path: root/core/Lossless_UDP.c

/* Lossless_UDP.c
 *
 * An implementation of the Lossless_UDP protocol as seen in docs/Lossless_UDP.txt
 *
 *  Copyright (C) 2013 Tox project All Rights Reserved.
 *
 *  This file is part of Tox.
 *
 *  Tox is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  Tox is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with Tox.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

/* TODO: clean this file a bit.
   There are a couple of useless variables to get rid of. */
#include "Lossless_UDP.h"

/* maximum data packets in sent and receive queues. */
#define MAX_QUEUE_NUM 16

/* maximum length of the data in the data packets */
/* #define MAX_DATA_SIZE 1024 */ /* defined in Lossless_UDP.h */

/* maximum number of data packets in the buffer */
#define BUFFER_PACKET_NUM (16-1)

/* Lossless UDP connection timeout.
timeout per connection is randomly set between CONNEXION_TIMEOUT and 2*CONNEXION_TIMEOUT */
#define CONNEXION_TIMEOUT 5

/* initial amount of sync/hanshake packets to send per second. */
#define SYNC_RATE 2

/* initial send rate of data. */
#define DATA_SYNC_RATE 30

typedef struct {
    uint8_t data[MAX_DATA_SIZE];
    uint16_t size;
} Data;

typedef struct {
    IP_Port ip_port;
    uint8_t status; /* 0 if connection is dead, 1 if attempting handshake,
                       2 if handshake is done (we start sending SYNC packets)
                       3 if we are sending SYNC packets and can send data
                       4 if the connection has timed out. */

    uint8_t inbound; /* 1 or 2 if connection was initiated by someone else, 0 if not.
                        2 if incoming_connection() has not returned it yet, 1 if it has. */

    uint16_t SYNC_rate; /* current SYNC packet send rate packets per second. */
    uint16_t data_rate; /* current data packet send rate packets per second. */
    uint64_t last_SYNC; /* time at which our last SYNC packet was sent. */
    uint64_t last_sent; /* time at which our last data or handshake packet was sent. */
    uint64_t last_recvSYNC; /* time at which we last received a SYNC packet from the other */
    uint64_t last_recvdata; /* time at which we last received a DATA packet from the other */
    uint64_t killat; /* time at which to kill the connection */
    Data sendbuffer[MAX_QUEUE_NUM]; /* packet send buffer. */
    Data recvbuffer[MAX_QUEUE_NUM]; /* packet receive buffer. */
    uint32_t handshake_id1;
    uint32_t handshake_id2;
    uint32_t recv_packetnum; /* number of data packets received (also used as handshake_id1) */
    uint32_t orecv_packetnum; /* number of packets received by the other peer */
    uint32_t sent_packetnum; /* number of data packets sent */
    uint32_t osent_packetnum; /* number of packets sent by the other peer. */
    uint32_t sendbuff_packetnum; /* number of latest packet written onto the sendbuffer */
    uint32_t successful_sent; /* we know all packets before that number were successfully sent */
    uint32_t successful_read; /* packet number of last packet read with the read_packet function */
    uint32_t req_packets[BUFFER_PACKET_NUM]; /* list of currently requested packet numbers(by the other person) */
    uint16_t num_req_paquets;  /* total number of currently requested packets(by the other person) */
    uint8_t recv_counter;
    uint8_t send_counter;
    uint8_t timeout; /* connection timeout in seconds. */
} Connection;


static Connection * connections;

static uint32_t connections_length; /* Length of connections array */
static uint32_t connections_number; /* Number of connections in connections array */

#define MAX_CONNECTIONS connections_length

/* Functions */

/* get connection id from IP_Port
   return -1 if there are no connections like we are looking for
   return id if it found it */
int getconnection_id(IP_Port ip_port)
{
    uint32_t i;
    for (i = 0; i < MAX_CONNECTIONS; ++i) {
        if (connections[i].ip_port.ip.i == ip_port.ip.i &&
            connections[i].ip_port.port == ip_port.port && connections[i].status > 0)
            return i;
    }
    return -1;
}

