// SPDX-License-Identifier: GPL-2.0-or-later
/* SCTP kernel implementation
 * (C) Copyright IBM Corp. 2001, 2004
 * Copyright (c) 1999-2000 Cisco, Inc.
 * Copyright (c) 1999-2001 Motorola, Inc.
 *
 * This file is part of the SCTP kernel implementation
 *
 * These functions handle output processing.
 *
 * Please send any bug reports or fixes you make to the
 * email address(es):
 *    lksctp developers <linux-sctp@vger.kernel.org>
 *
 * Written or modified by:
 *    La Monte H.P. Yarroll <piggy@acm.org>
 *    Karl Knutson          <karl@athena.chicago.il.us>
 *    Jon Grimm             <jgrimm@austin.ibm.com>
 *    Sridhar Samudrala     <sri@us.ibm.com>
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/wait.h>
#include <linux/time.h>
#include <linux/ip.h>
#include <linux/ipv6.h>
#include <linux/init.h>
#include <linux/slab.h>
#include <net/inet_ecn.h>
#include <net/ip.h>
#include <net/icmp.h>
#include <net/net_namespace.h>

#include <linux/socket.h> /* for sa_family_t */
#include <net/sock.h>

#include <net/sctp/sctp.h>
#include <net/sctp/sm.h>
#include <net/sctp/checksum.h>

/* Forward declarations for private helpers. */
static enum sctp_xmit __sctp_packet_append_chunk(struct sctp_packet *packet,
                                                 struct sctp_chunk *chunk);
static enum sctp_xmit sctp_packet_can_append_data(struct sctp_packet *packet,
                                                  struct sctp_chunk *chunk);
static void sctp_packet_append_data(struct sctp_packet *packet,
                                    struct sctp_chunk *chunk);
static enum sctp_xmit sctp_packet_will_fit(struct sctp_packet *packet,
                                           struct sctp_chunk *chunk,
                                           u16 chunk_len);

static void sctp_packet_reset(struct sctp_packet *packet)
{
        /* sctp_packet_transmit() relies on this to reset size to the
         * current overhead after sending packets.
         */
        packet->size = packet->overhead;

        packet->has_cookie_echo = 0;
        packet->has_sack = 0;
        packet->has_data = 0;
        packet->has_auth = 0;
        packet->ipfragok = 0;
        packet->auth = NULL;
}

/* Config a packet.
 * This appears to be a followup set of initializations.
 */
void sctp_packet_config(struct sctp_packet *packet, __u32 vtag,
                        int ecn_capable)
{
        struct sctp_transport *tp = packet->transport;
        struct sctp_association *asoc = tp->asoc;
        struct sctp_sock *sp = NULL;
        struct sock *sk;

        pr_debug("%s: packet:%p vtag:0x%x\n", __func__, packet, vtag);
        packet->vtag = vtag;

        /* do the following jobs only once for a flush schedule */
        if (!sctp_packet_empty(packet))
                return;

        /* set packet max_size with pathmtu, then calculate overhead */
        packet->max_size = tp->pathmtu;

        if (asoc) {
                sk = asoc->base.sk;
                sp = sctp_sk(sk);
        }
        packet->overhead = sctp_mtu_payload(sp, 0, 0);
        packet->size = packet->overhead;

        if (!asoc)
                return;

        /* update dst or transport pathmtu if in need */
        if (!sctp_transport_dst_check(tp)) {
                sctp_transport_route(tp, NULL, sp);
                if (asoc->param_flags & SPP_PMTUD_ENABLE)
                        sctp_assoc_sync_pmtu(asoc);
        } else if (!sctp_transport_pl_enabled(tp) &&
                   asoc->param_flags & SPP_PMTUD_ENABLE) {
                if (!sctp_transport_pmtu_check(tp))
                        sctp_assoc_sync_pmtu(asoc);
        }

        if (asoc->pmtu_pending) {
                if (asoc->param_flags & SPP_PMTUD_ENABLE)
                        sctp_assoc_sync_pmtu(asoc);
                asoc->pmtu_pending = 0;
        }

