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   1 /*
   2     utcp.c -- Userspace TCP
   3     Copyright (C) 2014 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 <errno.h>
  23 #include <stdio.h>
  24 #include <stdlib.h>
  25 #include <stdint.h>
  26 #include <stdbool.h>
  27 #include <string.h>
  28 #include <unistd.h>
  29 #include <sys/time.h>
  30 #include <sys/socket.h>
  31
  32 #define UTCP_INTERNAL
  33 #include "utcp.h"
  34
  35 #define PREP(l) char pkt[(l) + sizeof struct hdr]; struct hdr *hdr = &pkt;
  36
  37 #define SYN 1
  38 #define ACK 2
  39 #define FIN 4
  40 #define RST 8
  41
  42 struct hdr {
  43         uint16_t src; // Source port
  44         uint16_t dst; // Destination port
  45         uint32_t seq; // Sequence number
  46         uint32_t ack; // Acknowledgement number
  47         uint32_t wnd; // Window size
  48         uint16_t ctl; // Flags (SYN, ACK, FIN, RST)
  49         uint16_t aux; // other stuff
  50 };
  51
  52 enum state {
  53         CLOSED,
  54         LISTEN,
  55         SYN_SENT,
  56         SYN_RECEIVED,
  57         ESTABLISHED,
  58         FIN_WAIT_1,
  59         FIN_WAIT_2,
  60         CLOSE_WAIT,
  61         CLOSING,
  62         LAST_ACK,
  63         TIME_WAIT
  64 };
  65
  66 const char *strstate[] = {
  67         "CLOSED",
  68         "LISTEN",
  69         "SYN_SENT",
  70         "SYN_RECEIVED",
  71         "ESTABLISHED",
  72         "FIN_WAIT_1",
  73         "FIN_WAIT_2",
  74         "CLOSE_WAIT",
  75         "CLOSING",
  76         "LAST_ACK",
  77         "TIME_WAIT"
  78 };
  79
  80 struct utcp_connection {
  81         void *priv;
  82         struct utcp *utcp;
  83         bool reapable;
  84
  85         uint16_t src;
  86         uint16_t dst;
  87         enum state state;
  88
  89         // The following two structures form the TCB
  90
  91         struct {
  92                 uint32_t una;
  93                 uint32_t nxt;
  94                 uint32_t wnd;
  95                 uint32_t iss;
  96         } snd;
  97
  98         struct {
  99                 uint32_t nxt;
 100                 uint32_t wnd;
 101                 uint32_t irs;
 102         } rcv;
 103
 104         utcp_recv_t recv;
 105
 106         struct timeval conn_timeout;
 107         struct timeval rtrx_timeout;
 108
 109         char *sndbuf;
 110         uint32_t sndbufsize;
 111 };
 112
 113 struct utcp {
 114         void *priv;
 115
 116         utcp_accept_t accept;
 117         utcp_pre_accept_t pre_accept;
 118         utcp_send_t send;
 119
 120         uint16_t mtu;
 121         int timeout;
 122
 123         struct utcp_connection **connections;
 124         int nconnections;
 125         int nallocated;
 126 };
 127
 128 static void set_state(struct utcp_connection *c, enum state state) {
 129         c->state = state;
 130         if(state == ESTABLISHED)
 131                 timerclear(&c->conn_timeout);
 132         fprintf(stderr, "%p new state: %s\n", c->utcp, strstate[state]);
 133 }
 134
 135 static void print_packet(struct utcp *utcp, const char *dir, const void *pkt, size_t len) {
 136         struct hdr hdr;
 137         if(len < sizeof hdr) {
 138                 fprintf(stderr, "%p %s: short packet (%zu bytes)\n", utcp, dir, len);
 139                 return;
 140         }
 141
 142         memcpy(&hdr, pkt, sizeof hdr);
 143         fprintf (stderr, "%p %s: src=%u dst=%u seq=%u ack=%u wnd=%u ctl=", utcp, dir, hdr.src, hdr.dst, hdr.seq, hdr.ack, hdr.wnd);
 144         if(hdr.ctl & SYN)
 145                 fprintf(stderr, "SYN");
 146         if(hdr.ctl & RST)
 147                 fprintf(stderr, "RST");
 148         if(hdr.ctl & FIN)
 149                 fprintf(stderr, "FIN");
 150         if(hdr.ctl & ACK)
 151                 fprintf(stderr, "ACK");
 152
 153         if(len > sizeof hdr) {
 154                 fprintf(stderr, " data=");
 155                 for(int i = sizeof hdr; i < len; i++) {
 156                         const char *data = pkt;
 157                         fprintf(stderr, "%c", data[i] >= 32 ? data[i] : '.');
 158                 }
 159         }
 160
 161         fprintf(stderr, "\n");
 162 }
 163
 164 static inline void list_connections(struct utcp *utcp) {
 165         fprintf(stderr, "%p has %d connections:\n", utcp, utcp->nconnections);
 166         for(int i = 0; i < utcp->nconnections; i++)
 167                 fprintf(stderr, "  %u -> %u state %s\n", utcp->connections[i]->src, utcp->connections[i]->dst, strstate[utcp->connections[i]->state]);
 168 }
 169
 170 // Connections are stored in a sorted list.