/* table of random numbers used below. */
static uint32_t randtable[6][256];

/* generate a handshake_id which depends on the ip_port.
   this function will always give one unique handshake_id per ip_port.
   TODO: make this better */
uint32_t handshake_id(IP_Port source)
{
    uint32_t id = 0, i;
    for (i = 0; i < 6; ++i) {
        if(randtable[i][((uint8_t *)&source)[i]] == 0)
            randtable[i][((uint8_t *)&source)[i]] = random_int();
        id ^= randtable[i][((uint8_t *)&source)[i]];
    }
    if (id == 0) /* id can't be zero */
        id = 1;
    return id;
}

/* change the hnshake id associated with that ip_port
   TODO: make this better */
void change_handshake(IP_Port source)
{
    uint8_t rand = random_int() % 4;
    randtable[rand][((uint8_t *)&source)[rand]] = random_int();
}

/* initialize a new connection to ip_port
   returns an integer corresponding to the connection id.
   return -1 if it could not initialize the connection.
   if there already was an existing connection to that ip_port return its number. */
int new_connection(IP_Port ip_port)
{
    int connect = getconnection_id(ip_port);
    if (connect != -1)
        return connect;
    
    if(connections_number == connections_length) {
        Connection * temp;
        temp = realloc(connections, sizeof(Connection) * (connections_length + 1));
        if(temp == NULL)
            return -1;
        memset(&temp[connections_length], 0, sizeof(Connection));
        ++connections_length;
        connections = temp;
    }
    
    uint32_t i;
    for (i = 0; i < MAX_CONNECTIONS; ++i) {
        if(connections[i].status == 0) {
            memset(&connections[i], 0, sizeof(Connection));
            connections[i].ip_port = ip_port;
            connections[i].status = 1;
            connections[i].inbound = 0;
            connections[i].handshake_id1 = handshake_id(ip_port);
            connections[i].sent_packetnum = connections[i].handshake_id1;
            connections[i].sendbuff_packetnum = connections[i].handshake_id1;
            connections[i].successful_sent = connections[i].handshake_id1;
            connections[i].SYNC_rate = SYNC_RATE;
            connections[i].data_rate = DATA_SYNC_RATE;
            connections[i].last_recvSYNC = current_time();
            connections[i].last_sent = current_time();
            connections[i].killat = ~0;
            connections[i].send_counter = 0;
            /* add randomness to timeout to prevent connections getting stuck in a loop. */
            connections[i].timeout = CONNEXION_TIMEOUT + rand() % CONNEXION_TIMEOUT;
            ++connections_number;
            return i;
        }
    }
    return -1;
}

/* initialize a new inbound connection from ip_port
   returns an integer corresponding to the connection id.
   return -1 if it could not initialize the connection. */
int new_inconnection(IP_Port ip_port)
{
    if (getconnection_id(ip_port) != -1)
        return -1;
    
    if(connections_number == connections_length) {
        Connection * temp;
        temp = realloc(connections, sizeof(Connection) * (connections_length + 1));
        if(temp == NULL)
            return -1;
        memset(&temp[connections_length], 0, sizeof(Connection));
        ++connections_length;
        connections = temp;
    }
    
    uint32_t i;
    for (i = 0; i < MAX_CONNECTIONS; ++i) {
        if (connections[i].status == 0) {
            memset(&connections[i], 0, sizeof(Connection));
            connections[i].ip_port = ip_port;
            connections[i].status = 2;
            connections[i].inbound = 2;
            connections[i].SYNC_rate = SYNC_RATE;
            connections[i].data_rate = DATA_SYNC_RATE;
            connections[i].last_recvSYNC = current_time();
            connections[i].last_sent = current_time();
            /* add randomness to timeout to prevent connections getting stuck in a loop. */
            connections[i].timeout = CONNEXION_TIMEOUT + rand() % CONNEXION_TIMEOUT;
            /* if this connection isn't handled within the timeout kill it. */
            connections[i].killat = current_time() + 1000000UL*connections[i].timeout;
            connections[i].send_counter = 127;
            ++connections_number;
            return i;
        }
    }
    return -1;
}

