net/sctp/output.c

   1 /* SCTP kernel reference Implementation
   2  * (C) Copyright IBM Corp. 2001, 2004
   3  * Copyright (c) 1999-2000 Cisco, Inc.
   4  * Copyright (c) 1999-2001 Motorola, Inc.
   5  *
   6  * This file is part of the SCTP kernel reference Implementation
   7  *
   8  * These functions handle output processing.
   9  *
  10  * The SCTP reference implementation is free software;
  11  * you can redistribute it and/or modify it under the terms of
  12  * the GNU General Public License as published by
  13  * the Free Software Foundation; either version 2, or (at your option)
  14  * any later version.
  15  *
  16  * The SCTP reference implementation is distributed in the hope that it
  17  * will be useful, but WITHOUT ANY WARRANTY; without even the implied
  18  *                 ************************
  19  * warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  20  * See the GNU General Public License for more details.
  21  *
  22  * You should have received a copy of the GNU General Public License
  23  * along with GNU CC; see the file COPYING.  If not, write to
  24  * the Free Software Foundation, 59 Temple Place - Suite 330,
  25  * Boston, MA 02111-1307, USA.
  26  *
  27  * Please send any bug reports or fixes you make to the
  28  * email address(es):
  29  *    lksctp developers <lksctp-developers@lists.sourceforge.net>
  30  *
  31  * Or submit a bug report through the following website:
  32  *    http://www.sf.net/projects/lksctp
  33  *
  34  * Written or modified by:
  35  *    La Monte H.P. Yarroll <piggy@acm.org>
  36  *    Karl Knutson          <karl@athena.chicago.il.us>
  37  *    Jon Grimm             <jgrimm@austin.ibm.com>
  38  *    Sridhar Samudrala     <sri@us.ibm.com>
  39  *
  40  * Any bugs reported given to us we will try to fix... any fixes shared will
  41  * be incorporated into the next SCTP release.
  42  */
  43
  44 #include <linux/types.h>
  45 #include <linux/kernel.h>
  46 #include <linux/wait.h>
  47 #include <linux/time.h>
  48 #include <linux/ip.h>
  49 #include <linux/ipv6.h>
  50 #include <linux/init.h>
  51 #include <net/inet_ecn.h>
  52 #include <net/icmp.h>
  53
  54 #ifndef TEST_FRAME
  55 #include <net/tcp.h>
  56 #endif /* TEST_FRAME (not defined) */
  57
  58 #include <linux/socket.h> /* for sa_family_t */
  59 #include <net/sock.h>
  60
  61 #include <net/sctp/sctp.h>
  62 #include <net/sctp/sm.h>
  63
  64 /* Forward declarations for private helpers. */
  65 static sctp_xmit_t sctp_packet_append_data(struct sctp_packet *packet,
  66                                            struct sctp_chunk *chunk);
  67
  68 /* Config a packet.
  69  * This appears to be a followup set of initializations.
  70  */
  71 struct sctp_packet *sctp_packet_config(struct sctp_packet *packet,
  72                                        __u32 vtag, int ecn_capable)
  73 {
  74         struct sctp_chunk *chunk = NULL;
  75
  76         SCTP_DEBUG_PRINTK("%s: packet:%p vtag:0x%x\n", __FUNCTION__,
  77                           packet, vtag);
  78
  79         packet->vtag = vtag;
  80         packet->has_cookie_echo = 0;
  81         packet->has_sack = 0;
  82         packet->ipfragok = 0;
  83
  84         if (ecn_capable && sctp_packet_empty(packet)) {
  85                 chunk = sctp_get_ecne_prepend(packet->transport->asoc);
  86
  87                 /* If there a is a prepend chunk stick it on the list before
  88                  * any other chunks get appended.
