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   1 /*
   2     utcp.c -- Userspace TCP
   3     Copyright (C) 2014-2017 Guus Sliepen <guus@tinc-vpn.org>
   4
   5     This program is free software; you can redistribute it and/or modify
   6     it under the terms of the GNU General Public License as published by
   7     the Free Software Foundation; either version 2 of the License, or
   8     (at your option) any later version.
   9
  10     This program is distributed in the hope that it will be useful,
  11     but WITHOUT ANY WARRANTY; without even the implied warranty of
  12     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13     GNU General Public License for more details.
  14
  15     You should have received a copy of the GNU General Public License along
  16     with this program; if not, write to the Free Software Foundation, Inc.,
  17     51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  18 */
  19
  20 #define _GNU_SOURCE
  21
  22 #include <assert.h>
  23 #include <errno.h>
  24 #include <stdio.h>
  25 #include <stdlib.h>
  26 #include <stdint.h>
  27 #include <stdbool.h>
  28 #include <string.h>
  29 #include <unistd.h>
  30 #include <sys/time.h>
  31 #include <sys/socket.h>
  32
  33 #include "utcp_priv.h"
  34
  35 #ifndef EBADMSG
  36 #define EBADMSG         104
  37 #endif
  38
  39 #ifndef SHUT_RDWR
  40 #define SHUT_RDWR 2
  41 #endif
  42
  43 #ifdef poll
  44 #undef poll
  45 #endif
  46
  47 #ifndef timersub
  48 #define timersub(a, b, r)\
  49         do {\
  50                 (r)->tv_sec = (a)->tv_sec - (b)->tv_sec;\
  51                 (r)->tv_usec = (a)->tv_usec - (b)->tv_usec;\
  52                 if((r)->tv_usec < 0)\
  53                         (r)->tv_sec--, (r)->tv_usec += USEC_PER_SEC;\
  54         } while (0)
  55 #endif
  56
  57 static inline size_t max(size_t a, size_t b) {
  58         return a > b ? a : b;
  59 }
  60
  61 #ifdef UTCP_DEBUG
  62 #include <stdarg.h>
  63
  64 static void debug(const char *format, ...) {
  65         va_list ap;
  66         va_start(ap, format);
  67         vfprintf(stderr, format, ap);
  68         va_end(ap);
  69 }
  70
  71 static void print_packet(struct utcp *utcp, const char *dir, const void *pkt, size_t len) {
  72         struct hdr hdr;
  73
  74         if(len < sizeof(hdr)) {
  75                 debug("%p %s: short packet (%lu bytes)\n", utcp, dir, (unsigned long)len);
  76                 return;
  77         }
  78
  79         memcpy(&hdr, pkt, sizeof(hdr));
  80         debug("%p %s: len=%lu, src=%u dst=%u seq=%u ack=%u wnd=%u aux=%x ctl=", utcp, dir, (unsigned long)len, hdr.src, hdr.dst, hdr.seq, hdr.ack, hdr.wnd, hdr.aux);
  81
  82         if(hdr.ctl & SYN) {
  83                 debug("SYN");
  84         }
  85
  86         if(hdr.ctl & RST) {
  87                 debug("RST");
  88         }
  89
  90         if(hdr.ctl & FIN) {
  91                 debug("FIN");
  92         }
  93
  94         if(hdr.ctl & ACK) {
  95                 debug("ACK");
  96         }
  97
  98         if(len > sizeof(hdr)) {
  99                 uint32_t datalen = len - sizeof(hdr);
 100                 const uint8_t *data = (uint8_t *)pkt + sizeof(hdr);
 101                 char str[datalen * 2 + 1];
 102                 char *p = str;
 103
 104                 for(uint32_t i = 0; i < datalen; i++) {
 105                         *p++ = "0123456789ABCDEF"[data[i] >> 4];
 106                         *p++ = "0123456789ABCDEF"[data[i] & 15];
 107                 }
 108
 109                 *p = 0;
 110
 111                 debug(" data=%s", str);
 112         }
 113
 114         debug("\n");
 115 }
 116 #else
 117 #define debug(...) do {} while(0)
 118 #define print_packet(...) do {} while(0)
 119 #endif
 120
 121 static void set_state(struct utcp_connection *c, enum state state) {
 122         c->state = state;
 123
 124         if(state == ESTABLISHED) {
 125                 timerclear(&c->conn_timeout);
 126         }
 127
 128         debug("%p new state: %s\n", c->utcp, strstate[state]);
 129 }
 130
 131 static bool fin_wanted(struct utcp_connection *c, uint32_t seq) {
 132         if(seq != c->snd.last) {
 133                 return false;
 134         }
 135
 136         switch(c->state) {
 137         case FIN_WAIT_1:
 138         case CLOSING:
 139         case LAST_ACK:
 140                 return true;
 141
 142         default:
 143                 return false;
 144         }
 145 }
 146
 147 static bool is_reliable(struct utcp_connection *c) {
 148         return c->flags & UTCP_RELIABLE;
 149 }
 150
 151 static int32_t seqdiff(uint32_t a, uint32_t b) {
 152         return a - b;
 153 }
 154
 155 // Buffer functions
 156 // TODO: convert to ringbuffers to avoid memmove() operations.
 157
 158 // Store data into the buffer
 159 static ssize_t buffer_put_at(struct buffer *buf, size_t offset, const void *data, size_t len) {
 160         debug("buffer_put_at %lu %lu %lu\n", (unsigned long)buf->used, (unsigned long)offset, (unsigned long)len);
 161
 162         size_t required = offset + len;
 163
 164         if(required > buf->maxsize) {
 165                 if(offset >= buf->maxsize) {
 166                         return 0;
 167                 }
 168
 169                 len = buf->maxsize - offset;
 170                 required = buf->maxsize;
 171         }
 172
 173         if(required > buf->size) {
 174                 size_t newsize = buf->size;
 175
 176                 if(!newsize) {
 177                         newsize = required;
 178                 } else {
 179                         do {
 180                                 newsize *= 2;
 181                         } while(newsize < required);
 182                 }
 183
 184                 if(newsize > buf->maxsize) {
 185                         newsize = buf->maxsize;
 186                 }
 187
 188                 char *newdata = realloc(buf->data, newsize);
 189
 190                 if(!newdata) {
 191                         return -1;
 192                 }
 193
 194                 buf->data = newdata;
 195                 buf->size = newsize;
 196         }
 197
 198         memcpy(buf->data + offset, data, len);
 199
 200         if(required > buf->used) {
 201                 buf->used = required;
 202         }
 203
 204         return len;
 205 }
 206
 207 static ssize_t buffer_put(struct buffer *buf, const void *data, size_t len) {
 208         return buffer_put_at(buf, buf->used, data, len);
 209 }
 210
 211 // Get data from the buffer. data can be NULL.
 212 static ssize_t buffer_get(struct buffer *buf, void *data, size_t len) {
 213         if(len > buf->used) {
 214                 len = buf->used;
 215         }
 216
 217         if(data) {
 218                 memcpy(data, buf->data, len);
 219         }
 220
 221         if(len < buf->used) {
 222                 memmove(buf->data, buf->data + len, buf->used - len);
 223         }
 224
 225         buf->used -= len;
 226         return len;
 227 }
 228
 229 // Copy data from the buffer without removing it.
 230 static ssize_t buffer_copy(struct buffer *buf, void *data, size_t offset, size_t len) {
 231         if(offset >= buf->used) {
 232                 return 0;
 233         }
 234
 235         if(offset + len > buf->used) {
 236                 len = buf->used - offset;
 237         }
 238
 239         memcpy(data, buf->data + offset, len);
 240         return len;
 241 }
 242
 243 static bool buffer_init(struct buffer *buf, uint32_t len, uint32_t maxlen) {
 244         memset(buf, 0, sizeof(*buf));
 245
 246         if(len) {
 247                 buf->data = malloc(len);
 248
 249                 if(!buf->data) {
 250                         return false;
 251                 }
 252         }
 253
 254         buf->size = len;
 255         buf->maxsize = maxlen;
 256         return true;
 257 }
 258
 259 static void buffer_exit(struct buffer *buf) {
 260         free(buf->data);
 261         memset(buf, 0, sizeof(*buf));
 262 }
 263
 264 static uint32_t buffer_free(const struct buffer *buf) {
 265         return buf->maxsize - buf->used;
 266 }
 267
 268 // Connections are stored in a sorted list.
 269 // This gives O(log(N)) lookup time, O(N log(N)) insertion time and O(N) deletion time.