/* returns an integer corresponding to the next connection in our incoming connection list
   return -1 if there are no new incoming connections in the list. */
int incoming_connection()
{
    uint32_t i;
    for (i = 0; i < MAX_CONNECTIONS; ++i) {
        if (connections[i].inbound == 2) {
            connections[i].inbound = 1;
            return i;
        }
    }
    return -1;
}
/*Try to free some memory from the connections array.*/
static void free_connections()
{
    uint32_t i;
    for(i = connections_length; i != 0; --i)
        if (connections[i - 1].status != 0)
            break;

    if(connections_length == i)
        return;
    Connection * temp;
    temp = realloc(connections, sizeof(Connection) * i);
    if(temp == NULL && i != 0)
        return;
    connections = temp;
    connections_length = i;
}

/* return -1 if it could not kill the connection.
   return 0 if killed successfully */
int kill_connection(int connection_id)
{
    if (connection_id >= 0 && connection_id < MAX_CONNECTIONS) {
        if (connections[connection_id].status > 0) {
            connections[connection_id].status = 0;
            change_handshake(connections[connection_id].ip_port);
            --connections_number;
            free_connections();
            return 0;
        }
    }
    return -1;
}

/* kill connection in seconds seconds.
   return -1 if it can not kill the connection.
   return 0 if it will kill it */
int kill_connection_in(int connection_id, uint32_t seconds)
{
    if (connection_id >= 0 && connection_id < MAX_CONNECTIONS) {
        if (connections[connection_id].status > 0) {
            connections[connection_id].killat = current_time() + 1000000UL*seconds;
            return 0;
        }
    }
    return -1;
}

/* check if connection is connected
   return 0 no.
   return 1 if attempting handshake
   return 2 if handshake is done
   return 3 if fully connected
   return 4 if timed out and waiting to be killed */
int is_connected(int connection_id)
{
    if (connection_id >= 0 && connection_id < MAX_CONNECTIONS)
        return connections[connection_id].status;
    return 0;
}

/* returns the ip_port of the corresponding connection. */
IP_Port connection_ip(int connection_id)
{
    if (connection_id >= 0 && connection_id < MAX_CONNECTIONS)
        return connections[connection_id].ip_port;
    IP_Port zero = {{{0}}, 0};
    return zero;
}

/* returns the number of packets in the queue waiting to be successfully sent. */
uint32_t sendqueue(int connection_id)
{
    return connections[connection_id].sendbuff_packetnum - connections[connection_id].successful_sent;
}

/* returns the number of packets in the queue waiting to be successfully read with read_packet(...) */
uint32_t recvqueue(int connection_id)
{
    return connections[connection_id].recv_packetnum - connections[connection_id].successful_read;
}

/* returns the id of the next packet in the queue
   return -1 if no packet in queue */
char id_packet(int connection_id)
{
    if (recvqueue(connection_id) != 0 && connections[connection_id].status != 0)
        return connections[connection_id].recvbuffer[connections[connection_id].successful_read % MAX_QUEUE_NUM].data[0];
    return -1;
}

/* return 0 if there is no received data in the buffer.
   return length of received packet if successful */
int read_packet(int connection_id, uint8_t * data)
{
    if (recvqueue(connection_id) != 0) {
        uint16_t index = connections[connection_id].successful_read % MAX_QUEUE_NUM;
        uint16_t size = connections[connection_id].recvbuffer[index].size;
        memcpy(data, connections[connection_id].recvbuffer[index].data, size);
        ++connections[connection_id].successful_read;
        connections[connection_id].recvbuffer[index].size = 0;
        return size;
    }
    return 0;
}