        /* If there a is a prepend chunk stick it on the list before
         * any other chunks get appended.
         */
        if (ecn_capable) {
                struct sctp_chunk *chunk = sctp_get_ecne_prepend(asoc);

                if (chunk)
                        sctp_packet_append_chunk(packet, chunk);
        }

        if (!tp->dst)
                return;

        /* set packet max_size with gso_max_size if gso is enabled*/
        rcu_read_lock();
        if (__sk_dst_get(sk) != tp->dst) {
                dst_hold(tp->dst);
                sk_setup_caps(sk, tp->dst);
        }
        packet->max_size = sk_can_gso(sk) ? min(READ_ONCE(tp->dst->dev->gso_max_size),
                                                GSO_LEGACY_MAX_SIZE)
                                          : asoc->pathmtu;
        rcu_read_unlock();
}

/* Initialize the packet structure. */
void sctp_packet_init(struct sctp_packet *packet,
                      struct sctp_transport *transport,
                      __u16 sport, __u16 dport)
{
        pr_debug("%s: packet:%p transport:%p\n", __func__, packet, transport);

        packet->transport = transport;
        packet->source_port = sport;
        packet->destination_port = dport;
        INIT_LIST_HEAD(&packet->chunk_list);
        /* The overhead will be calculated by sctp_packet_config() */
        packet->overhead = 0;
        sctp_packet_reset(packet);
        packet->vtag = 0;
}

/* Free a packet.  */
void sctp_packet_free(struct sctp_packet *packet)
{
        struct sctp_chunk *chunk, *tmp;

        pr_debug("%s: packet:%p\n", __func__, packet);

        list_for_each_entry_safe(chunk, tmp, &packet->chunk_list, list) {
                list_del_init(&chunk->list);
                sctp_chunk_free(chunk);
        }
}

/* This routine tries to append the chunk to the offered packet. If adding
 * the chunk causes the packet to exceed the path MTU and COOKIE_ECHO chunk
 * is not present in the packet, it transmits the input packet.
 * Data can be bundled with a packet containing a COOKIE_ECHO chunk as long
 * as it can fit in the packet, but any more data that does not fit in this
 * packet can be sent only after receiving the COOKIE_ACK.
 */
enum sctp_xmit sctp_packet_transmit_chunk(struct sctp_packet *packet,
                                          struct sctp_chunk *chunk,
                                          int one_packet, gfp_t gfp)
{
        enum sctp_xmit retval;

        pr_debug("%s: packet:%p size:%zu chunk:%p size:%d\n", __func__,
                 packet, packet->size, chunk, chunk->skb ? chunk->skb->len : -1);

        switch ((retval = (sctp_packet_append_chunk(packet, chunk)))) {
        case SCTP_XMIT_PMTU_FULL:
                if (!packet->has_cookie_echo) {
                        int error = 0;

                        error = sctp_packet_transmit(packet, gfp);
                        if (error < 0)
                                chunk->skb->sk->sk_err = -error;

                        /* If we have an empty packet, then we can NOT ever
                         * return PMTU_FULL.
                         */
                        if (!one_packet)
                                retval = sctp_packet_append_chunk(packet,
                                                                  chunk);
                }
                break;

        case SCTP_XMIT_RWND_FULL:
        case SCTP_XMIT_OK:
        case SCTP_XMIT_DELAY:
                break;
        }

        return retval;
}

/* Try to bundle a pad chunk into a packet with a heartbeat chunk for PLPMTUTD probe */
static enum sctp_xmit sctp_packet_bundle_pad(struct sctp_packet *pkt, struct sctp_chunk *chunk)
{
        struct sctp_transport *t = pkt->transport;
        struct sctp_chunk *pad;
        int overhead = 0;

        if (!chunk->pmtu_probe)
                return SCTP_XMIT_OK;