 270
 271 static int compare(const void *va, const void *vb) {
 272         assert(va && vb);
 273
 274         const struct utcp_connection *a = *(struct utcp_connection **)va;
 275         const struct utcp_connection *b = *(struct utcp_connection **)vb;
 276
 277         assert(a && b);
 278         assert(a->src && b->src);
 279
 280         int c = (int)a->src - (int)b->src;
 281
 282         if(c) {
 283                 return c;
 284         }
 285
 286         c = (int)a->dst - (int)b->dst;
 287         return c;
 288 }
 289
 290 static struct utcp_connection *find_connection(const struct utcp *utcp, uint16_t src, uint16_t dst) {
 291         if(!utcp->nconnections) {
 292                 return NULL;
 293         }
 294
 295         struct utcp_connection key = {
 296                 .src = src,
 297                 .dst = dst,
 298         }, *keyp = &key;
 299         struct utcp_connection **match = bsearch(&keyp, utcp->connections, utcp->nconnections, sizeof(*utcp->connections), compare);
 300         return match ? *match : NULL;
 301 }
 302
 303 static void free_connection(struct utcp_connection *c) {
 304         struct utcp *utcp = c->utcp;
 305         struct utcp_connection **cp = bsearch(&c, utcp->connections, utcp->nconnections, sizeof(*utcp->connections), compare);
 306
 307         assert(cp);
 308
 309         int i = cp - utcp->connections;
 310         memmove(cp, cp + 1, (utcp->nconnections - i - 1) * sizeof(*cp));
 311         utcp->nconnections--;
 312
 313         buffer_exit(&c->rcvbuf);
 314         buffer_exit(&c->sndbuf);
 315         free(c);
 316 }
 317
 318 static struct utcp_connection *allocate_connection(struct utcp *utcp, uint16_t src, uint16_t dst) {
 319         // Check whether this combination of src and dst is free
 320
 321         if(src) {
 322                 if(find_connection(utcp, src, dst)) {
 323                         errno = EADDRINUSE;
 324                         return NULL;
 325                 }
 326         } else { // If src == 0, generate a random port number with the high bit set
 327                 if(utcp->nconnections >= 32767) {
 328                         errno = ENOMEM;
 329                         return NULL;
 330                 }
 331
 332                 src = rand() | 0x8000;
 333
 334                 while(find_connection(utcp, src, dst)) {
 335                         src++;
 336                 }
 337         }
 338
 339         // Allocate memory for the new connection
 340
 341         if(utcp->nconnections >= utcp->nallocated) {
 342                 if(!utcp->nallocated) {
 343                         utcp->nallocated = 4;
 344                 } else {
 345                         utcp->nallocated *= 2;
 346                 }
 347
 348                 struct utcp_connection **new_array = realloc(utcp->connections, utcp->nallocated * sizeof(*utcp->connections));
 349
 350                 if(!new_array) {
 351                         return NULL;
 352                 }
 353
 354                 utcp->connections = new_array;
 355         }
 356
 357         struct utcp_connection *c = calloc(1, sizeof(*c));
 358
 359         if(!c) {
 360                 return NULL;
 361         }
 362
 363         if(!buffer_init(&c->sndbuf, DEFAULT_SNDBUFSIZE, DEFAULT_MAXSNDBUFSIZE)) {
 364                 free(c);
 365                 return NULL;
 366         }
 367
 368         if(!buffer_init(&c->rcvbuf, DEFAULT_RCVBUFSIZE, DEFAULT_MAXRCVBUFSIZE)) {
 369                 buffer_exit(&c->sndbuf);
 370                 free(c);
 371                 return NULL;
 372         }
 373
 374         // Fill in the details
 375
 376         c->src = src;
 377         c->dst = dst;
 378 #ifdef UTCP_DEBUG
 379         c->snd.iss = 0;
 380 #else
 381         c->snd.iss = rand();
 382 #endif
 383         c->snd.una = c->snd.iss;
 384         c->snd.nxt = c->snd.iss + 1;
 385         c->rcv.wnd = utcp->mtu;
 386         c->snd.last = c->snd.nxt;
 387         c->snd.cwnd = utcp->mtu;
 388         c->utcp = utcp;
 389
 390         // Add it to the sorted list of connections
 391
 392         utcp->connections[utcp->nconnections++] = c;
 393         qsort(utcp->connections, utcp->nconnections, sizeof(*utcp->connections), compare);
 394
 395         return c;
 396 }
 397
 398 static inline uint32_t absdiff(uint32_t a, uint32_t b) {
 399         if(a > b) {
 400                 return a - b;
 401         } else {
 402                 return b - a;
 403         }
 404 }
 405
 406 // Update RTT variables. See RFC 6298.
 407 static void update_rtt(struct utcp_connection *c, uint32_t rtt) {
 408         if(!rtt) {
 409                 debug("invalid rtt\n");
 410                 return;
 411         }
 412
 413         struct utcp *utcp = c->utcp;
 414
 415         if(!utcp->srtt) {
 416                 utcp->srtt = rtt;
 417                 utcp->rttvar = rtt / 2;
 418                 utcp->rto = rtt + max(2 * rtt, CLOCK_GRANULARITY);
 419         } else {
 420                 utcp->rttvar = (utcp->rttvar * 3 + absdiff(utcp->srtt, rtt)) / 4;
 421                 utcp->srtt = (utcp->srtt * 7 + rtt) / 8;
 422                 utcp->rto = utcp->srtt + max(utcp->rttvar, CLOCK_GRANULARITY);
 423         }
 424
 425         if(utcp->rto > MAX_RTO) {
 426                 utcp->rto = MAX_RTO;
 427         }
 428
 429         debug("rtt %u srtt %u rttvar %u rto %u\n", rtt, utcp->srtt, utcp->rttvar, utcp->rto);
 430 }
 431
 432 static void start_retransmit_timer(struct utcp_connection *c) {
 433         gettimeofday(&c->rtrx_timeout, NULL);
 434         c->rtrx_timeout.tv_usec += c->utcp->rto;
 435
 436         while(c->rtrx_timeout.tv_usec >= 1000000) {
 437                 c->rtrx_timeout.tv_usec -= 1000000;
 438                 c->rtrx_timeout.tv_sec++;
 439         }
 440
 441         debug("timeout set to %lu.%06lu (%u)\n", c->rtrx_timeout.tv_sec, c->rtrx_timeout.tv_usec, c->utcp->rto);
 442 }
 443
 444 static void stop_retransmit_timer(struct utcp_connection *c) {
 445         timerclear(&c->rtrx_timeout);
 446         debug("timeout cleared\n");
 447 }
 448
 449 struct utcp_connection *utcp_connect_ex(struct utcp *utcp, uint16_t dst, utcp_recv_t recv, void *priv, uint32_t flags) {
 450         struct utcp_connection *c = allocate_connection(utcp, 0, dst);
 451
 452         if(!c) {
 453                 return NULL;
 454         }
 455
 456         assert((flags & ~0x1f) == 0);
 457
 458         c->flags = flags;
 459         c->recv = recv;
 460         c->priv = priv;
 461
 462         struct {
 463                 struct hdr hdr;
 464                 uint8_t init[4];
 465         } pkt;
 466
 467         pkt.hdr.src = c->src;
 468         pkt.hdr.dst = c->dst;
 469         pkt.hdr.seq = c->snd.iss;
 470         pkt.hdr.ack = 0;
 471         pkt.hdr.wnd = c->rcv.wnd;
 472         pkt.hdr.ctl = SYN;
 473         pkt.hdr.aux = 0x0101;
 474         pkt.init[0] = 1;
 475         pkt.init[1] = 0;
 476         pkt.init[2] = 0;
 477         pkt.init[3] = flags & 0x7;
 478
 479         set_state(c, SYN_SENT);
 480
 481         print_packet(utcp, "send", &pkt, sizeof(pkt));
 482         utcp->send(utcp, &pkt, sizeof(pkt));
 483
 484         gettimeofday(&c->conn_timeout, NULL);
 485         c->conn_timeout.tv_sec += utcp->timeout;
 486
 487         start_retransmit_timer(c);
 488
 489         return c;
 490 }
 491
 492 struct utcp_connection *utcp_connect(struct utcp *utcp, uint16_t dst, utcp_recv_t recv, void *priv) {
 493         return utcp_connect_ex(utcp, dst, recv, priv, UTCP_TCP);
 494 }
 495
 496 void utcp_accept(struct utcp_connection *c, utcp_recv_t recv, void *priv) {
 497         if(c->reapable || c->state != SYN_RECEIVED) {
 498                 debug("Error: accept() called on invalid connection %p in state %s\n", c, strstate[c->state]);
 499                 return;
 500         }
 501
 502         debug("%p accepted, %p %p\n", c, recv, priv);
 503         c->recv = recv;
 504         c->priv = priv;
 505         set_state(c, ESTABLISHED);
 506 }
 507
 508 static void ack(struct utcp_connection *c, bool sendatleastone) {
 509         int32_t left = seqdiff(c->snd.last, c->snd.nxt);
 510         int32_t cwndleft = c->snd.cwnd - seqdiff(c->snd.nxt, c->snd.una);