/* return 0 if data could not be put in packet queue
   return 1 if data was put into the queue */
int write_packet(int connection_id, uint8_t * data, uint32_t length)
{
    if (length > MAX_DATA_SIZE)
        return 0;
    if (length == 0)
        return 0;
    if (sendqueue(connection_id) <  BUFFER_PACKET_NUM) {
        uint32_t index = connections[connection_id].sendbuff_packetnum % MAX_QUEUE_NUM;
        memcpy(connections[connection_id].sendbuffer[index].data, data, length);
        connections[connection_id].sendbuffer[index].size = length;
        connections[connection_id].sendbuff_packetnum++;
        return 1;
    }
    return 0;
}

/* put the packet numbers the we are missing in requested and return the number */
uint32_t missing_packets(int connection_id, uint32_t * requested)
{
    uint32_t number = 0;
    uint32_t i;
    uint32_t temp;
    if (recvqueue(connection_id) >= (BUFFER_PACKET_NUM - 1)) /* don't request packets if the buffer is full. */
        return 0;
    for (i = connections[connection_id].recv_packetnum; i != connections[connection_id].osent_packetnum; i++) {
        if(connections[connection_id].recvbuffer[i % MAX_QUEUE_NUM].size == 0) {
            temp = htonl(i);
            memcpy(requested + number, &temp, 4);
            ++number;
        }
    }
    if(number == 0)
        connections[connection_id].recv_packetnum = connections[connection_id].osent_packetnum;
    return number;
}

/* Packet sending functions
  One per packet type.
  see docs/Lossless_UDP.txt for more information. */
int send_handshake(IP_Port ip_port, uint32_t handshake_id1, uint32_t handshake_id2)
{
    uint8_t packet[1 + 4 + 4];
    uint32_t temp;

    packet[0] = 16;
    temp = htonl(handshake_id1);
    memcpy(packet + 1, &temp, 4);
    temp = htonl(handshake_id2);
    memcpy(packet + 5, &temp, 4);
    return sendpacket(ip_port, packet, sizeof(packet));
}

int send_SYNC(uint32_t connection_id)
{

    uint8_t packet[(BUFFER_PACKET_NUM*4 + 4 + 4 + 2)];
    uint16_t index = 0;

    IP_Port ip_port = connections[connection_id].ip_port;
    uint8_t counter = connections[connection_id].send_counter;
    uint32_t recv_packetnum = htonl(connections[connection_id].recv_packetnum);
    uint32_t sent_packetnum = htonl(connections[connection_id].sent_packetnum);
    uint32_t requested[BUFFER_PACKET_NUM];
    uint32_t number = missing_packets(connection_id, requested);

    packet[0] = 17;
    index += 1;
    memcpy(packet + index, &counter, 1);
    index += 1;
    memcpy(packet + index, &recv_packetnum, 4);
    index += 4;
    memcpy(packet + index, &sent_packetnum, 4);
    index += 4;
    memcpy(packet + index, requested, 4 * number);

    return sendpacket(ip_port, packet, (number*4 + 4 + 4 + 2));

}

int send_data_packet(uint32_t connection_id, uint32_t packet_num)
{
    uint32_t index = packet_num % MAX_QUEUE_NUM;
    uint32_t temp;
    uint8_t packet[1 + 4 + MAX_DATA_SIZE];
    packet[0] = 18;
    temp = htonl(packet_num);
    memcpy(packet + 1, &temp, 4);
    memcpy(packet + 5, connections[connection_id].sendbuffer[index].data,
           connections[connection_id].sendbuffer[index].size);
    return sendpacket(connections[connection_id].ip_port, packet,
                      1 + 4 + connections[connection_id].sendbuffer[index].size);
}