  89                  */
  90                 if (chunk)
  91                         sctp_packet_append_chunk(packet, chunk);
  92         }
  93
  94         return packet;
  95 }
  96
  97 /* Initialize the packet structure. */
  98 struct sctp_packet *sctp_packet_init(struct sctp_packet *packet,
  99                                      struct sctp_transport *transport,
 100                                      __u16 sport, __u16 dport)
 101 {
 102         struct sctp_association *asoc = transport->asoc;
 103         size_t overhead;
 104
 105         SCTP_DEBUG_PRINTK("%s: packet:%p transport:%p\n", __FUNCTION__,
 106                           packet, transport);
 107
 108         packet->transport = transport;
 109         packet->source_port = sport;
 110         packet->destination_port = dport;
 111         INIT_LIST_HEAD(&packet->chunk_list);
 112         if (asoc) {
 113                 struct sctp_sock *sp = sctp_sk(asoc->base.sk);
 114                 overhead = sp->pf->af->net_header_len;
 115         } else {
 116                 overhead = sizeof(struct ipv6hdr);
 117         }
 118         overhead += sizeof(struct sctphdr);
 119         packet->overhead = overhead;
 120         packet->size = overhead;
 121         packet->vtag = 0;
 122         packet->has_cookie_echo = 0;
 123         packet->has_sack = 0;
 124         packet->ipfragok = 0;
 125         packet->malloced = 0;
 126         return packet;
 127 }
 128
 129 /* Free a packet.  */
 130 void sctp_packet_free(struct sctp_packet *packet)
 131 {
 132         struct sctp_chunk *chunk, *tmp;
 133
 134         SCTP_DEBUG_PRINTK("%s: packet:%p\n", __FUNCTION__, packet);
 135
 136         list_for_each_entry_safe(chunk, tmp, &packet->chunk_list, list) {
 137                 list_del_init(&chunk->list);
 138                 sctp_chunk_free(chunk);
 139         }
 140
 141         if (packet->malloced)
 142                 kfree(packet);
 143 }
 144
 145 /* This routine tries to append the chunk to the offered packet. If adding
 146  * the chunk causes the packet to exceed the path MTU and COOKIE_ECHO chunk
 147  * is not present in the packet, it transmits the input packet.
 148  * Data can be bundled with a packet containing a COOKIE_ECHO chunk as long
 149  * as it can fit in the packet, but any more data that does not fit in this
 150  * packet can be sent only after receiving the COOKIE_ACK.
 151  */
 152 sctp_xmit_t sctp_packet_transmit_chunk(struct sctp_packet *packet,
 153                                        struct sctp_chunk *chunk)
 154 {
 155         sctp_xmit_t retval;
 156         int error = 0;
 157
 158         SCTP_DEBUG_PRINTK("%s: packet:%p chunk:%p\n", __FUNCTION__,
 159                           packet, chunk);
 160
 161         switch ((retval = (sctp_packet_append_chunk(packet, chunk)))) {
 162         case SCTP_XMIT_PMTU_FULL:
 163                 if (!packet->has_cookie_echo) {
 164                         error = sctp_packet_transmit(packet);
 165                         if (error < 0)
 166                                 chunk->skb->sk->sk_err = -error;
 167
 168                         /* If we have an empty packet, then we can NOT ever
 169                          * return PMTU_FULL.
 170                          */
 171                         retval = sctp_packet_append_chunk(packet, chunk);
 172                 }
 173                 break;
 174
 175         case SCTP_XMIT_RWND_FULL:
 176         case SCTP_XMIT_OK:
 177         case SCTP_XMIT_NAGLE_DELAY:
 178                 break;
 179         };
 180
 181         return retval;
 182 }
 183
 184 /* Try to bundle a SACK with the packet. */
 185 static sctp_xmit_t sctp_packet_bundle_sack(struct sctp_packet *pkt,
 186                                            struct sctp_chunk *chunk)
 187 {
 188         sctp_xmit_t retval = SCTP_XMIT_OK;
 189
 190         /* If sending DATA and haven't aleady bundled a SACK, try to
 191          * bundle one in to the packet.