 511         debug("cwndleft = %d\n", cwndleft);
 512
 513         assert(left >= 0);
 514
 515         if(cwndleft <= 0) {
 516                 cwndleft = 0;
 517         }
 518
 519         if(cwndleft < left) {
 520                 left = cwndleft;
 521         }
 522
 523         if(!left && !sendatleastone) {
 524                 return;
 525         }
 526
 527         struct {
 528                 struct hdr hdr;
 529                 uint8_t data[];
 530         } *pkt;
 531
 532         pkt = malloc(sizeof(pkt->hdr) + c->utcp->mtu);
 533
 534         if(!pkt) {
 535                 return;
 536         }
 537
 538         pkt->hdr.src = c->src;
 539         pkt->hdr.dst = c->dst;
 540         pkt->hdr.ack = c->rcv.nxt;
 541         pkt->hdr.wnd = c->snd.wnd;
 542         pkt->hdr.ctl = ACK;
 543         pkt->hdr.aux = 0;
 544
 545         do {
 546                 uint32_t seglen = left > c->utcp->mtu ? c->utcp->mtu : left;
 547                 pkt->hdr.seq = c->snd.nxt;
 548
 549                 buffer_copy(&c->sndbuf, pkt->data, seqdiff(c->snd.nxt, c->snd.una), seglen);
 550
 551                 c->snd.nxt += seglen;
 552                 left -= seglen;
 553
 554                 if(seglen && fin_wanted(c, c->snd.nxt)) {
 555                         seglen--;
 556                         pkt->hdr.ctl |= FIN;
 557                 }
 558
 559                 if(!c->rtt_start.tv_sec) {
 560                         // Start RTT measurement
 561                         gettimeofday(&c->rtt_start, NULL);
 562                         c->rtt_seq = pkt->hdr.seq + seglen;
 563                         debug("Starting RTT measurement, expecting ack %u\n", c->rtt_seq);
 564                 }
 565
 566                 print_packet(c->utcp, "send", pkt, sizeof(pkt->hdr) + seglen);
 567                 c->utcp->send(c->utcp, pkt, sizeof(pkt->hdr) + seglen);
 568         } while(left);
 569
 570         free(pkt);
 571 }
 572
 573 ssize_t utcp_send(struct utcp_connection *c, const void *data, size_t len) {
 574         if(c->reapable) {
 575                 debug("Error: send() called on closed connection %p\n", c);
 576                 errno = EBADF;
 577                 return -1;
 578         }
 579
 580         switch(c->state) {
 581         case CLOSED:
 582         case LISTEN:
 583                 debug("Error: send() called on unconnected connection %p\n", c);
 584                 errno = ENOTCONN;
 585                 return -1;
 586
 587         case SYN_SENT:
 588         case SYN_RECEIVED:
 589         case ESTABLISHED:
 590         case CLOSE_WAIT:
 591                 break;
 592
 593         case FIN_WAIT_1:
 594         case FIN_WAIT_2:
 595         case CLOSING:
 596         case LAST_ACK:
 597         case TIME_WAIT:
 598                 debug("Error: send() called on closing connection %p\n", c);
 599                 errno = EPIPE;
 600                 return -1;
 601         }
 602
 603         // Exit early if we have nothing to send.
 604
 605         if(!len) {
 606                 return 0;
 607         }
 608
 609         if(!data) {
 610                 errno = EFAULT;
 611                 return -1;
 612         }
 613
 614         // Check if we need to be able to buffer all data
 615
 616         if(c->flags & UTCP_NO_PARTIAL) {
 617                 if(len > buffer_free(&c->sndbuf)) {
 618                         if(len > c->sndbuf.maxsize) {
 619                                 errno = EMSGSIZE;
 620                                 return -1;
 621                         } else {
 622                                 errno = EWOULDBLOCK;
 623                                 return 0;
 624                         }
 625                 }
 626         }
 627
 628         // Add data to send buffer.
 629
 630         if(is_reliable(c) || (c->state != SYN_SENT && c->state != SYN_RECEIVED)) {
 631                 len = buffer_put(&c->sndbuf, data, len);
 632         }
 633
 634         if(len <= 0) {
 635                 if(is_reliable(c)) {
 636                         errno = EWOULDBLOCK;
 637                         return 0;
 638                 } else {
 639                         return len;
 640                 }
 641         }
 642
 643         c->snd.last += len;
 644
 645         // Don't send anything yet if the connection has not fully established yet
 646
 647         if(c->state == SYN_SENT || c->state == SYN_RECEIVED) {
 648                 return len;
 649         }
 650
 651         ack(c, false);
 652
 653         if(!is_reliable(c)) {
 654                 c->snd.una = c->snd.nxt = c->snd.last;
 655                 buffer_get(&c->sndbuf, NULL, c->sndbuf.used);
 656         }
 657
 658         if(is_reliable(c) && !timerisset(&c->rtrx_timeout)) {
 659                 start_retransmit_timer(c);
 660         }
 661
 662         if(is_reliable(c) && !timerisset(&c->conn_timeout)) {
 663                 gettimeofday(&c->conn_timeout, NULL);
 664                 c->conn_timeout.tv_sec += c->utcp->timeout;
 665         }
 666
 667         return len;
 668 }
 669
 670 static void swap_ports(struct hdr *hdr) {
 671         uint16_t tmp = hdr->src;
 672         hdr->src = hdr->dst;
 673         hdr->dst = tmp;
 674 }
 675
 676 static void retransmit(struct utcp_connection *c) {
 677         if(c->state == CLOSED || c->snd.last == c->snd.una) {
 678                 debug("Retransmit() called but nothing to retransmit!\n");
 679                 stop_retransmit_timer(c);
 680                 return;
 681         }
 682
 683         struct utcp *utcp = c->utcp;
 684
 685         struct {
 686                 struct hdr hdr;
 687                 uint8_t data[];
 688         } *pkt;
 689
 690         pkt = malloc(sizeof(pkt->hdr) + c->utcp->mtu);
 691
 692         if(!pkt) {
 693                 return;
 694         }
 695
 696         pkt->hdr.src = c->src;
 697         pkt->hdr.dst = c->dst;
 698         pkt->hdr.wnd = c->rcv.wnd;
 699         pkt->hdr.aux = 0;
 700
 701         switch(c->state) {
 702         case SYN_SENT:
 703                 // Send our SYN again
 704                 pkt->hdr.seq = c->snd.iss;
 705                 pkt->hdr.ack = 0;
 706                 pkt->hdr.ctl = SYN;
 707                 pkt->hdr.aux = 0x0101;
 708                 pkt->data[0] = 1;
 709                 pkt->data[1] = 0;
 710                 pkt->data[2] = 0;
 711                 pkt->data[3] = c->flags & 0x7;
 712                 print_packet(c->utcp, "rtrx", pkt, sizeof(pkt->hdr) + 4);
 713                 utcp->send(utcp, pkt, sizeof(pkt->hdr) + 4);
 714                 break;
 715
 716         case SYN_RECEIVED:
 717                 // Send SYNACK again
 718                 pkt->hdr.seq = c->snd.nxt;
 719                 pkt->hdr.ack = c->rcv.nxt;
 720                 pkt->hdr.ctl = SYN | ACK;
 721                 print_packet(c->utcp, "rtrx", pkt, sizeof(pkt->hdr));
 722                 utcp->send(utcp, pkt, sizeof(pkt->hdr));
 723                 break;
 724
 725         case ESTABLISHED:
 726         case FIN_WAIT_1:
 727         case CLOSE_WAIT:
 728         case CLOSING:
 729         case LAST_ACK:
 730                 // Send unacked data again.
 731                 pkt->hdr.seq = c->snd.una;
 732                 pkt->hdr.ack = c->rcv.nxt;
 733                 pkt->hdr.ctl = ACK;
 734                 uint32_t len = seqdiff(c->snd.last, c->snd.una);
 735
 736                 if(len > utcp->mtu) {
 737                         len = utcp->mtu;
 738                 }
 739
 740                 if(fin_wanted(c, c->snd.una + len)) {
 741                         len--;
 742                         pkt->hdr.ctl |= FIN;
 743                 }
 744
 745                 c->snd.nxt = c->snd.una + len;
 746                 c->snd.cwnd = utcp->mtu; // reduce cwnd on retransmit
 747                 buffer_copy(&c->sndbuf, pkt->data, 0, len);
 748                 print_packet(c->utcp, "rtrx", pkt, sizeof(pkt->hdr) + len);
 749                 utcp->send(utcp, pkt, sizeof(pkt->hdr) + len);
 750                 break;
 751
 752         case CLOSED:
 753         case LISTEN:
 754         case TIME_WAIT:
 755         case FIN_WAIT_2:
 756                 // We shouldn't need to retransmit anything in this state.
