1 /* SCTP kernel implementation
2 * (C) Copyright IBM Corp. 2001, 2004
3 * Copyright (c) 1999-2000 Cisco, Inc.
4 * Copyright (c) 1999-2001 Motorola, Inc.
6 * This file is part of the SCTP kernel implementation
8 * These functions handle output processing.
10 * This SCTP 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)
16 * This SCTP 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.
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.
27 * Please send any bug reports or fixes you make to the
29 * lksctp developers <lksctp-developers@lists.sourceforge.net>
31 * Or submit a bug report through the following website:
32 * http://www.sf.net/projects/lksctp
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>
40 * Any bugs reported given to us we will try to fix... any fixes shared will
41 * be incorporated into the next SCTP release.
44 #include <linux/types.h>
45 #include <linux/kernel.h>
46 #include <linux/wait.h>
47 #include <linux/time.h>
49 #include <linux/ipv6.h>
50 #include <linux/init.h>
51 #include <net/inet_ecn.h>
54 #include <net/net_namespace.h>
56 #include <linux/socket.h> /* for sa_family_t */
59 #include <net/sctp/sctp.h>
60 #include <net/sctp/sm.h>
61 #include <net/sctp/checksum.h>
63 /* Forward declarations for private helpers. */
64 static sctp_xmit_t sctp_packet_can_append_data(struct sctp_packet *packet,
65 struct sctp_chunk *chunk);
66 static void sctp_packet_append_data(struct sctp_packet *packet,
67 struct sctp_chunk *chunk);
68 static sctp_xmit_t sctp_packet_will_fit(struct sctp_packet *packet,
69 struct sctp_chunk *chunk,
73 * This appears to be a followup set of initializations.
75 struct sctp_packet *sctp_packet_config(struct sctp_packet *packet,
76 __u32 vtag, int ecn_capable)
78 struct sctp_chunk *chunk = NULL;
80 SCTP_DEBUG_PRINTK("%s: packet:%p vtag:0x%x\n", __func__,
84 packet->has_cookie_echo = 0;
91 if (ecn_capable && sctp_packet_empty(packet)) {
92 chunk = sctp_get_ecne_prepend(packet->transport->asoc);
94 /* If there a is a prepend chunk stick it on the list before
95 * any other chunks get appended.
98 sctp_packet_append_chunk(packet, chunk);
104 /* Initialize the packet structure. */
105 struct sctp_packet *sctp_packet_init(struct sctp_packet *packet,
106 struct sctp_transport *transport,
107 __u16 sport, __u16 dport)
109 struct sctp_association *asoc = transport->asoc;
112 SCTP_DEBUG_PRINTK("%s: packet:%p transport:%p\n", __func__,
115 packet->transport = transport;
116 packet->source_port = sport;
117 packet->destination_port = dport;
118 INIT_LIST_HEAD(&packet->chunk_list);
120 struct sctp_sock *sp = sctp_sk(asoc->base.sk);
121 overhead = sp->pf->af->net_header_len;
123 overhead = sizeof(struct ipv6hdr);
125 overhead += sizeof(struct sctphdr);
126 packet->overhead = overhead;
127 packet->size = overhead;
129 packet->has_cookie_echo = 0;
130 packet->has_sack = 0;
131 packet->has_auth = 0;
132 packet->has_data = 0;
133 packet->ipfragok = 0;
134 packet->malloced = 0;
139 static void sctp_packet_reset(struct sctp_packet *packet)
141 packet->size = packet->overhead;
142 packet->has_cookie_echo = 0;
143 packet->has_sack = 0;
144 packet->has_data = 0;
145 packet->has_auth = 0;
146 packet->ipfragok = 0;
151 void sctp_packet_free(struct sctp_packet *packet)
153 struct sctp_chunk *chunk, *tmp;
155 SCTP_DEBUG_PRINTK("%s: packet:%p\n", __func__, packet);
157 list_for_each_entry_safe(chunk, tmp, &packet->chunk_list, list) {
158 list_del_init(&chunk->list);
159 sctp_chunk_free(chunk);
162 if (packet->malloced)
166 /* This routine tries to append the chunk to the offered packet. If adding
167 * the chunk causes the packet to exceed the path MTU and COOKIE_ECHO chunk
168 * is not present in the packet, it transmits the input packet.