/* sends 1 data packet */
int send_DATA(uint32_t connection_id)
{
    int ret;
    uint32_t buffer[BUFFER_PACKET_NUM];
    if (connections[connection_id].num_req_paquets > 0) {
        ret = send_data_packet(connection_id, connections[connection_id].req_packets[0]);
        connections[connection_id].num_req_paquets--;
        memcpy(buffer, connections[connection_id].req_packets + 1, connections[connection_id].num_req_paquets * 4);
        memcpy(connections[connection_id].req_packets, buffer, connections[connection_id].num_req_paquets * 4);
        return ret;
    }
    if (connections[connection_id].sendbuff_packetnum != connections[connection_id].sent_packetnum) {
        ret = send_data_packet(connection_id, connections[connection_id].sent_packetnum);
        connections[connection_id].sent_packetnum++;
        return ret;
    }
    return 0;
}

/* END of packet sending functions */

/* Packet handling functions
   One to handle each type of packets we receive
   return 0 if handled correctly, 1 if packet is bad. */
int handle_handshake(uint8_t * packet, uint32_t length, IP_Port source)
{
    if (length != (1 + 4 + 4))
        return 1;
    uint32_t temp;
    uint32_t handshake_id1, handshake_id2;
    int connection = getconnection_id(source);
    memcpy(&temp, packet + 1, 4);
    handshake_id1 = ntohl(temp);
    memcpy(&temp, packet + 5, 4);
    handshake_id2 = ntohl(temp);

    if (handshake_id2 == 0 && is_connected(connection) < 3) {
        send_handshake(source, handshake_id(source), handshake_id1);
        return 0;
    }
    if (is_connected(connection) != 1)
        return 1;
    if (handshake_id2 == connections[connection].handshake_id1) { /* if handshake_id2 is what we sent previously as handshake_id1 */
        connections[connection].status = 2;
        /* NOTE: is this necessary?
        connections[connection].handshake_id2 = handshake_id1; */
        connections[connection].orecv_packetnum = handshake_id2;
        connections[connection].osent_packetnum = handshake_id1;
        connections[connection].recv_packetnum = handshake_id1;
        connections[connection].successful_read = handshake_id1;
    }
    return 0;

}

/* returns 1 if sync packet is valid
   0 if not. */
int SYNC_valid(uint32_t length)
{
    if (length < 4 + 4 + 2)
        return 0;
    if (length > (BUFFER_PACKET_NUM*4 + 4 + 4 + 2) ||
        ((length - 4 - 4 - 2) % 4) != 0)
        return 0;
    return 1;
}

/* case 1: */
int handle_SYNC1(IP_Port source, uint32_t recv_packetnum, uint32_t sent_packetnum)
{
    if (handshake_id(source) == recv_packetnum) {
        int x = new_inconnection(source);
        if (x != -1) {
            connections[x].orecv_packetnum = recv_packetnum;
            connections[x].sent_packetnum = recv_packetnum;
            connections[x].sendbuff_packetnum = recv_packetnum;
            connections[x].successful_sent = recv_packetnum;
            connections[x].osent_packetnum = sent_packetnum;
            connections[x].recv_packetnum = sent_packetnum;
            connections[x].successful_read = sent_packetnum;