 171 // This gives O(log(N)) lookup time, O(N log(N)) insertion time and O(N) deletion time.
 172
 173 static int compare(const void *va, const void *vb) {
 174         const struct utcp_connection *a = *(struct utcp_connection **)va;
 175         const struct utcp_connection *b = *(struct utcp_connection **)vb;
 176         if(!a->src || !b->src)
 177                 abort();
 178         int c = (int)a->src - (int)b->src;
 179         if(c)
 180                 return c;
 181         c = (int)a->dst - (int)b->dst;
 182         return c;
 183 }
 184
 185 static struct utcp_connection *find_connection(const struct utcp *utcp, uint16_t src, uint16_t dst) {
 186         if(!utcp->nconnections)
 187                 return NULL;
 188         struct utcp_connection key = {
 189                 .src = src,
 190                 .dst = dst,
 191         }, *keyp = &key;
 192         struct utcp_connection **match = bsearch(&keyp, utcp->connections, utcp->nconnections, sizeof *utcp->connections, compare);
 193         return match ? *match : NULL;
 194 }
 195
 196 static void free_connection(struct utcp_connection *c) {
 197         struct utcp *utcp = c->utcp;
 198         struct utcp_connection **cp = bsearch(&c, utcp->connections, utcp->nconnections, sizeof *utcp->connections, compare);
 199         if(!cp)
 200                 abort();
 201
 202         int i = cp - utcp->connections;
 203         memmove(cp + i, cp + i + 1, (utcp->nconnections - i - 1) * sizeof *cp);
 204         utcp->nconnections--;
 205
 206         free(c);
 207 }
 208
 209 static struct utcp_connection *allocate_connection(struct utcp *utcp, uint16_t src, uint16_t dst) {
 210         // Check whether this combination of src and dst is free
 211
 212         if(src) {
 213                 if(find_connection(utcp, src, dst)) {
 214                         errno = EADDRINUSE;
 215                         return NULL;
 216                 }
 217         } else { // If src == 0, generate a random port number with the high bit set
 218                 if(utcp->nconnections >= 32767) {
 219                         errno = ENOMEM;
 220                         return NULL;
 221                 }
 222                 src = rand() | 0x8000;
 223                 while(find_connection(utcp, src, dst))
 224                         src++;
 225         }
 226
 227         // Allocate memory for the new connection
 228
 229         if(utcp->nconnections >= utcp->nallocated) {
 230                 if(!utcp->nallocated)
 231                         utcp->nallocated = 4;
 232                 else
 233                         utcp->nallocated *= 2;
 234                 struct utcp_connection **new_array = realloc(utcp->connections, utcp->nallocated * sizeof *utcp->connections);
 235                 if(!new_array) {
 236                         errno = ENOMEM;
 237                         return NULL;
 238                 }
 239                 utcp->connections = new_array;
 240         }
 241
 242         struct utcp_connection *c = calloc(1, sizeof *c);
 243         if(!c) {
 244                 errno = ENOMEM;
 245                 return NULL;
 246         }
 247
 248         // Fill in the details
 249
 250         c->src = src;
 251         c->dst = dst;
 252         c->snd.iss = rand();
 253         c->snd.una = c->snd.iss;
 254         c->snd.nxt = c->snd.iss + 1;
 255         c->rcv.wnd = utcp->mtu;
 256         c->utcp = utcp;
 257         c->sndbufsize = 65536;
 258         c->sndbuf = malloc(c->sndbufsize);
 259         if(!c->sndbuf)
 260                 c->sndbufsize = 0;
 261
 262         // Add it to the sorted list of connections
 263
 264         utcp->connections[utcp->nconnections++] = c;
 265         qsort(utcp->connections, utcp->nconnections, sizeof *utcp->connections, compare);
 266
 267         return c;
 268 }
 269
 270 struct utcp_connection *utcp_connect(struct utcp *utcp, uint16_t dst, utcp_recv_t recv, void *priv) {
 271         struct utcp_connection *c = allocate_connection(utcp, 0, dst);
 272         if(!c)
 273                 return NULL;
 274
 275         c->recv = recv;
 276
 277         struct hdr hdr;
 278
 279         hdr.src = c->src;
 280         hdr.dst = c->dst;
 281         hdr.seq = c->snd.iss;
 282         hdr.ack = 0;
 283         hdr.ctl = SYN;
 284         hdr.wnd = c->rcv.wnd;
 285
 286         set_state(c, SYN_SENT);
 287
 288         print_packet(utcp, "send", &hdr, sizeof hdr);
 289         utcp->send(utcp, &hdr, sizeof hdr);
 290
 291         gettimeofday(&c->conn_timeout, NULL);
 292         c->conn_timeout.tv_sec += utcp->timeout;
 293
 294         return c;
 295 }
 296
 297 void utcp_accept(struct utcp_connection *c, utcp_recv_t recv, void *priv) {
 298         if(c->reapable || c->state != SYN_RECEIVED) {