169 * Data can be bundled with a packet containing a COOKIE_ECHO chunk as long
170 * as it can fit in the packet, but any more data that does not fit in this
171 * packet can be sent only after receiving the COOKIE_ACK.
173 sctp_xmit_t sctp_packet_transmit_chunk(struct sctp_packet *packet,
174 struct sctp_chunk *chunk,
180 SCTP_DEBUG_PRINTK("%s: packet:%p chunk:%p\n", __func__,
183 switch ((retval = (sctp_packet_append_chunk(packet, chunk)))) {
184 case SCTP_XMIT_PMTU_FULL:
185 if (!packet->has_cookie_echo) {
186 error = sctp_packet_transmit(packet);
188 chunk->skb->sk->sk_err = -error;
190 /* If we have an empty packet, then we can NOT ever
194 retval = sctp_packet_append_chunk(packet,
199 case SCTP_XMIT_RWND_FULL:
201 case SCTP_XMIT_NAGLE_DELAY:
208 /* Try to bundle an auth chunk into the packet. */
209 static sctp_xmit_t sctp_packet_bundle_auth(struct sctp_packet *pkt,
210 struct sctp_chunk *chunk)
212 struct sctp_association *asoc = pkt->transport->asoc;
213 struct sctp_chunk *auth;
214 sctp_xmit_t retval = SCTP_XMIT_OK;
216 /* if we don't have an association, we can't do authentication */
220 /* See if this is an auth chunk we are bundling or if
221 * auth is already bundled.
223 if (chunk->chunk_hdr->type == SCTP_CID_AUTH || pkt->auth)
226 /* if the peer did not request this chunk to be authenticated,
232 auth = sctp_make_auth(asoc);
236 retval = sctp_packet_append_chunk(pkt, auth);
241 /* Try to bundle a SACK with the packet. */
242 static sctp_xmit_t sctp_packet_bundle_sack(struct sctp_packet *pkt,
243 struct sctp_chunk *chunk)
245 sctp_xmit_t retval = SCTP_XMIT_OK;
247 /* If sending DATA and haven't aleady bundled a SACK, try to
248 * bundle one in to the packet.
250 if (sctp_chunk_is_data(chunk) && !pkt->has_sack &&
251 !pkt->has_cookie_echo) {
252 struct sctp_association *asoc;
253 struct timer_list *timer;
254 asoc = pkt->transport->asoc;
255 timer = &asoc->timers[SCTP_EVENT_TIMEOUT_SACK];
257 /* If the SACK timer is running, we have a pending SACK */
258 if (timer_pending(timer)) {
259 struct sctp_chunk *sack;
260 asoc->a_rwnd = asoc->rwnd;
261 sack = sctp_make_sack(asoc);
263 retval = sctp_packet_append_chunk(pkt, sack);
264 asoc->peer.sack_needed = 0;
265 if (del_timer(timer))
266 sctp_association_put(asoc);
273 /* Append a chunk to the offered packet reporting back any inability to do
276 sctp_xmit_t sctp_packet_append_chunk(struct sctp_packet *packet,
277 struct sctp_chunk *chunk)
279 sctp_xmit_t retval = SCTP_XMIT_OK;
280 __u16 chunk_len = WORD_ROUND(ntohs(chunk->chunk_hdr->length));
282 SCTP_DEBUG_PRINTK("%s: packet:%p chunk:%p\n", __func__, packet,
285 /* Data chunks are special. Before seeing what else we can
286 * bundle into this packet, check to see if we are allowed to
289 if (sctp_chunk_is_data(chunk)) {
290 retval = sctp_packet_can_append_data(packet, chunk);
291 if (retval != SCTP_XMIT_OK)
295 /* Try to bundle AUTH chunk */
296 retval = sctp_packet_bundle_auth(packet, chunk);
297 if (retval != SCTP_XMIT_OK)
300 /* Try to bundle SACK chunk */
301 retval = sctp_packet_bundle_sack(packet, chunk);
302 if (retval != SCTP_XMIT_OK)
305 /* Check to see if this chunk will fit into the packet */
306 retval = sctp_packet_will_fit(packet, chunk, chunk_len);
307 if (retval != SCTP_XMIT_OK)
310 /* We believe that this chunk is OK to add to the packet */
311 switch (chunk->chunk_hdr->type) {
313 /* Account for the data being in the packet */
314 sctp_packet_append_data(packet, chunk);
315 /* Disallow SACK bundling after DATA. */
316 packet->has_sack = 1;
317 /* Disallow AUTH bundling after DATA */
318 packet->has_auth = 1;
319 /* Let it be knows that packet has DATA in it */
320 packet->has_data = 1;
321 /* timestamp the chunk for rtx purposes */
322 chunk->sent_at = jiffies;
324 case SCTP_CID_COOKIE_ECHO:
325 packet->has_cookie_echo = 1;
329 packet->has_sack = 1;
333 packet->has_auth = 1;
334 packet->auth = chunk;
338 /* It is OK to send this chunk. */
339 list_add_tail(&chunk->list, &packet->chunk_list);
340 packet->size += chunk_len;
341 chunk->transport = packet->transport;
346 /* All packets are sent to the network through this function from
349 * The return value is a normal kernel error return value.