        /* calculate the Padding Data size for the pad chunk */
        overhead += sizeof(struct sctphdr) + sizeof(struct sctp_chunkhdr);
        overhead += sizeof(struct sctp_sender_hb_info) + sizeof(struct sctp_pad_chunk);
        pad = sctp_make_pad(t->asoc, t->pl.probe_size - overhead);
        if (!pad)
                return SCTP_XMIT_DELAY;

        list_add_tail(&pad->list, &pkt->chunk_list);
        pkt->size += SCTP_PAD4(ntohs(pad->chunk_hdr->length));
        chunk->transport = t;

        return SCTP_XMIT_OK;
}

/* Try to bundle an auth chunk into the packet. */
static enum sctp_xmit sctp_packet_bundle_auth(struct sctp_packet *pkt,
                                              struct sctp_chunk *chunk)
{
        struct sctp_association *asoc = pkt->transport->asoc;
        enum sctp_xmit retval = SCTP_XMIT_OK;
        struct sctp_chunk *auth;

        /* if we don't have an association, we can't do authentication */
        if (!asoc)
                return retval;

        /* See if this is an auth chunk we are bundling or if
         * auth is already bundled.
         */
        if (chunk->chunk_hdr->type == SCTP_CID_AUTH || pkt->has_auth)
                return retval;

        /* if the peer did not request this chunk to be authenticated,
         * don't do it
         */
        if (!chunk->auth)
                return retval;

        auth = sctp_make_auth(asoc, chunk->shkey->key_id);
        if (!auth)
                return retval;

        auth->shkey = chunk->shkey;
        sctp_auth_shkey_hold(auth->shkey);

        retval = __sctp_packet_append_chunk(pkt, auth);

        if (retval != SCTP_XMIT_OK)
                sctp_chunk_free(auth);

        return retval;
}

/* Try to bundle a SACK with the packet. */
static enum sctp_xmit sctp_packet_bundle_sack(struct sctp_packet *pkt,
                                              struct sctp_chunk *chunk)
{
        enum sctp_xmit retval = SCTP_XMIT_OK;

        /* If sending DATA and haven't aleady bundled a SACK, try to
         * bundle one in to the packet.
         */
        if (sctp_chunk_is_data(chunk) && !pkt->has_sack &&
            !pkt->has_cookie_echo) {
                struct sctp_association *asoc;
                struct timer_list *timer;
                asoc = pkt->transport->asoc;
                timer = &asoc->timers[SCTP_EVENT_TIMEOUT_SACK];

                /* If the SACK timer is running, we have a pending SACK */
                if (timer_pending(timer)) {
                        struct sctp_chunk *sack;

                        if (pkt->transport->sack_generation !=
                            pkt->transport->asoc->peer.sack_generation)
                                return retval;

                        asoc->a_rwnd = asoc->rwnd;
                        sack = sctp_make_sack(asoc);
                        if (sack) {
                                retval = __sctp_packet_append_chunk(pkt, sack);
                                if (retval != SCTP_XMIT_OK) {
                                        sctp_chunk_free(sack);
                                        goto out;
                                }
                                SCTP_INC_STATS(asoc->base.net,
                                               SCTP_MIB_OUTCTRLCHUNKS);
                                asoc->stats.octrlchunks++;
                                asoc->peer.sack_needed = 0;
                                if (timer_delete(timer))
                                        sctp_association_put(asoc);
                        }
                }
        }
out:
        return retval;
}


/* Append a chunk to the offered packet reporting back any inability to do
 * so.
 */
static enum sctp_xmit __sctp_packet_append_chunk(struct sctp_packet *packet,
                                                 struct sctp_chunk *chunk)
{
        __u16 chunk_len = SCTP_PAD4(ntohs(chunk->chunk_hdr->length));
        enum sctp_xmit retval = SCTP_XMIT_OK;