            return x;
        }
    }
    return -1;
}

/* case 2: */
int handle_SYNC2(int connection_id, uint8_t counter, uint32_t recv_packetnum, uint32_t sent_packetnum)
{
    if (recv_packetnum == connections[connection_id].orecv_packetnum) {
        /* && sent_packetnum == connections[connection_id].osent_packetnum) */
        connections[connection_id].status =  3;
        connections[connection_id].recv_counter = counter;
        ++connections[connection_id].send_counter;
        send_SYNC(connection_id);
        return 0;
    }
    return 1;
}
/* case 3: */
int handle_SYNC3(int connection_id, uint8_t counter, uint32_t recv_packetnum, uint32_t sent_packetnum, uint32_t * req_packets,
                 uint16_t number)
{
    uint8_t comp_counter = (counter - connections[connection_id].recv_counter );
    uint32_t i, temp;
    /* uint32_t comp_1 = (recv_packetnum - connections[connection_id].successful_sent);
       uint32_t comp_2 = (sent_packetnum - connections[connection_id].successful_read); */
    uint32_t comp_1 = (recv_packetnum - connections[connection_id].orecv_packetnum);
    uint32_t comp_2 = (sent_packetnum - connections[connection_id].osent_packetnum);
    if (comp_1 <= BUFFER_PACKET_NUM && comp_2 <= BUFFER_PACKET_NUM && comp_counter < 10 && comp_counter != 0) { /* packet valid */
        connections[connection_id].orecv_packetnum = recv_packetnum;
        connections[connection_id].osent_packetnum = sent_packetnum;
        connections[connection_id].successful_sent = recv_packetnum;
        connections[connection_id].last_recvSYNC = current_time();
        connections[connection_id].recv_counter = counter;
        ++connections[connection_id].send_counter;
        for (i = 0; i < number; ++i) {
            temp = ntohl(req_packets[i]);
            memcpy(connections[connection_id].req_packets + i, &temp, 4 * number);
        }
        connections[connection_id].num_req_paquets = number;
        return 0;
    }
    return 1;
}

int handle_SYNC(uint8_t *packet, uint32_t length, IP_Port source)
{

    if (!SYNC_valid(length))
        return 1;
    int connection = getconnection_id(source);
    uint8_t counter;
    uint32_t temp;
    uint32_t recv_packetnum, sent_packetnum;
    uint32_t req_packets[BUFFER_PACKET_NUM];
    uint16_t number = (length - 4 - 4 - 2)/ 4;

    memcpy(&counter, packet + 1, 1);
    memcpy(&temp, packet + 2, 4);
    recv_packetnum = ntohl(temp);
    memcpy(&temp,packet + 6,  4);
    sent_packetnum = ntohl(temp);
    if (number != 0)
        memcpy(req_packets, packet + 10,  4 * number);
    if (connection == -1)
        return handle_SYNC1(source, recv_packetnum, sent_packetnum);
    if (connections[connection].status ==  2)
        return handle_SYNC2(connection, counter, recv_packetnum, sent_packetnum);
    if (connections[connection].status ==  3)
        return handle_SYNC3(connection, counter, recv_packetnum, sent_packetnum, req_packets, number);
    return 0;
}

/* add a packet to the received buffer and set the recv_packetnum of the connection to its proper value.
   return 1 if data was too big, 0 if not. */
int add_recv(int connection_id, uint32_t data_num, uint8_t *data, uint16_t size)
{
    if (size > MAX_DATA_SIZE)
        return 1;

    uint32_t i;
    uint32_t maxnum = connections[connection_id].successful_read + BUFFER_PACKET_NUM;
    uint32_t sent_packet = data_num - connections[connection_id].osent_packetnum;
    for (i = connections[connection_id].recv_packetnum; i != maxnum; ++i) {
        if (i == data_num) {
            memcpy(connections[connection_id].recvbuffer[i % MAX_QUEUE_NUM].data, data, size);
            connections[connection_id].recvbuffer[i % MAX_QUEUE_NUM].size = size;
            connections[connection_id].last_recvdata = current_time();
            if (sent_packet < BUFFER_PACKET_NUM)
                connections[connection_id].osent_packetnum = data_num;
            break;
        }
    }
    for (i = connections[connection_id].recv_packetnum; i != maxnum; ++i) {
        if (connections[connection_id].recvbuffer[i % MAX_QUEUE_NUM].size != 0)
            connections[connection_id].recv_packetnum = i;
        else
            break;
    }

    return 0;
}

int handle_data(uint8_t *packet, uint32_t length, IP_Port source)
{
    int connection = getconnection_id(source);

    if (connection == -1)
        return 1;

    if (connections[connection].status != 3) /* Drop the data packet if connection is not connected. */
        return 1;

    if (length > 1 + 4 + MAX_DATA_SIZE || length < 1 + 4 + 1)
        return 1;
    uint32_t temp;
    uint32_t number;
    uint16_t size = length - 1 - 4;

    memcpy(&temp, packet + 1, 4);
    number = ntohl(temp);
    return add_recv(connection, number, packet + 5, size);
}