 299                 fprintf(stderr, "Error: accept() called on invalid connection %p in state %s\n", c, strstate[c->state]);
 300                 return;
 301         }
 302
 303         fprintf(stderr, "%p accepted, %p %p\n", c, recv, priv);
 304         c->recv = recv;
 305         c->priv = priv;
 306         set_state(c, ESTABLISHED);
 307 }
 308
 309 ssize_t utcp_send(struct utcp_connection *c, const void *data, size_t len) {
 310         if(c->reapable) {
 311                 fprintf(stderr, "Error: send() called on closed connection %p\n", c);
 312                 errno = EBADF;
 313                 return -1;
 314         }
 315
 316         switch(c->state) {
 317         case CLOSED:
 318         case LISTEN:
 319         case SYN_SENT:
 320         case SYN_RECEIVED:
 321                 fprintf(stderr, "Error: send() called on unconnected connection %p\n", c);
 322                 errno = ENOTCONN;
 323                 return -1;
 324         case ESTABLISHED:
 325         case CLOSE_WAIT:
 326                 break;
 327         case FIN_WAIT_1:
 328         case FIN_WAIT_2:
 329         case CLOSING:
 330         case LAST_ACK:
 331         case TIME_WAIT:
 332                 fprintf(stderr, "Error: send() called on closing connection %p\n", c);
 333                 errno = EPIPE;
 334                 return -1;
 335         }
 336
 337         // Add data to send buffer
 338
 339         if(!len)
 340                 return 0;
 341
 342         if(!data) {
 343                 errno = EFAULT;
 344                 return -1;
 345         }
 346
 347         uint32_t bufused = c->snd.nxt - c->snd.una;
 348
 349         /* Check our send buffer.
 350          * - If it's big enough, just put the data in there.
 351          * - If not, decide whether to enlarge. (TODO, now we just always enlarge)
 352          * - Cap len so it doesn't overflow our buffer.
 353          */
 354
 355         if(len > c->sndbufsize - bufused) {
 356                 c->sndbufsize *= 2;
 357                 c->sndbuf = realloc(c->sndbuf, c->sndbufsize);
 358         }
 359
 360         if(len > c->sndbufsize - bufused)
 361                 len = c->sndbufsize - bufused;
 362
 363         if(!len) {
 364                 errno == EWOULDBLOCK;
 365                 return 0;
 366         }
 367
 368         memcpy(c->sndbuf + bufused, data, len);
 369
 370         // Send segments
 371
 372         struct {
 373                 struct hdr hdr;
 374                 char data[c->utcp->mtu];
 375         } pkt;
 376
 377         pkt.hdr.src = c->src;
 378         pkt.hdr.dst = c->dst;
 379         pkt.hdr.ack = c->rcv.nxt;
 380         pkt.hdr.wnd = c->snd.wnd;
 381         pkt.hdr.ctl = ACK;
 382
 383         uint32_t left = len;
 384
 385         while(left) {
 386                 uint32_t seglen = left > c->utcp->mtu ? c->utcp->mtu : left;
 387                 pkt.hdr.seq = c->snd.nxt;
 388
 389                 memcpy(pkt.data, data, seglen);
 390
 391                 c->snd.nxt += seglen;
 392                 data += seglen;
 393                 left -= seglen;
 394
 395                 print_packet(c->utcp, "send", &pkt, sizeof pkt.hdr + seglen);
 396                 c->utcp->send(c->utcp, &pkt, sizeof pkt.hdr + seglen);
 397         }
 398
 399         fprintf(stderr, "len=%zu\n", len);
 400         return len;
 401 }
 402
 403 static void swap_ports(struct hdr *hdr) {
 404         uint16_t tmp = hdr->src;
 405         hdr->src = hdr->dst;
 406         hdr->dst = tmp;
 407 }
 408
 409 static int32_t seqdiff(uint32_t a, uint32_t b) {
 410         return a - b;
 411 }
 412
 413 int utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 414         if(!utcp) {
 415                 errno = EFAULT;
 416                 return -1;
 417         }
 418
 419         if(!len)
 420                 return 0;
 421
 422         if(!data) {
 423                 errno = EFAULT;
 424                 return -1;
 425         }
 426
 427         print_packet(utcp, "recv", data, len);
 428
 429         // Drop packets smaller than the header
 430
 431         struct hdr hdr;
 432         if(len < sizeof hdr) {
 433                 errno = EBADMSG;
 434                 return -1;
 435         }
 436
 437         // Make a copy from the potentially unaligned data to a struct hdr
 438
 439         memcpy(&hdr, data, sizeof hdr);
 440         data += sizeof hdr;
 441         len -= sizeof hdr;
 442
 443         // Drop packets with an unknown CTL flag
 444
 445         if(hdr.ctl & ~(SYN | ACK | RST | FIN)) {
 446                 errno = EBADMSG;
 447                 return -1;
 448         }
 449
 450         // Try to match the packet to an existing connection
 451
 452         struct utcp_connection *c = find_connection(utcp, hdr.dst, hdr.src);
 453
 454         // Is it for a new connection?