 757 #ifdef UTCP_DEBUG
 758                 abort();
 759 #endif
 760                 stop_retransmit_timer(c);
 761                 goto cleanup;
 762         }
 763
 764         start_retransmit_timer(c);
 765         utcp->rto *= 2;
 766
 767         if(utcp->rto > MAX_RTO) {
 768                 utcp->rto = MAX_RTO;
 769         }
 770
 771         c->rtt_start.tv_sec = 0; // invalidate RTT timer
 772
 773 cleanup:
 774         free(pkt);
 775 }
 776
 777 /* Update receive buffer and SACK entries after consuming data.
 778  *
 779  * Situation:
 780  *
 781  * |.....0000..1111111111.....22222......3333|
 782  * |---------------^
 783  *
 784  * 0..3 represent the SACK entries. The ^ indicates up to which point we want
 785  * to remove data from the receive buffer. The idea is to substract "len"
 786  * from the offset of all the SACK entries, and then remove/cut down entries
 787  * that are shifted to before the start of the receive buffer.
 788  *
 789  * There are three cases:
 790  * - the SACK entry is after ^, in that case just change the offset.
 791  * - the SACK entry starts before and ends after ^, so we have to
 792  *   change both its offset and size.
 793  * - the SACK entry is completely before ^, in that case delete it.
 794  */
 795 static void sack_consume(struct utcp_connection *c, size_t len) {
 796         debug("sack_consume %lu\n", (unsigned long)len);
 797
 798         if(len > c->rcvbuf.used) {
 799                 debug("All SACK entries consumed");
 800                 c->sacks[0].len = 0;
 801                 return;
 802         }
 803
 804         buffer_get(&c->rcvbuf, NULL, len);
 805
 806         for(int i = 0; i < NSACKS && c->sacks[i].len;) {
 807                 if(len < c->sacks[i].offset) {
 808                         c->sacks[i].offset -= len;
 809                         i++;
 810                 } else if(len < c->sacks[i].offset + c->sacks[i].len) {
 811                         c->sacks[i].len -= len - c->sacks[i].offset;
 812                         c->sacks[i].offset = 0;
 813                         i++;
 814                 } else {
 815                         if(i < NSACKS - 1) {
 816                                 memmove(&c->sacks[i], &c->sacks[i + 1], (NSACKS - 1 - i) * sizeof(c->sacks)[i]);
 817                                 c->sacks[NSACKS - 1].len = 0;
 818                         } else {
 819                                 c->sacks[i].len = 0;
 820                                 break;
 821                         }
 822                 }
 823         }
 824
 825         for(int i = 0; i < NSACKS && c->sacks[i].len; i++) {
 826                 debug("SACK[%d] offset %u len %u\n", i, c->sacks[i].offset, c->sacks[i].len);
 827         }
 828 }
 829
 830 static void handle_out_of_order(struct utcp_connection *c, uint32_t offset, const void *data, size_t len) {
 831         debug("out of order packet, offset %u\n", offset);
 832         // Packet loss or reordering occured. Store the data in the buffer.
 833         ssize_t rxd = buffer_put_at(&c->rcvbuf, offset, data, len);
 834
 835         if(rxd < 0 || (size_t)rxd < len) {
 836                 abort();
 837         }
 838
 839         // Make note of where we put it.
 840         for(int i = 0; i < NSACKS; i++) {
 841                 if(!c->sacks[i].len) { // nothing to merge, add new entry
 842                         debug("New SACK entry %d\n", i);
 843                         c->sacks[i].offset = offset;
 844                         c->sacks[i].len = rxd;
 845                         break;
 846                 } else if(offset < c->sacks[i].offset) {
 847                         if(offset + rxd < c->sacks[i].offset) { // insert before
 848                                 if(!c->sacks[NSACKS - 1].len) { // only if room left
 849                                         debug("Insert SACK entry at %d\n", i);
 850                                         memmove(&c->sacks[i + 1], &c->sacks[i], (NSACKS - i - 1) * sizeof(c->sacks)[i]);
 851                                         c->sacks[i].offset = offset;
 852                                         c->sacks[i].len = rxd;
 853                                 } else {
 854                                         debug("SACK entries full, dropping packet\n");
 855                                 }
 856
 857                                 break;
 858                         } else { // merge
 859                                 debug("Merge with start of SACK entry at %d\n", i);
 860                                 c->sacks[i].offset = offset;
 861                                 break;
 862                         }
 863                 } else if(offset <= c->sacks[i].offset + c->sacks[i].len) {
 864                         if(offset + rxd > c->sacks[i].offset + c->sacks[i].len) { // merge
 865                                 debug("Merge with end of SACK entry at %d\n", i);
 866                                 c->sacks[i].len = offset + rxd - c->sacks[i].offset;
 867                                 // TODO: handle potential merge with next entry
 868                         }
 869
 870                         break;
 871                 }
 872         }
 873
 874         for(int i = 0; i < NSACKS && c->sacks[i].len; i++) {
 875                 debug("SACK[%d] offset %u len %u\n", i, c->sacks[i].offset, c->sacks[i].len);
 876         }
 877 }
 878
 879 static void handle_in_order(struct utcp_connection *c, const void *data, size_t len) {
 880         // Check if we can process out-of-order data now.
 881         if(c->sacks[0].len && len >= c->sacks[0].offset) { // TODO: handle overlap with second SACK
 882                 debug("incoming packet len %lu connected with SACK at %u\n", (unsigned long)len, c->sacks[0].offset);
 883                 buffer_put_at(&c->rcvbuf, 0, data, len); // TODO: handle return value
 884                 len = max(len, c->sacks[0].offset + c->sacks[0].len);
 885                 data = c->rcvbuf.data;
 886         }
 887
 888         if(c->recv) {
 889                 ssize_t rxd = c->recv(c, data, len);
 890
 891                 if(rxd < 0 || (size_t)rxd != len) {
 892                         // TODO: handle the application not accepting all data.
 893                         abort();
 894                 }
 895         }
 896
 897         if(c->rcvbuf.used) {
 898                 sack_consume(c, len);
 899         }
 900
 901         c->rcv.nxt += len;
 902 }
 903
 904
 905 static void handle_incoming_data(struct utcp_connection *c, uint32_t seq, const void *data, size_t len) {
 906         if(!is_reliable(c)) {
 907                 c->recv(c, data, len);
 908                 c->rcv.nxt = seq + len;
 909                 return;
 910         }
 911
 912         uint32_t offset = seqdiff(seq, c->rcv.nxt);
 913
 914         if(offset + len > c->rcvbuf.maxsize) {
 915                 abort();
 916         }
 917
 918         if(offset) {
 919                 handle_out_of_order(c, offset, data, len);
 920         } else {
 921                 handle_in_order(c, data, len);
 922         }
 923 }
 924
 925
 926 ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 927         const uint8_t *ptr = data;
 928
 929         if(!utcp) {
 930                 errno = EFAULT;
 931                 return -1;
 932         }
 933
 934         if(!len) {
 935                 return 0;
 936         }
 937
 938         if(!data) {
 939                 errno = EFAULT;
 940                 return -1;
 941         }
 942
 943         print_packet(utcp, "recv", data, len);
 944
 945         // Drop packets smaller than the header
 946
 947         struct hdr hdr;
 948
 949         if(len < sizeof(hdr)) {
 950                 errno = EBADMSG;
 951                 return -1;
 952         }
 953
 954         // Make a copy from the potentially unaligned data to a struct hdr
 955
 956         memcpy(&hdr, ptr, sizeof(hdr));
 957         ptr += sizeof(hdr);
 958         len -= sizeof(hdr);
 959
 960         // Drop packets with an unknown CTL flag
 961
 962         if(hdr.ctl & ~(SYN | ACK | RST | FIN)) {
 963                 errno = EBADMSG;
 964                 return -1;
 965         }
 966
 967         // Check for auxiliary headers
 968
 969         const uint8_t *init = NULL;
 970
 971         uint16_t aux = hdr.aux;
 972
 973         while(aux) {
 974                 size_t auxlen = 4 * (aux >> 8) & 0xf;
 975                 uint8_t auxtype = aux & 0xff;
 976
 977                 if(len < auxlen) {
 978                         errno = EBADMSG;
 979                         return -1;
 980                 }
 981
 982                 switch(auxtype) {
 983                 case AUX_INIT:
 984                         if(!(hdr.ctl & SYN) || auxlen != 4) {
 985                                 errno = EBADMSG;
 986                                 return -1;
 987                         }
 988
 989                         init = ptr;
 990                         break;
 991
 992                 default:
 993                         errno = EBADMSG;
 994                         return -1;
 995                 }
 996
 997                 len -= auxlen;
 998                 ptr += auxlen;
 999
1000                 if(!(aux & 0x800)) {
1001                         break;
1002                 }
1003
1004                 if(len < 2) {
1005                         errno = EBADMSG;
1006                         return -1;
1007                 }
1008
1009                 memcpy(&aux, ptr, 2);
1010                 len -= 2;
1011                 ptr += 2;
1012         }
1013
1014         // Try to match the packet to an existing connection
1015
1016         struct utcp_connection *c = find_connection(utcp, hdr.dst, hdr.src);
1017
1018         // Is it for a new connection?