/* END of packet handling functions */

int LosslessUDP_handlepacket(uint8_t *packet, uint32_t length, IP_Port source)
{
    switch (packet[0]) { //TODO: check if no break statement is correct???
    case 16:
        return handle_handshake(packet, length, source);

    case 17:
        return handle_SYNC(packet, length, source);

    case 18:
        return handle_data(packet, length, source);

    default:
        return 1;
    }

    return 0;
}

/* Send handshake requests
   handshake packets are sent at the same rate as SYNC packets */
void doNew()
{
    uint32_t i;
    uint64_t temp_time = current_time();
    for (i = 0; i < MAX_CONNECTIONS; ++i) {
        if (connections[i].status == 1)
            if ((connections[i].last_sent + (1000000UL/connections[i].SYNC_rate)) <= temp_time) {
                send_handshake(connections[i].ip_port, connections[i].handshake_id1, 0);
                connections[i].last_sent = temp_time;
            }

        /* kill all timed out connections */
        if ( connections[i].status > 0 && (connections[i].last_recvSYNC + connections[i].timeout * 1000000UL) < temp_time &&
             connections[i].status != 4)
            /* kill_connection(i); */
            connections[i].status = 4;
        if (connections[i].status > 0 && connections[i].killat < temp_time)
            kill_connection(i);
    }
}

void doSYNC()
{
    uint32_t i;
    uint64_t temp_time = current_time();
    for (i = 0; i < MAX_CONNECTIONS; ++i) {
        if (connections[i].status == 2 || connections[i].status == 3)
            if ((connections[i].last_SYNC + (1000000UL/connections[i].SYNC_rate)) <= temp_time) {
                send_SYNC(i);
                connections[i].last_SYNC = temp_time;
            }
    }
}

void doData()
{
    uint32_t i;
    uint64_t j;
    uint64_t temp_time = current_time();
    for (i = 0; i < MAX_CONNECTIONS; ++i)
        if (connections[i].status == 3 && sendqueue(i) != 0)
            if ((connections[i].last_sent + (1000000UL/connections[i].data_rate)) <= temp_time) {
                for (j = connections[i].last_sent; j < temp_time; j +=  (1000000UL/connections[i].data_rate))
                    send_DATA(i);
                connections[i].last_sent = temp_time;
            }
}

/* TODO: flow control.
   automatically adjusts send rates of packets for optimal transmission. */

#define MAX_SYNC_RATE 10

void adjustRates()
{
    uint32_t i;
    uint64_t temp_time = current_time();
    for (i = 0; i < MAX_CONNECTIONS; ++i) {
        if (connections[i].status == 1 || connections[i].status == 2)
            connections[i].SYNC_rate = MAX_SYNC_RATE;
        if (connections[i].status == 3) {
            if (sendqueue(i) != 0) {
                connections[i].data_rate = (BUFFER_PACKET_NUM - connections[i].num_req_paquets) * MAX_SYNC_RATE;
                connections[i].SYNC_rate = MAX_SYNC_RATE;
            } else if (connections[i].last_recvdata + 1000000UL > temp_time)
                connections[i].SYNC_rate = MAX_SYNC_RATE;
            else
                connections[i].SYNC_rate = SYNC_RATE;
        }
    }
}

/* Call this function a couple times per second
   It's the main loop. */
void doLossless_UDP()
{
    doNew();
    doSYNC();
    doData();
    adjustRates();
}