        /* Check to see if this chunk will fit into the packet */
        retval = sctp_packet_will_fit(packet, chunk, chunk_len);
        if (retval != SCTP_XMIT_OK)
                goto finish;

        /* We believe that this chunk is OK to add to the packet */
        switch (chunk->chunk_hdr->type) {
        case SCTP_CID_DATA:
        case SCTP_CID_I_DATA:
                /* Account for the data being in the packet */
                sctp_packet_append_data(packet, chunk);
                /* Disallow SACK bundling after DATA. */
                packet->has_sack = 1;
                /* Disallow AUTH bundling after DATA */
                packet->has_auth = 1;
                /* Let it be knows that packet has DATA in it */
                packet->has_data = 1;
                /* timestamp the chunk for rtx purposes */
                chunk->sent_at = jiffies;
                /* Mainly used for prsctp RTX policy */
                chunk->sent_count++;
                break;
        case SCTP_CID_COOKIE_ECHO:
                packet->has_cookie_echo = 1;
                break;

        case SCTP_CID_SACK:
                packet->has_sack = 1;
                if (chunk->asoc)
                        chunk->asoc->stats.osacks++;
                break;

        case SCTP_CID_AUTH:
                packet->has_auth = 1;
                packet->auth = chunk;
                break;
        }

        /* It is OK to send this chunk.  */
        list_add_tail(&chunk->list, &packet->chunk_list);
        packet->size += chunk_len;
        chunk->transport = packet->transport;
finish:
        return retval;
}

/* Append a chunk to the offered packet reporting back any inability to do
 * so.
 */
enum sctp_xmit sctp_packet_append_chunk(struct sctp_packet *packet,
                                        struct sctp_chunk *chunk)
{
        enum sctp_xmit retval = SCTP_XMIT_OK;

        pr_debug("%s: packet:%p chunk:%p\n", __func__, packet, chunk);

        /* Data chunks are special.  Before seeing what else we can
         * bundle into this packet, check to see if we are allowed to
         * send this DATA.
         */
        if (sctp_chunk_is_data(chunk)) {
                retval = sctp_packet_can_append_data(packet, chunk);
                if (retval != SCTP_XMIT_OK)
                        goto finish;
        }

        /* Try to bundle AUTH chunk */
        retval = sctp_packet_bundle_auth(packet, chunk);
        if (retval != SCTP_XMIT_OK)
                goto finish;

        /* Try to bundle SACK chunk */
        retval = sctp_packet_bundle_sack(packet, chunk);
        if (retval != SCTP_XMIT_OK)
                goto finish;

        retval = __sctp_packet_append_chunk(packet, chunk);
        if (retval != SCTP_XMIT_OK)
                goto finish;

        retval = sctp_packet_bundle_pad(packet, chunk);

finish:
        return retval;
}

static void sctp_packet_gso_append(struct sk_buff *head, struct sk_buff *skb)
{
        if (SCTP_OUTPUT_CB(head)->last == head)
                skb_shinfo(head)->frag_list = skb;
        else
                SCTP_OUTPUT_CB(head)->last->next = skb;
        SCTP_OUTPUT_CB(head)->last = skb;

        head->truesize += skb->truesize;
        head->data_len += skb->len;
        head->len += skb->len;
        refcount_add(skb->truesize, &head->sk->sk_wmem_alloc);

        __skb_header_release(skb);
}

static int sctp_packet_pack(struct sctp_packet *packet,
                            struct sk_buff *head, int gso, gfp_t gfp)
{
        struct sctp_transport *tp = packet->transport;
        struct sctp_auth_chunk *auth = NULL;
        struct sctp_chunk *chunk, *tmp;
        int pkt_count = 0, pkt_size;
        struct sock *sk = head->sk;
        struct sk_buff *nskb;
        int auth_len = 0;

        if (gso) {
                skb_shinfo(head)->gso_type = sk->sk_gso_type;
                SCTP_OUTPUT_CB(head)->last = head;
        } else {
                nskb = head;
                pkt_size = packet->size;
                goto merge;
        }

        do {
                /* calculate the pkt_size and alloc nskb */
                pkt_size = packet->overhead;
                list_for_each_entry_safe(chunk, tmp, &packet->chunk_list,
                                         list) {
                        int padded = SCTP_PAD4(chunk->skb->len);