1019
1020         if(!c) {
1021                 // Ignore RST packets
1022
1023                 if(hdr.ctl & RST) {
1024                         return 0;
1025                 }
1026
1027                 // Is it a SYN packet and are we LISTENing?
1028
1029                 if(hdr.ctl & SYN && !(hdr.ctl & ACK) && utcp->accept) {
1030                         // If we don't want to accept it, send a RST back
1031                         if((utcp->pre_accept && !utcp->pre_accept(utcp, hdr.dst))) {
1032                                 len = 1;
1033                                 goto reset;
1034                         }
1035
1036                         // Try to allocate memory, otherwise send a RST back
1037                         c = allocate_connection(utcp, hdr.dst, hdr.src);
1038
1039                         if(!c) {
1040                                 len = 1;
1041                                 goto reset;
1042                         }
1043
1044                         // Parse auxilliary information
1045                         if(init) {
1046                                 if(init[0] < 1) {
1047                                         len = 1;
1048                                         goto reset;
1049                                 }
1050
1051                                 c->flags = init[3] & 0x7;
1052                         } else {
1053                                 c->flags = UTCP_TCP;
1054                         }
1055
1056                         // Return SYN+ACK, go to SYN_RECEIVED state
1057                         c->snd.wnd = hdr.wnd;
1058                         c->rcv.irs = hdr.seq;
1059                         c->rcv.nxt = c->rcv.irs + 1;
1060                         set_state(c, SYN_RECEIVED);
1061
1062                         struct {
1063                                 struct hdr hdr;
1064                                 uint8_t data[4];
1065                         } pkt;
1066
1067                         pkt.hdr.src = c->src;
1068                         pkt.hdr.dst = c->dst;
1069                         pkt.hdr.ack = c->rcv.irs + 1;
1070                         pkt.hdr.seq = c->snd.iss;
1071                         pkt.hdr.wnd = c->rcv.wnd;
1072                         pkt.hdr.ctl = SYN | ACK;
1073
1074                         if(init) {
1075                                 pkt.hdr.aux = 0x0101;
1076                                 pkt.data[0] = 1;
1077                                 pkt.data[1] = 0;
1078                                 pkt.data[2] = 0;
1079                                 pkt.data[3] = c->flags & 0x7;
1080                                 print_packet(c->utcp, "send", &pkt, sizeof(hdr) + 4);
1081                                 utcp->send(utcp, &pkt, sizeof(hdr) + 4);
1082                         } else {
1083                                 pkt.hdr.aux = 0;
1084                                 print_packet(c->utcp, "send", &pkt, sizeof(hdr));
1085                                 utcp->send(utcp, &pkt, sizeof(hdr));
1086                         }
1087                 } else {
1088                         // No, we don't want your packets, send a RST back
1089                         len = 1;
1090                         goto reset;
1091                 }
1092
1093                 return 0;
1094         }
1095
1096         debug("%p state %s\n", c->utcp, strstate[c->state]);
1097
1098         // In case this is for a CLOSED connection, ignore the packet.
1099         // TODO: make it so incoming packets can never match a CLOSED connection.
1100
1101         if(c->state == CLOSED) {
1102                 debug("Got packet for closed connection\n");
1103                 return 0;
1104         }
1105
1106         // It is for an existing connection.
1107
1108         uint32_t prevrcvnxt = c->rcv.nxt;
1109
1110         // 1. Drop invalid packets.
1111
1112         // 1a. Drop packets that should not happen in our current state.
1113
1114         switch(c->state) {
1115         case SYN_SENT:
1116         case SYN_RECEIVED:
1117         case ESTABLISHED:
1118         case FIN_WAIT_1:
1119         case FIN_WAIT_2:
1120         case CLOSE_WAIT:
1121         case CLOSING:
1122         case LAST_ACK:
1123         case TIME_WAIT:
1124                 break;
1125
1126         default:
1127 #ifdef UTCP_DEBUG
1128                 abort();
1129 #endif
1130                 break;
1131         }
1132
1133         // 1b. Drop packets with a sequence number not in our receive window.
1134
1135         if(is_reliable(c)) {
1136                 bool acceptable;
1137
1138                 if(c->state == SYN_SENT) {
1139                         acceptable = true;
1140                 } else if(len == 0) {
1141                         acceptable = seqdiff(hdr.seq, c->rcv.nxt) >= 0;
1142                 } else {
1143                         int32_t rcv_offset = seqdiff(hdr.seq, c->rcv.nxt);
1144
1145                         // cut already accepted front overlapping
1146                         if(rcv_offset < 0) {
1147                                 acceptable = len > (size_t) - rcv_offset;
1148
1149                                 if(acceptable) {
1150                                         ptr -= rcv_offset;
1151                                         len += rcv_offset;
1152                                         hdr.seq -= rcv_offset;
1153                                 }
1154                         } else {
1155                                 acceptable = seqdiff(hdr.seq, c->rcv.nxt) >= 0 && seqdiff(hdr.seq, c->rcv.nxt) + len <= c->rcvbuf.maxsize;
1156                         }
1157                 }
1158
1159                 if(!acceptable) {
1160                         debug("Packet not acceptable, %u <= %u + %lu < %u\n", c->rcv.nxt, hdr.seq, (unsigned long)len, c->rcv.nxt + c->rcvbuf.maxsize);
1161
1162                         // Ignore unacceptable RST packets.
1163                         if(hdr.ctl & RST) {
1164                                 return 0;
1165                         }
1166
1167                         // Otherwise, continue processing.
1168                         len = 0;
1169                 }
1170         }
1171
1172         c->snd.wnd = hdr.wnd; // TODO: move below
1173
1174         // 1c. Drop packets with an invalid ACK.
1175         // ackno should not roll back, and it should also not be bigger than what we ever could have sent
1176         // (= snd.una + c->sndbuf.used).
1177
1178         if(!is_reliable(c)) {
1179                 if(hdr.ack != c->snd.last && c->state >= ESTABLISHED) {
1180                         hdr.ack = c->snd.una;
1181                 }
1182         }
1183
1184         if(hdr.ctl & ACK && (seqdiff(hdr.ack, c->snd.last) > 0 || seqdiff(hdr.ack, c->snd.una) < 0)) {
1185                 debug("Packet ack seqno out of range, %u <= %u < %u\n", c->snd.una, hdr.ack, c->snd.una + c->sndbuf.used);
1186
1187                 // Ignore unacceptable RST packets.
1188                 if(hdr.ctl & RST) {
1189                         return 0;
1190                 }
1191
1192                 goto reset;
1193         }
1194
1195         // 2. Handle RST packets
1196
1197         if(hdr.ctl & RST) {
1198                 switch(c->state) {
1199                 case SYN_SENT:
1200                         if(!(hdr.ctl & ACK)) {
1201                                 return 0;
1202                         }
1203
1204                         // The peer has refused our connection.
1205                         set_state(c, CLOSED);
1206                         errno = ECONNREFUSED;
1207
1208                         if(c->recv) {
1209                                 c->recv(c, NULL, 0);
1210                         }
1211
1212                         if(c->poll && !c->reapable) {
1213                                 c->poll(c, 0);
1214                         }
1215
1216                         return 0;
1217
1218                 case SYN_RECEIVED:
1219                         if(hdr.ctl & ACK) {
1220                                 return 0;
1221                         }
1222
1223                         // We haven't told the application about this connection yet. Silently delete.
1224                         free_connection(c);
1225                         return 0;
1226
1227                 case ESTABLISHED:
1228                 case FIN_WAIT_1:
1229                 case FIN_WAIT_2:
1230                 case CLOSE_WAIT:
1231                         if(hdr.ctl & ACK) {
1232                                 return 0;
1233                         }
1234
1235                         // The peer has aborted our connection.
1236                         set_state(c, CLOSED);
1237                         errno = ECONNRESET;
1238
1239                         if(c->recv) {
1240                                 c->recv(c, NULL, 0);
1241                         }
1242
1243                         if(c->poll && !c->reapable) {
1244                                 c->poll(c, 0);
1245                         }
1246
1247                         return 0;
1248
1249                 case CLOSING:
1250                 case LAST_ACK:
1251                 case TIME_WAIT:
1252                         if(hdr.ctl & ACK) {
1253                                 return 0;
1254                         }
1255
1256                         // As far as the application is concerned, the connection has already been closed.
1257                         // If it has called utcp_close() already, we can immediately free this connection.