 455
 456         if(!c) {
 457                 // Ignore RST packets
 458
 459                 if(hdr.ctl & RST)
 460                         return 0;
 461
 462                 // Is it a SYN packet and are we LISTENing?
 463
 464                 if(hdr.ctl & SYN && !(hdr.ctl & ACK) && utcp->accept) {
 465                         // If we don't want to accept it, send a RST back
 466                         if((utcp->pre_accept && !utcp->pre_accept(utcp, hdr.dst))) {
 467                                 len = 1;
 468                                 goto reset;
 469                         }
 470
 471                         // Try to allocate memory, otherwise send a RST back
 472                         c = allocate_connection(utcp, hdr.dst, hdr.src);
 473                         if(!c) {
 474                                 len = 1;
 475                                 goto reset;
 476                         }
 477
 478                         // Return SYN+ACK, go to SYN_RECEIVED state
 479                         c->snd.wnd = hdr.wnd;
 480                         c->rcv.irs = hdr.seq;
 481                         c->rcv.nxt = c->rcv.irs + 1;
 482                         set_state(c, SYN_RECEIVED);
 483
 484                         hdr.dst = c->dst;
 485                         hdr.src = c->src;
 486                         hdr.ack = c->rcv.irs + 1;
 487                         hdr.seq = c->snd.iss;
 488                         hdr.ctl = SYN | ACK;
 489                         print_packet(c->utcp, "send", &hdr, sizeof hdr);
 490                         utcp->send(utcp, &hdr, sizeof hdr);
 491                 } else {
 492                         // No, we don't want your packets, send a RST back
 493                         len = 1;
 494                         goto reset;
 495                 }
 496
 497                 return 0;
 498         }
 499
 500         fprintf(stderr, "%p state %s\n", c->utcp, strstate[c->state]);
 501
 502         // In case this is for a CLOSED connection, ignore the packet.
 503         // TODO: make it so incoming packets can never match a CLOSED connection.
 504
 505         if(c->state == CLOSED)
 506                 return 0;
 507
 508         // It is for an existing connection.
 509
 510         // 1. Drop invalid packets.
 511
 512         // 1a. Drop packets that should not happen in our current state.
 513
 514         switch(c->state) {
 515         case SYN_SENT:
 516         case SYN_RECEIVED:
 517         case ESTABLISHED:
 518         case FIN_WAIT_1:
 519         case FIN_WAIT_2:
 520         case CLOSE_WAIT:
 521         case CLOSING:
 522         case LAST_ACK:
 523         case TIME_WAIT:
 524                 break;
 525         default:
 526                 abort();
 527         }
 528
 529         // 1b. Drop packets with a sequence number not in our receive window.
 530
 531         bool acceptable;
 532
 533         if(c->state == SYN_SENT)
 534                 acceptable = true;
 535
 536         // TODO: handle packets overlapping c->rcv.nxt.
 537 #if 0
 538         // Only use this when accepting out-of-order packets.
 539         else if(len == 0)
 540                 if(c->rcv.wnd == 0)
 541                         acceptable = hdr.seq == c->rcv.nxt;
 542                 else
 543                         acceptable = (seqdiff(hdr.seq, c->rcv.nxt) >= 0 && seqdiff(hdr.seq, c->rcv.nxt + c->rcv.wnd) < 0);
 544         else
 545                 if(c->rcv.wnd == 0)
 546                         // We don't accept data when the receive window is zero.