351 int sctp_packet_transmit(struct sctp_packet *packet)
353 struct sctp_transport *tp = packet->transport;
354 struct sctp_association *asoc = tp->asoc;
356 struct sk_buff *nskb;
357 struct sctp_chunk *chunk, *tmp;
360 int padding; /* How much padding do we need? */
362 struct dst_entry *dst = tp->dst;
363 unsigned char *auth = NULL; /* pointer to auth in skb data */
364 __u32 cksum_buf_len = sizeof(struct sctphdr);
366 SCTP_DEBUG_PRINTK("%s: packet:%p\n", __func__, packet);
368 /* Do NOT generate a chunkless packet. */
369 if (list_empty(&packet->chunk_list))
372 /* Set up convenience variables... */
373 chunk = list_entry(packet->chunk_list.next, struct sctp_chunk, list);
376 /* Allocate the new skb. */
377 nskb = alloc_skb(packet->size + LL_MAX_HEADER, GFP_ATOMIC);
381 /* Make sure the outbound skb has enough header room reserved. */
382 skb_reserve(nskb, packet->overhead + LL_MAX_HEADER);
384 /* Set the owning socket so that we know where to get the
385 * destination IP address.
387 skb_set_owner_w(nskb, sk);
389 /* The 'obsolete' field of dst is set to 2 when a dst is freed. */
390 if (!dst || (dst->obsolete > 1)) {
392 sctp_transport_route(tp, NULL, sctp_sk(sk));
393 if (asoc && (asoc->param_flags & SPP_PMTUD_ENABLE)) {
394 sctp_assoc_sync_pmtu(asoc);
397 dst = dst_clone(tp->dst);
398 skb_dst_set(nskb, dst);
402 /* Build the SCTP header. */
403 sh = (struct sctphdr *)skb_push(nskb, sizeof(struct sctphdr));
404 skb_reset_transport_header(nskb);
405 sh->source = htons(packet->source_port);
406 sh->dest = htons(packet->destination_port);
408 /* From 6.8 Adler-32 Checksum Calculation:
409 * After the packet is constructed (containing the SCTP common
410 * header and one or more control or DATA chunks), the
413 * 1) Fill in the proper Verification Tag in the SCTP common
414 * header and initialize the checksum field to 0's.
416 sh->vtag = htonl(packet->vtag);
422 * An endpoint bundles chunks by simply including multiple
423 * chunks in one outbound SCTP packet. ...
427 * 3.2 Chunk Field Descriptions
429 * The total length of a chunk (including Type, Length and
430 * Value fields) MUST be a multiple of 4 bytes. If the length
431 * of the chunk is not a multiple of 4 bytes, the sender MUST
432 * pad the chunk with all zero bytes and this padding is not
433 * included in the chunk length field. The sender should
434 * never pad with more than 3 bytes.
436 * [This whole comment explains WORD_ROUND() below.]