1258                         if(c->reapable) {
1259                                 free_connection(c);
1260                                 return 0;
1261                         }
1262
1263                         // Otherwise, immediately move to the CLOSED state.
1264                         set_state(c, CLOSED);
1265                         return 0;
1266
1267                 default:
1268 #ifdef UTCP_DEBUG
1269                         abort();
1270 #endif
1271                         break;
1272                 }
1273         }
1274
1275         uint32_t advanced;
1276
1277         if(!(hdr.ctl & ACK)) {
1278                 advanced = 0;
1279                 goto skip_ack;
1280         }
1281
1282         // 3. Advance snd.una
1283
1284         advanced = seqdiff(hdr.ack, c->snd.una);
1285         prevrcvnxt = c->rcv.nxt;
1286
1287         if(advanced) {
1288                 // RTT measurement
1289                 if(c->rtt_start.tv_sec) {
1290                         if(c->rtt_seq == hdr.ack) {
1291                                 struct timeval now, diff;
1292                                 gettimeofday(&now, NULL);
1293                                 timersub(&now, &c->rtt_start, &diff);
1294                                 update_rtt(c, diff.tv_sec * 1000000 + diff.tv_usec);
1295                                 c->rtt_start.tv_sec = 0;
1296                         } else if(c->rtt_seq < hdr.ack) {
1297                                 debug("Cancelling RTT measurement: %u < %u\n", c->rtt_seq, hdr.ack);
1298                                 c->rtt_start.tv_sec = 0;
1299                         }
1300                 }
1301
1302                 int32_t data_acked = advanced;
1303
1304                 switch(c->state) {
1305                 case SYN_SENT:
1306                 case SYN_RECEIVED:
1307                         data_acked--;
1308                         break;
1309
1310                 // TODO: handle FIN as well.
1311                 default:
1312                         break;
1313                 }
1314
1315                 assert(data_acked >= 0);
1316
1317                 int32_t bufused = seqdiff(c->snd.last, c->snd.una);
1318                 assert(data_acked <= bufused);
1319
1320                 if(data_acked) {
1321                         buffer_get(&c->sndbuf, NULL, data_acked);
1322                 }
1323
1324                 // Also advance snd.nxt if possible
1325                 if(seqdiff(c->snd.nxt, hdr.ack) < 0) {
1326                         c->snd.nxt = hdr.ack;
1327                 }
1328
1329                 c->snd.una = hdr.ack;
1330
1331                 c->dupack = 0;
1332                 c->snd.cwnd += utcp->mtu;
1333
1334                 if(c->snd.cwnd > c->sndbuf.maxsize) {
1335                         c->snd.cwnd = c->sndbuf.maxsize;
1336                 }
1337
1338                 // Check if we have sent a FIN that is now ACKed.
1339                 switch(c->state) {
1340                 case FIN_WAIT_1:
1341                         if(c->snd.una == c->snd.last) {
1342                                 set_state(c, FIN_WAIT_2);
1343                         }
1344
1345                         break;
1346
1347                 case CLOSING:
1348                         if(c->snd.una == c->snd.last) {
1349                                 gettimeofday(&c->conn_timeout, NULL);
1350                                 c->conn_timeout.tv_sec += 60;
1351                                 set_state(c, TIME_WAIT);
1352                         }
1353
1354                         break;
1355
1356                 default:
1357                         break;
1358                 }
1359         } else {
1360                 if(!len && is_reliable(c)) {
1361                         c->dupack++;
1362
1363                         if(c->dupack == 3) {
1364                                 debug("Triplicate ACK\n");
1365                                 //TODO: Resend one packet and go to fast recovery mode. See RFC 6582.
1366                                 //We do a very simple variant here; reset the nxt pointer to the last acknowledged packet from the peer.
1367                                 //Reset the congestion window so we wait for ACKs.
1368                                 c->snd.nxt = c->snd.una;
1369                                 c->snd.cwnd = utcp->mtu;
1370                                 start_retransmit_timer(c);
1371                         }
1372                 }
1373         }
1374
1375         // 4. Update timers
1376
1377         if(advanced) {
1378                 if(c->snd.una == c->snd.last) {
1379                         stop_retransmit_timer(c);
1380                         timerclear(&c->conn_timeout);
1381                 } else if(is_reliable(c)) {
1382                         start_retransmit_timer(c);
1383                         gettimeofday(&c->conn_timeout, NULL);
1384                         c->conn_timeout.tv_sec += utcp->timeout;
1385                 }
1386         }
1387
1388 skip_ack:
1389         // 5. Process SYN stuff
1390
1391         if(hdr.ctl & SYN) {
1392                 switch(c->state) {
1393                 case SYN_SENT:
1394
1395                         // This is a SYNACK. It should always have ACKed the SYN.
1396                         if(!advanced) {
1397                                 goto reset;
1398                         }
1399
1400                         c->rcv.irs = hdr.seq;
1401                         c->rcv.nxt = hdr.seq;
1402
1403                         if(c->shut_wr) {
1404                                 c->snd.last++;
1405                                 set_state(c, FIN_WAIT_1);
1406                         } else {
1407                                 set_state(c, ESTABLISHED);
1408                         }
1409
1410                         // TODO: notify application of this somehow.
1411                         break;
1412
1413                 case SYN_RECEIVED:
1414                 case ESTABLISHED:
1415                 case FIN_WAIT_1:
1416                 case FIN_WAIT_2:
1417                 case CLOSE_WAIT:
1418                 case CLOSING:
1419                 case LAST_ACK:
1420                 case TIME_WAIT:
1421                         // Ehm, no. We should never receive a second SYN.
1422                         return 0;
1423
1424                 default:
1425 #ifdef UTCP_DEBUG
1426                         abort();
1427 #endif
1428                         return 0;
1429                 }
1430
1431                 // SYN counts as one sequence number
1432                 c->rcv.nxt++;
1433         }
1434
1435         // 6. Process new data
1436
1437         if(c->state == SYN_RECEIVED) {
1438                 // This is the ACK after the SYNACK. It should always have ACKed the SYNACK.
1439                 if(!advanced) {
1440                         goto reset;
1441                 }
1442
1443                 // Are we still LISTENing?
1444                 if(utcp->accept) {
1445                         utcp->accept(c, c->src);
1446                 }
1447
1448                 if(c->state != ESTABLISHED) {
1449                         set_state(c, CLOSED);
1450                         c->reapable = true;
1451                         goto reset;
1452                 }
1453         }
1454
1455         if(len) {
1456                 switch(c->state) {
1457                 case SYN_SENT:
1458                 case SYN_RECEIVED:
1459                         // This should never happen.
1460 #ifdef UTCP_DEBUG
1461                         abort();
1462 #endif
1463                         return 0;
1464
1465                 case ESTABLISHED:
1466                 case FIN_WAIT_1:
1467                 case FIN_WAIT_2:
1468                         break;
1469
1470                 case CLOSE_WAIT:
1471                 case CLOSING:
1472                 case LAST_ACK:
1473                 case TIME_WAIT:
1474                         // Ehm no, We should never receive more data after a FIN.
1475                         goto reset;
1476
1477                 default:
1478 #ifdef UTCP_DEBUG
1479                         abort();
1480 #endif
1481                         return 0;
1482                 }
1483
1484                 handle_incoming_data(c, hdr.seq, ptr, len);
1485         }
1486
1487         // 7. Process FIN stuff
1488
1489         if((hdr.ctl & FIN) && (!is_reliable(c) || hdr.seq + len == c->rcv.nxt)) {
1490                 switch(c->state) {
1491                 case SYN_SENT:
1492                 case SYN_RECEIVED:
1493                         // This should never happen.
1494 #ifdef UTCP_DEBUG
1495                         abort();
1496 #endif
1497                         break;
1498
1499                 case ESTABLISHED:
1500                         set_state(c, CLOSE_WAIT);
1501                         break;
1502
1503                 case FIN_WAIT_1:
1504                         set_state(c, CLOSING);
1505                         break;
1506
1507                 case FIN_WAIT_2:
1508                         gettimeofday(&c->conn_timeout, NULL);
1509                         c->conn_timeout.tv_sec += 60;
1510                         set_state(c, TIME_WAIT);
1511                         break;
1512
1513                 case CLOSE_WAIT:
1514                 case CLOSING:
1515                 case LAST_ACK:
1516                 case TIME_WAIT:
1517                         // Ehm, no. We should never receive a second FIN.
1518                         goto reset;
1519
1520                 default:
1521 #ifdef UTCP_DEBUG
1522                         abort();
1523 #endif
1524                         break;
1525                 }
1526
1527                 // FIN counts as one sequence number
1528                 c->rcv.nxt++;
1529                 len++;
1530
1531                 // Inform the application that the peer closed its end of the connection.