 192          */
 193         if (sctp_chunk_is_data(chunk) && !pkt->has_sack &&
 194             !pkt->has_cookie_echo) {
 195                 struct sctp_association *asoc;
 196                 asoc = pkt->transport->asoc;
 197
 198                 if (asoc->a_rwnd > asoc->rwnd) {
 199                         struct sctp_chunk *sack;
 200                         asoc->a_rwnd = asoc->rwnd;
 201                         sack = sctp_make_sack(asoc);
 202                         if (sack) {
 203                                 struct timer_list *timer;
 204                                 retval = sctp_packet_append_chunk(pkt, sack);
 205                                 asoc->peer.sack_needed = 0;
 206                                 timer = &asoc->timers[SCTP_EVENT_TIMEOUT_SACK];
 207                                 if (timer_pending(timer) && del_timer(timer))
 208                                         sctp_association_put(asoc);
 209                         }
 210                 }
 211         }
 212         return retval;
 213 }
 214
 215 /* Append a chunk to the offered packet reporting back any inability to do
 216  * so.
 217  */
 218 sctp_xmit_t sctp_packet_append_chunk(struct sctp_packet *packet,
 219                                      struct sctp_chunk *chunk)
 220 {
 221         sctp_xmit_t retval = SCTP_XMIT_OK;
 222         __u16 chunk_len = WORD_ROUND(ntohs(chunk->chunk_hdr->length));
 223         size_t psize;
 224         size_t pmtu;
 225         int too_big;
 226
 227         SCTP_DEBUG_PRINTK("%s: packet:%p chunk:%p\n", __FUNCTION__, packet,
 228                           chunk);
 229
 230         retval = sctp_packet_bundle_sack(packet, chunk);
 231         psize = packet->size;
 232
 233         if (retval != SCTP_XMIT_OK)
 234                 goto finish;
 235
 236         pmtu  = ((packet->transport->asoc) ?
 237                  (packet->transport->asoc->pathmtu) :
 238                  (packet->transport->pathmtu));
 239
 240         too_big = (psize + chunk_len > pmtu);
 241
 242         /* Decide if we need to fragment or resubmit later. */
 243         if (too_big) {
 244                 /* Both control chunks and data chunks with TSNs are
 245                  * non-fragmentable.
 246                  */
 247                 if (sctp_packet_empty(packet) || !sctp_chunk_is_data(chunk)) {
 248                         /* We no longer do re-fragmentation.
 249                          * Just fragment at the IP layer, if we
 250                          * actually hit this condition
 251                          */
 252                         packet->ipfragok = 1;
 253                         goto append;
 254
 255                 } else {
 256                         retval = SCTP_XMIT_PMTU_FULL;
 257                         goto finish;
 258                 }
 259         }
 260
 261 append:
 262         /* We believe that this chunk is OK to add to the packet (as
 263          * long as we have the cwnd for it).
 264          */
 265
 266         /* DATA is a special case since we must examine both rwnd and cwnd
 267          * before we send DATA.
 268          */
 269         if (sctp_chunk_is_data(chunk)) {
 270                 retval = sctp_packet_append_data(packet, chunk);
 271                 /* Disallow SACK bundling after DATA. */
 272                 packet->has_sack = 1;
 273                 if (SCTP_XMIT_OK != retval)
 274                         goto finish;
 275         } else if (SCTP_CID_COOKIE_ECHO == chunk->chunk_hdr->type)
 276                 packet->has_cookie_echo = 1;
 277         else if (SCTP_CID_SACK == chunk->chunk_hdr->type)
 278                 packet->has_sack = 1;
 279
 280         /* It is OK to send this chunk.  */
 281         list_add_tail(&chunk->list, &packet->chunk_list);
 282         packet->size += chunk_len;
 283         chunk->transport = packet->transport;
 284 finish:
 285         return retval;
 286 }
 287
 288 /* All packets are sent to the network through this function from
 289  * sctp_outq_tail().
 290  *
 291  * The return value is a normal kernel error return value.