 547                         acceptable = false;
 548                 else
 549                         // Both start and end of packet must be within the receive window
 550                         acceptable = (seqdiff(hdr.seq, c->rcv.nxt) >= 0 && seqdiff(hdr.seq, c->rcv.nxt + c->rcv.wnd) < 0)
 551                                 || (seqdiff(hdr.seq + len + 1, c->rcv.nxt) >= 0 && seqdiff(hdr.seq + len - 1, c->rcv.nxt + c->rcv.wnd) < 0);
 552 #else
 553         if(c->state != SYN_SENT)
 554                 acceptable = hdr.seq == c->rcv.nxt;
 555 #endif
 556
 557         if(!acceptable) {
 558                 fprintf(stderr, "Packet not acceptable, %u  <= %u + %zu < %u\n", c->rcv.nxt, hdr.seq, len, c->rcv.nxt + c->rcv.wnd);
 559                 // Ignore unacceptable RST packets.
 560                 if(hdr.ctl & RST)
 561                         return 0;
 562                 // Otherwise, send an ACK back in the hope things improve.
 563                 goto ack;
 564         }
 565
 566         c->snd.wnd = hdr.wnd; // TODO: move below
 567
 568         // 1c. Drop packets with an invalid ACK.
 569         // ackno should not roll back, and it should also not be bigger than snd.nxt.
 570
 571         if(hdr.ctl & ACK && (seqdiff(hdr.ack, c->snd.nxt) > 0 || seqdiff(hdr.ack, c->snd.una) < 0)) {
 572                 fprintf(stderr, "Packet ack seqno out of range, %u %u %u\n", hdr.ack, c->snd.una, c->snd.nxt);
 573                 // Ignore unacceptable RST packets.
 574                 if(hdr.ctl & RST)
 575                         return 0;
 576                 goto reset;
 577         }
 578
 579         // 2. Handle RST packets
 580
 581         if(hdr.ctl & RST) {
 582                 switch(c->state) {
 583                 case SYN_SENT:
 584                         if(!(hdr.ctl & ACK))
 585                                 return 0;
 586                         // The peer has refused our connection.
 587                         set_state(c, CLOSED);
 588                         errno = ECONNREFUSED;
 589                         if(c->recv)
 590                                 c->recv(c, NULL, 0);
 591                         return 0;
 592                 case SYN_RECEIVED:
 593                         if(hdr.ctl & ACK)
 594                                 return 0;
 595                         // We haven't told the application about this connection yet. Silently delete.
 596                         free_connection(c);
 597                         return 0;
 598                 case ESTABLISHED:
 599                 case FIN_WAIT_1:
 600                 case FIN_WAIT_2:
 601                 case CLOSE_WAIT:
 602                         if(hdr.ctl & ACK)
 603                                 return 0;
 604                         // The peer has aborted our connection.
 605                         set_state(c, CLOSED);
 606                         errno = ECONNRESET;
 607                         if(c->recv)
 608                                 c->recv(c, NULL, 0);
 609                         return 0;
 610                 case CLOSING:
 611                 case LAST_ACK:
 612                 case TIME_WAIT:
 613                         if(hdr.ctl & ACK)
 614                                 return 0;
 615                         // As far as the application is concerned, the connection has already been closed.
 616                         // If it has called utcp_close() already, we can immediately free this connection.
 617                         if(c->reapable) {
 618                                 free_connection(c);
 619                                 return 0;
 620                         }
 621                         // Otherwise, immediately move to the CLOSED state.
 622                         set_state(c, CLOSED);
 623                         return 0;
 624                 default:
 625                         abort();
 626                 }
 627         }
 628
 629         // 3. Advance snd.una
 630
 631         uint32_t advanced = seqdiff(hdr.ack, c->snd.una);
 632         c->snd.una = hdr.ack;
 633
 634         if(advanced) {
 635                 fprintf(stderr, "%p advanced %u\n", utcp, advanced);
 636                 // Make room in the send buffer.
 637                 // TODO: try to avoid memmoving too much. Circular buffer?
 638                 uint32_t left = seqdiff(c->snd.nxt, hdr.ack);
 639                 if(left)
 640                         memmove(c->sndbuf, c->sndbuf + advanced, left);
 641         }
 642
 643         // 4. Update timers
 644
 645         if(advanced) {
 646                 timerclear(&c->conn_timeout); // It should be set anew in utcp_timeout() if c->snd.una != c->snd.nxt.
 647                 if(c->snd.una == c->snd.nxt)
 648                         timerclear(&c->rtrx_timeout);
 649         }
 650
 651         // 5. Process SYN stuff
 652
 653         if(hdr.ctl & SYN) {
 654                 switch(c->state) {
 655                 case SYN_SENT:
 656                         // This is a SYNACK. It should always have ACKed the SYN.
 657                         if(!advanced)
 658                                 goto reset;
 659                         c->rcv.irs = hdr.seq;
 660                         c->rcv.nxt = hdr.seq;
 661                         set_state(c, ESTABLISHED);
 662                         // TODO: notify application of this somehow.