1532                 if(c->recv) {
1533                         errno = 0;
1534                         c->recv(c, NULL, 0);
1535                 }
1536         }
1537
1538         // Now we send something back if:
1539         // - we advanced rcv.nxt (ie, we got some data that needs to be ACKed)
1540         //   -> sendatleastone = true
1541         // - or we got an ack, so we should maybe send a bit more data
1542         //   -> sendatleastone = false
1543
1544         if(is_reliable(c) || hdr.ctl & SYN || hdr.ctl & FIN) {
1545                 ack(c, len || prevrcvnxt != c->rcv.nxt);
1546         }
1547
1548         return 0;
1549
1550 reset:
1551         swap_ports(&hdr);
1552         hdr.wnd = 0;
1553         hdr.aux = 0;
1554
1555         if(hdr.ctl & ACK) {
1556                 hdr.seq = hdr.ack;
1557                 hdr.ctl = RST;
1558         } else {
1559                 hdr.ack = hdr.seq + len;
1560                 hdr.seq = 0;
1561                 hdr.ctl = RST | ACK;
1562         }
1563
1564         print_packet(utcp, "send", &hdr, sizeof(hdr));
1565         utcp->send(utcp, &hdr, sizeof(hdr));
1566         return 0;
1567
1568 }
1569
1570 int utcp_shutdown(struct utcp_connection *c, int dir) {
1571         debug("%p shutdown %d at %u\n", c ? c->utcp : NULL, dir, c ? c->snd.last : 0);
1572
1573         if(!c) {
1574                 errno = EFAULT;
1575                 return -1;
1576         }
1577
1578         if(c->reapable) {
1579                 debug("Error: shutdown() called on closed connection %p\n", c);
1580                 errno = EBADF;
1581                 return -1;
1582         }
1583
1584         if(!(dir == UTCP_SHUT_RD || dir == UTCP_SHUT_WR || dir == UTCP_SHUT_RDWR)) {
1585                 errno = EINVAL;
1586                 return -1;
1587         }
1588
1589         // TCP does not have a provision for stopping incoming packets.
1590         // The best we can do is to just ignore them.
1591         if(dir == UTCP_SHUT_RD || dir == UTCP_SHUT_RDWR) {
1592                 c->recv = NULL;
1593         }
1594
1595         // The rest of the code deals with shutting down writes.
1596         if(dir == UTCP_SHUT_RD) {
1597                 return 0;
1598         }
1599
1600         // Only process shutting down writes once.
1601         if(c->shut_wr) {
1602                 return 0;
1603         }
1604
1605         c->shut_wr = true;
1606
1607         switch(c->state) {
1608         case CLOSED:
1609         case LISTEN:
1610                 errno = ENOTCONN;
1611                 return -1;
1612
1613         case SYN_SENT:
1614                 return 0;
1615
1616         case SYN_RECEIVED:
1617         case ESTABLISHED:
1618                 set_state(c, FIN_WAIT_1);
1619                 break;
1620
1621         case FIN_WAIT_1:
1622         case FIN_WAIT_2:
1623                 return 0;
1624
1625         case CLOSE_WAIT:
1626                 set_state(c, CLOSING);
1627                 break;
1628
1629         case CLOSING:
1630         case LAST_ACK:
1631         case TIME_WAIT:
1632                 return 0;
1633         }
1634
1635         c->snd.last++;
1636
1637         ack(c, false);
1638
1639         if(!timerisset(&c->rtrx_timeout)) {
1640                 start_retransmit_timer(c);
1641         }
1642
1643         return 0;
1644 }
1645
1646 static bool reset_connection(struct utcp_connection *c) {
1647         if(!c) {
1648                 errno = EFAULT;
1649                 return false;
1650         }
1651
1652         if(c->reapable) {
1653                 debug("Error: abort() called on closed connection %p\n", c);
1654                 errno = EBADF;
1655                 return false;
1656         }
1657
1658         c->recv = NULL;
1659         c->poll = NULL;
1660
1661         switch(c->state) {
1662         case CLOSED:
1663                 return true;
1664
1665         case LISTEN:
1666         case SYN_SENT:
1667         case CLOSING:
1668         case LAST_ACK:
1669         case TIME_WAIT:
1670                 set_state(c, CLOSED);
1671                 return true;
1672
1673         case SYN_RECEIVED:
1674         case ESTABLISHED:
1675         case FIN_WAIT_1:
1676         case FIN_WAIT_2:
1677         case CLOSE_WAIT:
1678                 set_state(c, CLOSED);
1679                 break;
1680         }
1681
1682         // Send RST
1683
1684         struct hdr hdr;
1685
1686         hdr.src = c->src;
1687         hdr.dst = c->dst;
1688         hdr.seq = c->snd.nxt;
1689         hdr.ack = 0;
1690         hdr.wnd = 0;
1691         hdr.ctl = RST;
1692
1693         print_packet(c->utcp, "send", &hdr, sizeof(hdr));
1694         c->utcp->send(c->utcp, &hdr, sizeof(hdr));
1695         return true;
1696 }
1697
1698 // Closes all the opened connections
1699 void utcp_abort_all_connections(struct utcp *utcp) {
1700         if(!utcp) {
1701                 errno = EINVAL;
1702                 return;
1703         }
1704
1705         for(int i = 0; i < utcp->nconnections; i++) {
1706                 struct utcp_connection *c = utcp->connections[i];
1707
1708                 if(c->reapable || c->state == CLOSED) {
1709                         continue;
1710                 }
1711
1712                 utcp_recv_t old_recv = c->recv;
1713                 utcp_poll_t old_poll = c->poll;
1714
1715                 reset_connection(c);
1716
1717                 if(old_recv) {
1718                         errno = 0;
1719                         old_recv(c, NULL, 0);
1720                 }
1721
1722                 if(old_poll && !c->reapable) {
1723                         errno = 0;
1724                         old_poll(c, 0);
1725                 }
1726         }
1727
1728         return;
1729 }
1730
1731 int utcp_close(struct utcp_connection *c) {
1732         if(utcp_shutdown(c, SHUT_RDWR) && errno != ENOTCONN) {
1733                 return -1;
1734         }
1735
1736         c->recv = NULL;
1737         c->poll = NULL;
1738         c->reapable = true;
1739         return 0;
1740 }
1741
1742 int utcp_abort(struct utcp_connection *c) {
1743         if(!reset_connection(c)) {
1744                 return -1;
1745         }
1746
1747         c->reapable = true;
1748         return 0;
1749 }
1750
1751 /* Handle timeouts.
1752  * One call to this function will loop through all connections,
1753  * checking if something needs to be resent or not.
1754  * The return value is the time to the next timeout in milliseconds,
1755  * or maybe a negative value if the timeout is infinite.
1756  */
1757 struct timeval utcp_timeout(struct utcp *utcp) {
1758         struct timeval now;
1759         gettimeofday(&now, NULL);
1760         struct timeval next = {now.tv_sec + 3600, now.tv_usec};
1761
1762         for(int i = 0; i < utcp->nconnections; i++) {
1763                 struct utcp_connection *c = utcp->connections[i];
1764
1765                 if(!c) {
1766                         continue;
1767                 }
1768
1769                 // delete connections that have been utcp_close()d.