 292  */
 293 int sctp_packet_transmit(struct sctp_packet *packet)
 294 {
 295         struct sctp_transport *tp = packet->transport;
 296         struct sctp_association *asoc = tp->asoc;
 297         struct sctphdr *sh;
 298         __u32 crc32 = 0;
 299         struct sk_buff *nskb;
 300         struct sctp_chunk *chunk, *tmp;
 301         struct sock *sk;
 302         int err = 0;
 303         int padding;            /* How much padding do we need?  */
 304         __u8 has_data = 0;
 305         struct dst_entry *dst = tp->dst;
 306
 307         SCTP_DEBUG_PRINTK("%s: packet:%p\n", __FUNCTION__, packet);
 308
 309         /* Do NOT generate a chunkless packet. */
 310         if (list_empty(&packet->chunk_list))
 311                 return err;
 312
 313         /* Set up convenience variables... */
 314         chunk = list_entry(packet->chunk_list.next, struct sctp_chunk, list);
 315         sk = chunk->skb->sk;
 316
 317         /* Allocate the new skb.  */
 318         nskb = alloc_skb(packet->size + LL_MAX_HEADER, GFP_ATOMIC);
 319         if (!nskb)
 320                 goto nomem;
 321
 322         /* Make sure the outbound skb has enough header room reserved. */
 323         skb_reserve(nskb, packet->overhead + LL_MAX_HEADER);
 324
 325         /* Set the owning socket so that we know where to get the
 326          * destination IP address.
 327          */
 328         skb_set_owner_w(nskb, sk);
 329
 330         /* The 'obsolete' field of dst is set to 2 when a dst is freed. */
 331         if (!dst || (dst->obsolete > 1)) {
 332                 dst_release(dst);
 333                 sctp_transport_route(tp, NULL, sctp_sk(sk));
 334                 if (asoc && (asoc->param_flags & SPP_PMTUD_ENABLE)) {
 335                         sctp_assoc_sync_pmtu(asoc);
 336                 }
 337         }
 338         nskb->dst = dst_clone(tp->dst);
 339         if (!nskb->dst)
 340                 goto no_route;
 341         dst = nskb->dst;
 342
 343         /* Build the SCTP header.  */
 344         sh = (struct sctphdr *)skb_push(nskb, sizeof(struct sctphdr));
 345         sh->source = htons(packet->source_port);
 346         sh->dest   = htons(packet->destination_port);
 347
 348         /* From 6.8 Adler-32 Checksum Calculation:
 349          * After the packet is constructed (containing the SCTP common
 350          * header and one or more control or DATA chunks), the
 351          * transmitter shall:
 352          *
 353          * 1) Fill in the proper Verification Tag in the SCTP common
 354          *    header and initialize the checksum field to 0's.
 355          */
 356         sh->vtag     = htonl(packet->vtag);
 357         sh->checksum = 0;
 358
 359         /* 2) Calculate the Adler-32 checksum of the whole packet,
 360          *    including the SCTP common header and all the
 361          *    chunks.
 362          *
 363          * Note: Adler-32 is no longer applicable, as has been replaced
 364          * by CRC32-C as described in <draft-ietf-tsvwg-sctpcsum-02.txt>.
 365          */
 366         if (!(dst->dev->features & NETIF_F_NO_CSUM))
 367                 crc32 = sctp_start_cksum((__u8 *)sh, sizeof(struct sctphdr));
 368
 369         /**
 370          * 6.10 Bundling
 371          *
 372          *    An endpoint bundles chunks by simply including multiple
 373          *    chunks in one outbound SCTP packet.  ...
 374          */
 375
 376         /**
 377          * 3.2  Chunk Field Descriptions
 378          *
 379          * The total length of a chunk (including Type, Length and
 380          * Value fields) MUST be a multiple of 4 bytes.  If the length
 381          * of the chunk is not a multiple of 4 bytes, the sender MUST
 382          * pad the chunk with all zero bytes and this padding is not
 383          * included in the chunk length field.  The sender should
 384          * never pad with more than 3 bytes.