 663                         break;
 664                 case SYN_RECEIVED:
 665                 case ESTABLISHED:
 666                 case FIN_WAIT_1:
 667                 case FIN_WAIT_2:
 668                 case CLOSE_WAIT:
 669                 case CLOSING:
 670                 case LAST_ACK:
 671                 case TIME_WAIT:
 672                         // Ehm, no. We should never receive a second SYN.
 673                         goto reset;
 674                 default:
 675                         abort();
 676                 }
 677
 678                 // SYN counts as one sequence number
 679                 c->rcv.nxt++;
 680         }
 681
 682         // 6. Process new data
 683
 684         if(c->state == SYN_RECEIVED) {
 685                 // This is the ACK after the SYNACK. It should always have ACKed the SYNACK.
 686                 if(!advanced)
 687                         goto reset;
 688
 689                 // Are we still LISTENing?
 690                 if(utcp->accept)
 691                         utcp->accept(c, c->src);
 692
 693                 if(c->state != ESTABLISHED) {
 694                         set_state(c, CLOSED);
 695                         c->reapable = true;
 696                         goto reset;
 697                 }
 698         }
 699
 700         if(len) {
 701                 switch(c->state) {
 702                 case SYN_SENT:
 703                 case SYN_RECEIVED:
 704                         // This should never happen.
 705                         abort();
 706                 case ESTABLISHED:
 707                 case FIN_WAIT_1:
 708                 case FIN_WAIT_2:
 709                         break;
 710                 case CLOSE_WAIT:
 711                 case CLOSING:
 712                 case LAST_ACK:
 713                 case TIME_WAIT:
 714                         // Ehm no, We should never receive more data after a FIN.
 715                         goto reset;
 716                 default:
 717                         abort();
 718                 }
 719
 720                 int rxd;
 721
 722                 if(c->recv) {
 723                         rxd = c->recv(c, data, len);
 724                         if(rxd < 0)
 725                                 rxd = 0;
 726                         else if(rxd > len)
 727                                 rxd = len; // Bad application, bad!
 728                 } else {
 729                         rxd = len;
 730                 }
 731
 732                 c->rcv.nxt += len;
 733         }
 734
 735         // 7. Process FIN stuff
 736
 737         if(hdr.ctl & FIN) {
 738                 switch(c->state) {
 739                 case SYN_SENT:
 740                 case SYN_RECEIVED:
 741                         // This should never happen.
 742                         abort();
 743                 case ESTABLISHED:
 744                         set_state(c, CLOSE_WAIT);
 745                         break;
 746                 case FIN_WAIT_1:
 747                         set_state(c, CLOSING);
 748                         break;
 749                 case FIN_WAIT_2:
 750                         set_state(c, TIME_WAIT);
 751                         break;
 752                 case CLOSE_WAIT:
 753                 case CLOSING:
 754                 case LAST_ACK:
 755                 case TIME_WAIT:
 756                         // Ehm, no. We should never receive a second FIN.
 757                         goto reset;
 758                 default:
 759                         abort();
 760                 }
 761
 762                 // FIN counts as one sequence number
 763                 c->rcv.nxt++;
 764
 765                 // Inform the application that the peer closed the connection.
 766                 if(c->recv) {
 767                         errno = 0;
 768                         c->recv(c, NULL, 0);
 769                 }
 770         }
 771
 772         if(!len && !advanced)
 773                 return 0;
 774
 775         if(!len && !(hdr.ctl & SYN) && !(hdr.ctl & FIN))
 776                 return 0;
 777
 778 ack:
 779         hdr.src = c->src;
 780         hdr.dst = c->dst;
 781         hdr.seq = c->snd.nxt;
 782         hdr.ack = c->rcv.nxt;
 783         hdr.ctl = ACK;
 784         print_packet(c->utcp, "send", &hdr, sizeof hdr);
 785         utcp->send(utcp, &hdr, sizeof hdr);
 786         return 0;
 787
 788 reset:
 789         swap_ports(&hdr);
 790         hdr.wnd = 0;
 791         if(hdr.ctl & ACK) {
 792                 hdr.seq = hdr.ack;
 793                 hdr.ctl = RST;
 794         } else {
 795                 hdr.ack = hdr.seq + len;
 796                 hdr.seq = 0;
 797                 hdr.ctl = RST | ACK;
 798         }
 799         print_packet(utcp, "send", &hdr, sizeof hdr);
 800         utcp->send(utcp, &hdr, sizeof hdr);
 801         return 0;
 802
 803 }
 804
 805 int utcp_shutdown(struct utcp_connection *c, int dir) {
 806         fprintf(stderr, "%p shutdown %d\n", c->utcp, dir);
 807         if(!c) {
 808                 errno = EFAULT;
 809                 return -1;
 810         }
 811
 812         if(c->reapable) {
 813                 fprintf(stderr, "Error: shutdown() called on closed connection %p\n", c);
 814                 errno = EBADF;
 815                 return -1;