438 SCTP_DEBUG_PRINTK("***sctp_transmit_packet***\n");
439 list_for_each_entry_safe(chunk, tmp, &packet->chunk_list, list) {
440 list_del_init(&chunk->list);
441 if (sctp_chunk_is_data(chunk)) {
443 if (!chunk->has_tsn) {
444 sctp_chunk_assign_ssn(chunk);
445 sctp_chunk_assign_tsn(chunk);
447 /* 6.3.1 C4) When data is in flight and when allowed
448 * by rule C5, a new RTT measurement MUST be made each
449 * round trip. Furthermore, new RTT measurements
450 * SHOULD be made no more than once per round-trip
451 * for a given destination transport address.
454 if (!tp->rto_pending) {
455 chunk->rtt_in_progress = 1;
464 padding = WORD_ROUND(chunk->skb->len) - chunk->skb->len;
466 memset(skb_put(chunk->skb, padding), 0, padding);
468 /* if this is the auth chunk that we are adding,
469 * store pointer where it will be added and put
470 * the auth into the packet.
472 if (chunk == packet->auth)
473 auth = skb_tail_pointer(nskb);
475 cksum_buf_len += chunk->skb->len;
476 memcpy(skb_put(nskb, chunk->skb->len),
477 chunk->skb->data, chunk->skb->len);
479 SCTP_DEBUG_PRINTK("%s %p[%s] %s 0x%x, %s %d, %s %d, %s %d\n",
481 sctp_cname(SCTP_ST_CHUNK(
482 chunk->chunk_hdr->type)),
483 chunk->has_tsn ? "TSN" : "No TSN",
485 ntohl(chunk->subh.data_hdr->tsn) : 0,
486 "length", ntohs(chunk->chunk_hdr->length),
487 "chunk->skb->len", chunk->skb->len,
488 "rtt_in_progress", chunk->rtt_in_progress);
491 * If this is a control chunk, this is our last
492 * reference. Free data chunks after they've been
493 * acknowledged or have failed.
495 if (!sctp_chunk_is_data(chunk))
496 sctp_chunk_free(chunk);
499 /* SCTP-AUTH, Section 6.2
500 * The sender MUST calculate the MAC as described in RFC2104 [2]
501 * using the hash function H as described by the MAC Identifier and
502 * the shared association key K based on the endpoint pair shared key
503 * described by the shared key identifier. The 'data' used for the
504 * computation of the AUTH-chunk is given by the AUTH chunk with its
505 * HMAC field set to zero (as shown in Figure 6) followed by all
506 * chunks that are placed after the AUTH chunk in the SCTP packet.
509 sctp_auth_calculate_hmac(asoc, nskb,
510 (struct sctp_auth_chunk *)auth,
513 /* 2) Calculate the Adler-32 checksum of the whole packet,
514 * including the SCTP common header and all the
517 * Note: Adler-32 is no longer applicable, as has been replaced
518 * by CRC32-C as described in <draft-ietf-tsvwg-sctpcsum-02.txt>.
520 if (!sctp_checksum_disable &&
521 !(dst->dev->features & (NETIF_F_NO_CSUM | NETIF_F_SCTP_CSUM))) {
522 __u32 crc32 = sctp_start_cksum((__u8 *)sh, cksum_buf_len);
524 /* 3) Put the resultant value into the checksum field in the
525 * common header, and leave the rest of the bits unchanged.
527 sh->checksum = sctp_end_cksum(crc32);
529 if (dst->dev->features & NETIF_F_SCTP_CSUM) {
530 /* no need to seed psuedo checksum for SCTP */
531 nskb->ip_summed = CHECKSUM_PARTIAL;
532 nskb->csum_start = (skb_transport_header(nskb) -
534 nskb->csum_offset = offsetof(struct sctphdr, checksum);
536 nskb->ip_summed = CHECKSUM_UNNECESSARY;
540 /* IP layer ECN support
542 * "The ECN-Capable Transport (ECT) bit would be set by the
543 * data sender to indicate that the end-points of the
544 * transport protocol are ECN-capable."
546 * Now setting the ECT bit all the time, as it should not cause
547 * any problems protocol-wise even if our peer ignores it.