 385          *
 386          * [This whole comment explains WORD_ROUND() below.]
 387          */
 388         SCTP_DEBUG_PRINTK("***sctp_transmit_packet***\n");
 389         list_for_each_entry_safe(chunk, tmp, &packet->chunk_list, list) {
 390                 list_del_init(&chunk->list);
 391                 if (sctp_chunk_is_data(chunk)) {
 392
 393                         if (!chunk->has_tsn) {
 394                                 sctp_chunk_assign_ssn(chunk);
 395                                 sctp_chunk_assign_tsn(chunk);
 396
 397                         /* 6.3.1 C4) When data is in flight and when allowed
 398                          * by rule C5, a new RTT measurement MUST be made each
 399                          * round trip.  Furthermore, new RTT measurements
 400                          * SHOULD be made no more than once per round-trip
 401                          * for a given destination transport address.
 402                          */
 403
 404                                 if (!tp->rto_pending) {
 405                                         chunk->rtt_in_progress = 1;
 406                                         tp->rto_pending = 1;
 407                                 }
 408                         } else
 409                                 chunk->resent = 1;
 410
 411                         chunk->sent_at = jiffies;
 412                         has_data = 1;
 413                 }
 414
 415                 padding = WORD_ROUND(chunk->skb->len) - chunk->skb->len;
 416                 if (padding)
 417                         memset(skb_put(chunk->skb, padding), 0, padding);
 418
 419                 if (dst->dev->features & NETIF_F_NO_CSUM)
 420                         memcpy(skb_put(nskb, chunk->skb->len),
 421                                chunk->skb->data, chunk->skb->len);
 422                 else
 423                         crc32 = sctp_update_copy_cksum(skb_put(nskb,
 424                                                         chunk->skb->len),
 425                                                 chunk->skb->data,
 426                                                 chunk->skb->len, crc32);
 427
 428                 SCTP_DEBUG_PRINTK("%s %p[%s] %s 0x%x, %s %d, %s %d, %s %d\n",
 429                                   "*** Chunk", chunk,
 430                                   sctp_cname(SCTP_ST_CHUNK(
 431                                           chunk->chunk_hdr->type)),
 432                                   chunk->has_tsn ? "TSN" : "No TSN",
 433                                   chunk->has_tsn ?
 434                                   ntohl(chunk->subh.data_hdr->tsn) : 0,
 435                                   "length", ntohs(chunk->chunk_hdr->length),
 436                                   "chunk->skb->len", chunk->skb->len,
 437                                   "rtt_in_progress", chunk->rtt_in_progress);
 438
 439                 /*
 440                  * If this is a control chunk, this is our last
 441                  * reference. Free data chunks after they've been
 442                  * acknowledged or have failed.
 443                  */
 444                 if (!sctp_chunk_is_data(chunk))
 445                         sctp_chunk_free(chunk);
 446         }
 447
 448         /* Perform final transformation on checksum. */
 449         if (!(dst->dev->features & NETIF_F_NO_CSUM))
 450                 crc32 = sctp_end_cksum(crc32);
 451
 452         /* 3) Put the resultant value into the checksum field in the
 453          *    common header, and leave the rest of the bits unchanged.
 454          */
 455         sh->checksum = htonl(crc32);
 456
 457         /* IP layer ECN support
 458          * From RFC 2481
 459          *  "The ECN-Capable Transport (ECT) bit would be set by the
 460          *   data sender to indicate that the end-points of the
 461          *   transport protocol are ECN-capable."
 462          *
 463          * Now setting the ECT bit all the time, as it should not cause
 464          * any problems protocol-wise even if our peer ignores it.
 465          *
 466          * Note: The works for IPv6 layer checks this bit too later
 467          * in transmission.  See IP6_ECN_flow_xmit().
 468          */
 469         INET_ECN_xmit(nskb->sk);
 470
 471         /* Set up the IP options.  */
 472         /* BUG: not implemented
 473          * For v4 this all lives somewhere in sk->sk_opt...