 816         }
 817
 818         // TODO: handle dir
 819
 820         switch(c->state) {
 821         case CLOSED:
 822                 return 0;
 823         case LISTEN:
 824         case SYN_SENT:
 825                 set_state(c, CLOSED);
 826                 return 0;
 827
 828         case SYN_RECEIVED:
 829         case ESTABLISHED:
 830                 set_state(c, FIN_WAIT_1);
 831                 break;
 832         case FIN_WAIT_1:
 833         case FIN_WAIT_2:
 834                 return 0;
 835         case CLOSE_WAIT:
 836                 set_state(c, CLOSING);
 837                 break;
 838
 839         case CLOSING:
 840         case LAST_ACK:
 841         case TIME_WAIT:
 842                 return 0;
 843         }
 844
 845         // Send FIN
 846
 847         struct hdr hdr;
 848
 849         hdr.src = c->src;
 850         hdr.dst = c->dst;
 851         hdr.seq = c->snd.nxt;
 852         hdr.ack = c->rcv.nxt;
 853         hdr.wnd = c->snd.wnd;
 854         hdr.ctl = FIN | ACK;
 855
 856         c->snd.nxt += 1;
 857
 858         print_packet(c->utcp, "send", &hdr, sizeof hdr);
 859         c->utcp->send(c->utcp, &hdr, sizeof hdr);
 860         return 0;
 861 }
 862
 863 int utcp_close(struct utcp_connection *c) {
 864         if(utcp_shutdown(c, SHUT_RDWR))
 865                 return -1;
 866         c->reapable = true;
 867         return 0;
 868 }
 869
 870 int utcp_abort(struct utcp_connection *c) {
 871         if(!c) {
 872                 errno = EFAULT;
 873                 return -1;
 874         }
 875
 876         if(c->reapable) {
 877                 fprintf(stderr, "Error: abort() called on closed connection %p\n", c);
 878                 errno = EBADF;
 879                 return -1;
 880         }
 881
 882         c->reapable = true;
 883
 884         switch(c->state) {
 885         case CLOSED:
 886                 return 0;
 887         case LISTEN:
 888         case SYN_SENT:
 889         case CLOSING:
 890         case LAST_ACK:
 891         case TIME_WAIT:
 892                 set_state(c, CLOSED);
 893                 return 0;
 894
 895         case SYN_RECEIVED:
 896         case ESTABLISHED:
 897         case FIN_WAIT_1:
 898         case FIN_WAIT_2:
 899         case CLOSE_WAIT:
 900                 set_state(c, CLOSED);
 901                 break;
 902         }
 903
 904         // Send RST
 905
 906         struct hdr hdr;
 907
 908         hdr.src = c->src;
 909         hdr.dst = c->dst;
 910         hdr.seq = c->snd.nxt;
 911         hdr.ack = 0;
 912         hdr.wnd = 0;
 913         hdr.ctl = RST;
 914
 915         print_packet(c->utcp, "send", &hdr, sizeof hdr);
 916         c->utcp->send(c->utcp, &hdr, sizeof hdr);
 917         return 0;
 918 }
 919
 920 static void retransmit(struct utcp_connection *c) {
 921         if(c->state == CLOSED || c->snd.nxt == c->snd.una)
 922                 return;
 923
 924         struct utcp *utcp = c->utcp;
 925
 926         struct {
 927                 struct hdr hdr;
 928                 char data[c->utcp->mtu];
 929         } pkt;
 930
 931         pkt.hdr.src = c->src;
 932         pkt.hdr.dst = c->dst;
 933
 934         switch(c->state) {
 935                 case LISTEN:
 936                         // TODO: this should not happen
 937                         break;
 938
 939                 case SYN_SENT:
 940                         pkt.hdr.seq = c->snd.iss;
 941                         pkt.hdr.ack = 0;
 942                         pkt.hdr.wnd = c->rcv.wnd;
 943                         pkt.hdr.ctl = SYN;
 944                         print_packet(c->utcp, "rtrx", &pkt, sizeof pkt.hdr);
 945                         utcp->send(utcp, &pkt, sizeof pkt.hdr);
 946                         break;
 947
 948                 case SYN_RECEIVED:
 949                         pkt.hdr.seq = c->snd.nxt;
 950                         pkt.hdr.ack = c->rcv.nxt;
 951                         pkt.hdr.ctl = SYN | ACK;
 952                         print_packet(c->utcp, "rtrx", &pkt, sizeof pkt.hdr);
 953                         utcp->send(utcp, &pkt, sizeof pkt.hdr);
 954                         break;
 955
 956                 case ESTABLISHED:
 957                 case FIN_WAIT_1:
 958                         pkt.hdr.seq = c->snd.una;
 959                         pkt.hdr.ack = c->rcv.nxt;
 960                         pkt.hdr.ctl = ACK;
 961                         uint32_t len = seqdiff(c->snd.nxt, c->snd.una);
 962                         if(c->state == FIN_WAIT_1)
 963                                 len--;
 964                         if(len > utcp->mtu)
 965                                 len = utcp->mtu;
 966                         else {
 967                                 if(c->state == FIN_WAIT_1)
 968                                         pkt.hdr.ctl |= FIN;
 969                         }
 970                         memcpy(pkt.data, c->sndbuf, len);
 971                         print_packet(c->utcp, "rtrx", &pkt, sizeof pkt.hdr + len);
 972                         utcp->send(utcp, &pkt, sizeof pkt.hdr + len);
 973                         break;
 974
 975                 default:
 976                         // TODO: implement
 977                         abort();
 978         }
 979 }
 980
 981 /* Handle timeouts.