549 * Note: The works for IPv6 layer checks this bit too later
550 * in transmission. See IP6_ECN_flow_xmit().
552 (*tp->af_specific->ecn_capable)(nskb->sk);
554 /* Set up the IP options. */
555 /* BUG: not implemented
556 * For v4 this all lives somewhere in sk->sk_opt...
559 /* Dump that on IP! */
560 if (asoc && asoc->peer.last_sent_to != tp) {
561 /* Considering the multiple CPU scenario, this is a
562 * "correcter" place for last_sent_to. --xguo
564 asoc->peer.last_sent_to = tp;
568 struct timer_list *timer;
569 unsigned long timeout;
571 tp->last_time_used = jiffies;
573 /* Restart the AUTOCLOSE timer when sending data. */
574 if (sctp_state(asoc, ESTABLISHED) && asoc->autoclose) {
575 timer = &asoc->timers[SCTP_EVENT_TIMEOUT_AUTOCLOSE];
576 timeout = asoc->timeouts[SCTP_EVENT_TIMEOUT_AUTOCLOSE];
578 if (!mod_timer(timer, jiffies + timeout))
579 sctp_association_hold(asoc);
583 SCTP_DEBUG_PRINTK("***sctp_transmit_packet*** skb len %d\n",
586 nskb->local_df = packet->ipfragok;
587 (*tp->af_specific->sctp_xmit)(nskb, tp);
590 sctp_packet_reset(packet);
594 IP_INC_STATS_BH(&init_net, IPSTATS_MIB_OUTNOROUTES);
596 /* FIXME: Returning the 'err' will effect all the associations
597 * associated with a socket, although only one of the paths of the
598 * association is unreachable.
599 * The real failure of a transport or association can be passed on
600 * to the user via notifications. So setting this error may not be
603 /* err = -EHOSTUNREACH; */
605 /* Control chunks are unreliable so just drop them. DATA chunks
606 * will get resent or dropped later.
609 list_for_each_entry_safe(chunk, tmp, &packet->chunk_list, list) {
610 list_del_init(&chunk->list);
611 if (!sctp_chunk_is_data(chunk))
612 sctp_chunk_free(chunk);
620 /********************************************************************
621 * 2nd Level Abstractions
622 ********************************************************************/
624 /* This private function check to see if a chunk can be added */
625 static sctp_xmit_t sctp_packet_can_append_data(struct sctp_packet *packet,
626 struct sctp_chunk *chunk)
628 sctp_xmit_t retval = SCTP_XMIT_OK;
629 size_t datasize, rwnd, inflight, flight_size;
630 struct sctp_transport *transport = packet->transport;
631 __u32 max_burst_bytes;
632 struct sctp_association *asoc = transport->asoc;
633 struct sctp_outq *q = &asoc->outqueue;
635 /* RFC 2960 6.1 Transmission of DATA Chunks
637 * A) At any given time, the data sender MUST NOT transmit new data to
638 * any destination transport address if its peer's rwnd indicates
639 * that the peer has no buffer space (i.e. rwnd is 0, see Section
640 * 6.2.1). However, regardless of the value of rwnd (including if it
641 * is 0), the data sender can always have one DATA chunk in flight to
642 * the receiver if allowed by cwnd (see rule B below). This rule
643 * allows the sender to probe for a change in rwnd that the sender
644 * missed due to the SACK having been lost in transit from the data
645 * receiver to the data sender.
648 rwnd = asoc->peer.rwnd;
649 inflight = q->outstanding_bytes;
650 flight_size = transport->flight_size;
652 datasize = sctp_data_size(chunk);
654 if (datasize > rwnd) {
656 /* We have (at least) one data chunk in flight,
657 * so we can't fall back to rule 6.1 B).
659 retval = SCTP_XMIT_RWND_FULL;
664 /* sctpimpguide-05 2.14.2
665 * D) When the time comes for the sender to
666 * transmit new DATA chunks, the protocol parameter Max.Burst MUST
667 * first be applied to limit how many new DATA chunks may be sent.