 474          */
 475
 476         /* Dump that on IP!  */
 477         if (asoc && asoc->peer.last_sent_to != tp) {
 478                 /* Considering the multiple CPU scenario, this is a
 479                  * "correcter" place for last_sent_to.  --xguo
 480                  */
 481                 asoc->peer.last_sent_to = tp;
 482         }
 483
 484         if (has_data) {
 485                 struct timer_list *timer;
 486                 unsigned long timeout;
 487
 488                 tp->last_time_used = jiffies;
 489
 490                 /* Restart the AUTOCLOSE timer when sending data. */
 491                 if (sctp_state(asoc, ESTABLISHED) && asoc->autoclose) {
 492                         timer = &asoc->timers[SCTP_EVENT_TIMEOUT_AUTOCLOSE];
 493                         timeout = asoc->timeouts[SCTP_EVENT_TIMEOUT_AUTOCLOSE];
 494
 495                         if (!mod_timer(timer, jiffies + timeout))
 496                                 sctp_association_hold(asoc);
 497                 }
 498         }
 499
 500         SCTP_DEBUG_PRINTK("***sctp_transmit_packet*** skb len %d\n",
 501                           nskb->len);
 502
 503         if (tp->param_flags & SPP_PMTUD_ENABLE)
 504                 (*tp->af_specific->sctp_xmit)(nskb, tp, packet->ipfragok);
 505         else
 506                 (*tp->af_specific->sctp_xmit)(nskb, tp, 1);
 507
 508 out:
 509         packet->size = packet->overhead;
 510         return err;
 511 no_route:
 512         kfree_skb(nskb);
 513         IP_INC_STATS_BH(IPSTATS_MIB_OUTNOROUTES);
 514
 515         /* FIXME: Returning the 'err' will effect all the associations
 516          * associated with a socket, although only one of the paths of the
 517          * association is unreachable.
 518          * The real failure of a transport or association can be passed on
 519          * to the user via notifications. So setting this error may not be
 520          * required.
 521          */
 522          /* err = -EHOSTUNREACH; */
 523 err:
 524         /* Control chunks are unreliable so just drop them.  DATA chunks
 525          * will get resent or dropped later.
 526          */
 527
 528         list_for_each_entry_safe(chunk, tmp, &packet->chunk_list, list) {
 529                 list_del_init(&chunk->list);
 530                 if (!sctp_chunk_is_data(chunk))
 531                         sctp_chunk_free(chunk);
 532         }
 533         goto out;
 534 nomem:
 535         err = -ENOMEM;
 536         goto err;
 537 }
 538
 539 /********************************************************************
 540  * 2nd Level Abstractions
 541  ********************************************************************/
 542
 543 /* This private function handles the specifics of appending DATA chunks.  */
 544 static sctp_xmit_t sctp_packet_append_data(struct sctp_packet *packet,
 545                                            struct sctp_chunk *chunk)
 546 {
 547         sctp_xmit_t retval = SCTP_XMIT_OK;
 548         size_t datasize, rwnd, inflight;
 549         struct sctp_transport *transport = packet->transport;
 550         __u32 max_burst_bytes;
 551         struct sctp_association *asoc = transport->asoc;
 552         struct sctp_sock *sp = sctp_sk(asoc->base.sk);
 553         struct sctp_outq *q = &asoc->outqueue;
 554
 555         /* RFC 2960 6.1  Transmission of DATA Chunks
 556          *
 557          * A) At any given time, the data sender MUST NOT transmit new data to
 558          * any destination transport address if its peer's rwnd indicates
 559          * that the peer has no buffer space (i.e. rwnd is 0, see Section
 560          * 6.2.1).  However, regardless of the value of rwnd (including if it
 561          * is 0), the data sender can always have one DATA chunk in flight to
 562          * the receiver if allowed by cwnd (see rule B below).  This rule
 563          * allows the sender to probe for a change in rwnd that the sender
 564          * missed due to the SACK having been lost in transit from the data
 565          * receiver to the data sender.