 982  * One call to this function will loop through all connections,
 983  * checking if something needs to be resent or not.
 984  * The return value is the time to the next timeout in milliseconds,
 985  * or maybe a negative value if the timeout is infinite.
 986  */
 987 int utcp_timeout(struct utcp *utcp) {
 988         struct timeval now;
 989         gettimeofday(&now, NULL);
 990         struct timeval next = {now.tv_sec + 3600, now.tv_usec};
 991
 992         for(int i = 0; i < utcp->nconnections; i++) {
 993                 struct utcp_connection *c = utcp->connections[i];
 994                 if(!c)
 995                         continue;
 996
 997                 if(c->state == CLOSED) {
 998                         if(c->reapable) {
 999                                 fprintf(stderr, "Reaping %p\n", c);
1000                                 free_connection(c);
1001                                 i--;
1002                         }
1003                         continue;
1004                 }
1005
1006                 if(timerisset(&c->conn_timeout) && timercmp(&c->conn_timeout, &now, <)) {
1007                         errno = ETIMEDOUT;
1008                         c->state = CLOSED;
1009                         if(c->recv)
1010                                 c->recv(c, NULL, 0);
1011                         continue;
1012                 }
1013
1014                 if(timerisset(&c->rtrx_timeout) && timercmp(&c->rtrx_timeout, &now, <)) {
1015                         retransmit(c);
1016                 }
1017
1018                 if(timerisset(&c->conn_timeout) && timercmp(&c->conn_timeout, &next, <))
1019                         next = c->conn_timeout;
1020
1021                 if(c->snd.nxt != c->snd.una) {
1022                         c->rtrx_timeout = now;
1023                         c->rtrx_timeout.tv_sec++;
1024                 } else {
1025                         timerclear(&c->rtrx_timeout);
1026                 }
1027
1028                 if(timerisset(&c->rtrx_timeout) && timercmp(&c->rtrx_timeout, &next, <))
1029                         next = c->rtrx_timeout;
1030         }
1031
1032         struct timeval diff;
1033         timersub(&next, &now, &diff);
1034         if(diff.tv_sec < 0)
1035                 return 0;
1036         return diff.tv_sec * 1000 + diff.tv_usec / 1000;
1037 }
1038
1039 struct utcp *utcp_init(utcp_accept_t accept, utcp_pre_accept_t pre_accept, utcp_send_t send, void *priv) {
1040         struct utcp *utcp = calloc(1, sizeof *utcp);
1041         if(!utcp)
1042                 return NULL;
1043
1044         if(!send) {
1045                 errno = EFAULT;
1046                 return NULL;
1047         }
1048
1049         utcp->accept = accept;
1050         utcp->pre_accept = pre_accept;
1051         utcp->send = send;
1052         utcp->priv = priv;
1053         utcp->mtu = 1000;
1054         utcp->timeout = 60;
1055
1056         return utcp;
1057 }
1058
1059 void utcp_exit(struct utcp *utcp) {
1060         if(!utcp)
1061                 return;
1062         for(int i = 0; i < utcp->nconnections; i++)
1063                 free_connection(utcp->connections[i]);
1064         free(utcp);
1065 }
1066
1067 void utcp_set_mtu(struct utcp *utcp, uint16_t mtu) {
1068         // TODO: handle overhead of the header
1069         utcp->mtu = mtu;
1070 }
1071
1072 int utcp_set_connection_timeout(struct utcp *u, int timeout) {
1073         int prev = u->timeout;
1074         u->timeout = timeout;
1075         return prev;
1076 }