1770                 if(c->state == CLOSED) {
1771                         if(c->reapable) {
1772                                 debug("Reaping %p\n", c);
1773                                 free_connection(c);
1774                                 i--;
1775                         }
1776
1777                         continue;
1778                 }
1779
1780                 if(timerisset(&c->conn_timeout) && timercmp(&c->conn_timeout, &now, <)) {
1781                         errno = ETIMEDOUT;
1782                         c->state = CLOSED;
1783
1784                         if(c->recv) {
1785                                 c->recv(c, NULL, 0);
1786                         }
1787
1788                         if(c->poll && !c->reapable) {
1789                                 c->poll(c, 0);
1790                         }
1791
1792                         continue;
1793                 }
1794
1795                 if(timerisset(&c->rtrx_timeout) && timercmp(&c->rtrx_timeout, &now, <)) {
1796                         debug("retransmit()\n");
1797                         retransmit(c);
1798                 }
1799
1800                 if(c->poll) {
1801                         if((c->state == ESTABLISHED || c->state == CLOSE_WAIT)) {
1802                                 uint32_t len =  buffer_free(&c->sndbuf);
1803
1804                                 if(len) {
1805                                         c->poll(c, len);
1806                                 }
1807                         } else if(c->state == CLOSED) {
1808                                 c->poll(c, 0);
1809                         }
1810                 }
1811
1812                 if(timerisset(&c->conn_timeout) && timercmp(&c->conn_timeout, &next, <)) {
1813                         next = c->conn_timeout;
1814                 }
1815
1816                 if(timerisset(&c->rtrx_timeout) && timercmp(&c->rtrx_timeout, &next, <)) {
1817                         next = c->rtrx_timeout;
1818                 }
1819         }
1820
1821         struct timeval diff;
1822
1823         timersub(&next, &now, &diff);
1824
1825         return diff;
1826 }
1827
1828 bool utcp_is_active(struct utcp *utcp) {
1829         if(!utcp) {
1830                 return false;
1831         }
1832
1833         for(int i = 0; i < utcp->nconnections; i++)
1834                 if(utcp->connections[i]->state != CLOSED && utcp->connections[i]->state != TIME_WAIT) {
1835                         return true;
1836                 }
1837
1838         return false;
1839 }
1840
1841 struct utcp *utcp_init(utcp_accept_t accept, utcp_pre_accept_t pre_accept, utcp_send_t send, void *priv) {
1842         if(!send) {
1843                 errno = EFAULT;
1844                 return NULL;
1845         }
1846
1847         struct utcp *utcp = calloc(1, sizeof(*utcp));
1848
1849         if(!utcp) {
1850                 return NULL;
1851         }
1852
1853         utcp->accept = accept;
1854         utcp->pre_accept = pre_accept;
1855         utcp->send = send;
1856         utcp->priv = priv;
1857         utcp->mtu = DEFAULT_MTU;
1858         utcp->timeout = DEFAULT_USER_TIMEOUT; // sec
1859         utcp->rto = START_RTO; // usec
1860
1861         return utcp;
1862 }
1863
1864 void utcp_exit(struct utcp *utcp) {
1865         if(!utcp) {
1866                 return;
1867         }
1868
1869         for(int i = 0; i < utcp->nconnections; i++) {
1870                 struct utcp_connection *c = utcp->connections[i];
1871
1872                 if(!c->reapable) {
1873                         if(c->recv) {
1874                                 c->recv(c, NULL, 0);
1875                         }
1876
1877                         if(c->poll && !c->reapable) {
1878                                 c->poll(c, 0);
1879                         }
1880                 }
1881
1882                 buffer_exit(&c->rcvbuf);
1883                 buffer_exit(&c->sndbuf);
1884                 free(c);
1885         }
1886
1887         free(utcp->connections);
1888         free(utcp);
1889 }
1890
1891 uint16_t utcp_get_mtu(struct utcp *utcp) {
1892         return utcp ? utcp->mtu : 0;
1893 }
1894
1895 void utcp_set_mtu(struct utcp *utcp, uint16_t mtu) {
1896         // TODO: handle overhead of the header
1897         if(utcp) {
1898                 utcp->mtu = mtu;
1899         }
1900 }
1901
1902 void utcp_reset_timers(struct utcp *utcp) {
1903         if(!utcp) {
1904                 return;
1905         }
1906
1907         struct timeval now, then;
1908
1909         gettimeofday(&now, NULL);
1910
1911         then = now;
1912
1913         then.tv_sec += utcp->timeout;
1914
1915         for(int i = 0; i < utcp->nconnections; i++) {
1916                 struct utcp_connection *c = utcp->connections[i];
1917
1918                 if(c->reapable) {
1919                         continue;
1920                 }
1921
1922                 c->rtrx_timeout = now;
1923                 c->conn_timeout = then;
1924                 c->rtt_start.tv_sec = 0;
1925         }
1926
1927         if(utcp->rto > START_RTO) {
1928                 utcp->rto = START_RTO;
1929         }
1930 }
1931
1932 int utcp_get_user_timeout(struct utcp *u) {
1933         return u ? u->timeout : 0;
1934 }
1935
1936 void utcp_set_user_timeout(struct utcp *u, int timeout) {
1937         if(u) {
1938                 u->timeout = timeout;
1939         }
1940 }
1941
1942 size_t utcp_get_sndbuf(struct utcp_connection *c) {
1943         return c ? c->sndbuf.maxsize : 0;
1944 }
1945
1946 size_t utcp_get_sndbuf_free(struct utcp_connection *c) {
1947         if(!c) {
1948                 return 0;
1949         }
1950
1951         switch(c->state) {
1952         case SYN_SENT:
1953         case SYN_RECEIVED:
1954         case ESTABLISHED:
1955         case CLOSE_WAIT:
1956                 return buffer_free(&c->sndbuf);
1957
1958         default:
1959                 return 0;
1960         }
1961 }
1962
1963 void utcp_set_sndbuf(struct utcp_connection *c, size_t size) {
1964         if(!c) {
1965                 return;
1966         }
1967
1968         c->sndbuf.maxsize = size;
1969
1970         if(c->sndbuf.maxsize != size) {
1971                 c->sndbuf.maxsize = -1;
1972         }
1973 }
1974
1975 size_t utcp_get_rcvbuf(struct utcp_connection *c) {
1976         return c ? c->rcvbuf.maxsize : 0;
1977 }
1978
1979 size_t utcp_get_rcvbuf_free(struct utcp_connection *c) {
1980         if(c && (c->state == ESTABLISHED || c->state == CLOSE_WAIT)) {
1981                 return buffer_free(&c->rcvbuf);
1982         } else {
1983                 return 0;
1984         }
1985 }
1986
1987 void utcp_set_rcvbuf(struct utcp_connection *c, size_t size) {
1988         if(!c) {
1989                 return;
1990         }
1991
1992         c->rcvbuf.maxsize = size;
1993
1994         if(c->rcvbuf.maxsize != size) {
1995                 c->rcvbuf.maxsize = -1;
1996         }
1997 }
1998
1999 size_t utcp_get_sendq(struct utcp_connection *c) {
2000         return c->sndbuf.used;
2001 }
2002
2003 size_t utcp_get_recvq(struct utcp_connection *c) {
2004         return c->rcvbuf.used;
2005 }
2006
2007 bool utcp_get_nodelay(struct utcp_connection *c) {
2008         return c ? c->nodelay : false;
2009 }
2010
2011 void utcp_set_nodelay(struct utcp_connection *c, bool nodelay) {
2012         if(c) {
2013                 c->nodelay = nodelay;
2014         }
2015 }
2016
2017 bool utcp_get_keepalive(struct utcp_connection *c) {
2018         return c ? c->keepalive : false;
2019 }
2020
2021 void utcp_set_keepalive(struct utcp_connection *c, bool keepalive) {
2022         if(c) {
2023                 c->keepalive = keepalive;
2024         }
2025 }
2026
2027 size_t utcp_get_outq(struct utcp_connection *c) {
2028         return c ? seqdiff(c->snd.nxt, c->snd.una) : 0;
2029 }
2030
2031 void utcp_set_recv_cb(struct utcp_connection *c, utcp_recv_t recv) {
2032         if(c) {
2033                 c->recv = recv;
2034         }
2035 }
2036
2037 void utcp_set_poll_cb(struct utcp_connection *c, utcp_poll_t poll) {
2038         if(c) {
2039                 c->poll = poll;
2040         }
2041 }
2042
2043 void utcp_set_accept_cb(struct utcp *utcp, utcp_accept_t accept, utcp_pre_accept_t pre_accept) {
2044         if(utcp) {
2045                 utcp->accept = accept;
2046                 utcp->pre_accept = pre_accept;
2047         }
2048 }
2049
2050 void utcp_expect_data(struct utcp_connection *c, bool expect) {
2051         if(!c || c->reapable) {
2052                 return;
2053         }
2054
2055         if(!(c->state == ESTABLISHED || c->state == FIN_WAIT_1 || c->state == FIN_WAIT_2)) {
2056                 return;
2057         }
2058
2059         if(expect) {
2060                 // If we expect data, start the connection timer.
2061                 if(!timerisset(&c->conn_timeout)) {
2062                         gettimeofday(&c->conn_timeout, NULL);
2063                         c->conn_timeout.tv_sec += c->utcp->timeout;
2064                 }
2065         } else {
2066                 // If we want to cancel expecting data, only clear the timer when there is no unACKed data.
2067                 if(c->snd.una == c->snd.last) {
2068                         timerclear(&c->conn_timeout);
2069                 }
2070         }
2071 }
2072
2073 void utcp_offline(struct utcp *utcp, bool offline) {
2074         for(int i = 0; i < utcp->nconnections; i++) {
2075                 struct utcp_connection *c = utcp->connections[i];
2076
2077                 if(!c->reapable) {
2078                         utcp_expect_data(c, offline);
2079
2080                         // If we are online again, reset the retransmission timers, but keep the connection timeout as it is,
2081                         // to prevent peers toggling online/offline state frequently from keeping connections alive
2082                         // if there is no progress in sending actual data.
2083                         if(!offline) {
2084                                 gettimeofday(&utcp->connections[i]->rtrx_timeout, NULL);
2085                                 utcp->connections[i]->rtt_start.tv_sec = 0;
2086                         }
2087                 }
2088         }
2089
2090         if(!offline && utcp->rto > START_RTO) {
2091                 utcp->rto = START_RTO;
2092         }
2093 }