 566          */
 567
 568         rwnd = asoc->peer.rwnd;
 569         inflight = asoc->outqueue.outstanding_bytes;
 570
 571         datasize = sctp_data_size(chunk);
 572
 573         if (datasize > rwnd) {
 574                 if (inflight > 0) {
 575                         /* We have (at least) one data chunk in flight,
 576                          * so we can't fall back to rule 6.1 B).
 577                          */
 578                         retval = SCTP_XMIT_RWND_FULL;
 579                         goto finish;
 580                 }
 581         }
 582
 583         /* sctpimpguide-05 2.14.2
 584          * D) When the time comes for the sender to
 585          * transmit new DATA chunks, the protocol parameter Max.Burst MUST
 586          * first be applied to limit how many new DATA chunks may be sent.
 587          * The limit is applied by adjusting cwnd as follows:
 588          *      if ((flightsize + Max.Burst * MTU) < cwnd)
 589          *              cwnd = flightsize + Max.Burst * MTU
 590          */
 591         max_burst_bytes = asoc->max_burst * asoc->pathmtu;
 592         if ((transport->flight_size + max_burst_bytes) < transport->cwnd) {
 593                 transport->cwnd = transport->flight_size + max_burst_bytes;
 594                 SCTP_DEBUG_PRINTK("%s: cwnd limited by max_burst: "
 595                                   "transport: %p, cwnd: %d, "
 596                                   "ssthresh: %d, flight_size: %d, "
 597                                   "pba: %d\n",
 598                                   __FUNCTION__, transport,
 599                                   transport->cwnd,
 600                                   transport->ssthresh,
 601                                   transport->flight_size,
 602                                   transport->partial_bytes_acked);
 603         }
 604
 605         /* RFC 2960 6.1  Transmission of DATA Chunks
 606          *
 607          * B) At any given time, the sender MUST NOT transmit new data
 608          * to a given transport address if it has cwnd or more bytes
 609          * of data outstanding to that transport address.
 610          */
 611         /* RFC 7.2.4 & the Implementers Guide 2.8.
 612          *
 613          * 3) ...
 614          *    When a Fast Retransmit is being performed the sender SHOULD
 615          *    ignore the value of cwnd and SHOULD NOT delay retransmission.
 616          */
 617         if (chunk->fast_retransmit <= 0)
 618                 if (transport->flight_size >= transport->cwnd) {
 619                         retval = SCTP_XMIT_RWND_FULL;
 620                         goto finish;
 621                 }
 622
 623         /* Nagle's algorithm to solve small-packet problem:
 624          * Inhibit the sending of new chunks when new outgoing data arrives
 625          * if any previously transmitted data on the connection remains
 626          * unacknowledged.
 627          */
 628         if (!sp->nodelay && sctp_packet_empty(packet) &&
 629             q->outstanding_bytes && sctp_state(asoc, ESTABLISHED)) {
 630                 unsigned len = datasize + q->out_qlen;
 631
 632                 /* Check whether this chunk and all the rest of pending
 633                  * data will fit or delay in hopes of bundling a full
 634                  * sized packet.
 635                  */
 636                 if (len < asoc->pathmtu - packet->overhead) {
 637                         retval = SCTP_XMIT_NAGLE_DELAY;
 638                         goto finish;
 639                 }
 640         }
 641
 642         /* Keep track of how many bytes are in flight over this transport. */
 643         transport->flight_size += datasize;
 644
 645         /* Keep track of how many bytes are in flight to the receiver. */
 646         asoc->outqueue.outstanding_bytes += datasize;
 647
 648         /* Update our view of the receiver's rwnd. */
 649         if (datasize < rwnd)
 650                 rwnd -= datasize;
 651         else
 652                 rwnd = 0;
 653
 654         asoc->peer.rwnd = rwnd;
 655         /* Has been accepted for transmission. */
 656         if (!asoc->peer.prsctp_capable)
 657                 chunk->msg->can_abandon = 0;
 658
 659 finish:
 660         return retval;
 661 }