        return socket->get_packet_size();
}

void UTP_GetPeerName(UTPSocket *conn, struct sockaddr *addr, socklen_t *addrlen)
{
        assert(conn);

        socklen_t len;
        const SOCKADDR_STORAGE sa = conn->addr.get_sockaddr_storage(&len);
        *addrlen = min(len, *addrlen);
        memcpy(addr, &sa, *addrlen);
}

void UTP_GetDelays(UTPSocket *conn, int32 *ours, int32 *theirs, uint32 *age)
{
        assert(conn);

        if (ours) *ours = conn->our_hist.get_value();
        if (theirs) *theirs = conn->their_hist.get_value();
        if (age) *age = g_current_ms - conn->last_measured_delay;
}

#ifdef _DEBUG
void UTP_GetStats(UTPSocket *conn, UTPStats *stats)
{
        assert(conn);

        *stats = conn->_stats;
}
#endif // _DEBUG

void UTP_GetGlobalStats(UTPGlobalStats *stats)
{
        *stats = _global_stats;
}

// Close the UTP socket.
// It is not valid for the upper layer to refer to socket after it is closed.
// Data will keep to try being delivered after the close.
void UTP_Close(UTPSocket *conn)
{
        assert(conn);

        assert(conn->state != CS_DESTROY_DELAY && conn->state != CS_FIN_SENT && conn->state != CS_DESTROY);

        LOG_UTPV("0x%08x: UTP_Close in state:%s", conn, statenames[conn->state]);

        switch(conn->state) {
        case CS_CONNECTED:
        case CS_CONNECTED_FULL:
                conn->state = CS_FIN_SENT;
                conn->write_outgoing_packet(0, ST_FIN);
                break;

        case CS_SYN_SENT:
                conn->rto_timeout = UTP_GetMilliseconds() + min<uint>(conn->rto * 2, 60);
        case CS_GOT_FIN:
                conn->state = CS_DESTROY_DELAY;
                break;

        default:
                conn->state = CS_DESTROY;
                break;
        }
}