668 * The limit is applied by adjusting cwnd as follows:
669 * if ((flightsize + Max.Burst * MTU) < cwnd)
670 * cwnd = flightsize + Max.Burst * MTU
672 max_burst_bytes = asoc->max_burst * asoc->pathmtu;
673 if ((flight_size + max_burst_bytes) < transport->cwnd) {
674 transport->cwnd = flight_size + max_burst_bytes;
675 SCTP_DEBUG_PRINTK("%s: cwnd limited by max_burst: "
676 "transport: %p, cwnd: %d, "
677 "ssthresh: %d, flight_size: %d, "
682 transport->flight_size,
683 transport->partial_bytes_acked);
686 /* RFC 2960 6.1 Transmission of DATA Chunks
688 * B) At any given time, the sender MUST NOT transmit new data
689 * to a given transport address if it has cwnd or more bytes
690 * of data outstanding to that transport address.
692 /* RFC 7.2.4 & the Implementers Guide 2.8.
695 * When a Fast Retransmit is being performed the sender SHOULD
696 * ignore the value of cwnd and SHOULD NOT delay retransmission.
698 if (chunk->fast_retransmit != SCTP_NEED_FRTX)
699 if (flight_size >= transport->cwnd) {
700 retval = SCTP_XMIT_RWND_FULL;
704 /* Nagle's algorithm to solve small-packet problem:
705 * Inhibit the sending of new chunks when new outgoing data arrives
706 * if any previously transmitted data on the connection remains
709 if (!sctp_sk(asoc->base.sk)->nodelay && sctp_packet_empty(packet) &&
710 inflight && sctp_state(asoc, ESTABLISHED)) {
711 unsigned max = transport->pathmtu - packet->overhead;
712 unsigned len = chunk->skb->len + q->out_qlen;
714 /* Check whether this chunk and all the rest of pending
715 * data will fit or delay in hopes of bundling a full
717 * Don't delay large message writes that may have been
718 * fragmeneted into small peices.
720 if ((len < max) && (chunk->msg->msg_size < max)) {
721 retval = SCTP_XMIT_NAGLE_DELAY;
730 /* This private function does management things when adding DATA chunk */
731 static void sctp_packet_append_data(struct sctp_packet *packet,
732 struct sctp_chunk *chunk)
734 struct sctp_transport *transport = packet->transport;
735 size_t datasize = sctp_data_size(chunk);
736 struct sctp_association *asoc = transport->asoc;
737 u32 rwnd = asoc->peer.rwnd;
739 /* Keep track of how many bytes are in flight over this transport. */
740 transport->flight_size += datasize;
742 /* Keep track of how many bytes are in flight to the receiver. */
743 asoc->outqueue.outstanding_bytes += datasize;
745 /* Update our view of the receiver's rwnd. Include sk_buff overhead
746 * while updating peer.rwnd so that it reduces the chances of a
747 * receiver running out of receive buffer space even when receive
748 * window is still open. This can happen when a sender is sending
749 * sending small messages.
751 datasize += sizeof(struct sk_buff);
757 asoc->peer.rwnd = rwnd;
758 /* Has been accepted for transmission. */
759 if (!asoc->peer.prsctp_capable)
760 chunk->msg->can_abandon = 0;
763 static sctp_xmit_t sctp_packet_will_fit(struct sctp_packet *packet,
764 struct sctp_chunk *chunk,
770 sctp_xmit_t retval = SCTP_XMIT_OK;
772 psize = packet->size;
773 pmtu = ((packet->transport->asoc) ?
774 (packet->transport->asoc->pathmtu) :
775 (packet->transport->pathmtu));
777 too_big = (psize + chunk_len > pmtu);
779 /* Decide if we need to fragment or resubmit later. */
781 /* It's OK to fragmet at IP level if any one of the following
783 * 1. The packet is empty (meaning this chunk is greater
785 * 2. The chunk we are adding is a control chunk
786 * 3. The packet doesn't have any data in it yet and data
787 * requires authentication.
789 if (sctp_packet_empty(packet) || !sctp_chunk_is_data(chunk) ||
790 (!packet->has_data && chunk->auth)) {
791 /* We no longer do re-fragmentation.
792 * Just fragment at the IP layer, if we
793 * actually hit this condition
795 packet->ipfragok = 1;
797 retval = SCTP_XMIT_PMTU_FULL;