                        if (chunk == packet->auth)
                                auth_len = padded;
                        else if (auth_len + padded + packet->overhead >
                                 tp->pathmtu)
                                return 0;
                        else if (pkt_size + padded > tp->pathmtu)
                                break;
                        pkt_size += padded;
                }
                nskb = alloc_skb(pkt_size + MAX_HEADER, gfp);
                if (!nskb)
                        return 0;
                skb_reserve(nskb, packet->overhead + MAX_HEADER);

merge:
                /* merge chunks into nskb and append nskb into head list */
                pkt_size -= packet->overhead;
                list_for_each_entry_safe(chunk, tmp, &packet->chunk_list, list) {
                        int padding;

                        list_del_init(&chunk->list);
                        if (sctp_chunk_is_data(chunk)) {
                                if (!sctp_chunk_retransmitted(chunk) &&
                                    !tp->rto_pending) {
                                        chunk->rtt_in_progress = 1;
                                        tp->rto_pending = 1;
                                }
                        }

                        padding = SCTP_PAD4(chunk->skb->len) - chunk->skb->len;
                        if (padding)
                                skb_put_zero(chunk->skb, padding);

                        if (chunk == packet->auth)
                                auth = (struct sctp_auth_chunk *)
                                                        skb_tail_pointer(nskb);

                        skb_put_data(nskb, chunk->skb->data, chunk->skb->len);

                        pr_debug("*** Chunk:%p[%s] %s 0x%x, length:%d, chunk->skb->len:%d, rtt_in_progress:%d\n",
                                 chunk,
                                 sctp_cname(SCTP_ST_CHUNK(chunk->chunk_hdr->type)),
                                 chunk->has_tsn ? "TSN" : "No TSN",
                                 chunk->has_tsn ? ntohl(chunk->subh.data_hdr->tsn) : 0,
                                 ntohs(chunk->chunk_hdr->length), chunk->skb->len,
                                 chunk->rtt_in_progress);

                        pkt_size -= SCTP_PAD4(chunk->skb->len);

                        if (!sctp_chunk_is_data(chunk) && chunk != packet->auth)
                                sctp_chunk_free(chunk);

                        if (!pkt_size)
                                break;
                }

                if (auth) {
                        sctp_auth_calculate_hmac(tp->asoc, nskb, auth,
                                                 packet->auth->shkey, gfp);
                        /* free auth if no more chunks, or add it back */
                        if (list_empty(&packet->chunk_list))
                                sctp_chunk_free(packet->auth);
                        else
                                list_add(&packet->auth->list,
                                         &packet->chunk_list);
                }

                if (gso)
                        sctp_packet_gso_append(head, nskb);

                pkt_count++;
        } while (!list_empty(&packet->chunk_list));

        if (gso) {
                memset(head->cb, 0, max(sizeof(struct inet_skb_parm),
                                        sizeof(struct inet6_skb_parm)));
                skb_shinfo(head)->gso_segs = pkt_count;
                skb_shinfo(head)->gso_size = GSO_BY_FRAGS;
                goto chksum;
        }

        if (sctp_checksum_disable)
                return 1;

        if (!(tp->dst->dev->features & NETIF_F_SCTP_CRC) ||
            dst_xfrm(tp->dst) || packet->ipfragok || tp->encap_port) {
                struct sctphdr *sh =
                        (struct sctphdr *)skb_transport_header(head);

                sh->checksum = sctp_compute_cksum(head, 0);
        } else {
chksum:
                head->ip_summed = CHECKSUM_PARTIAL;
                head->csum_not_inet = 1;
                head->csum_start = skb_transport_header(head) - head->head;
                head->csum_offset = offsetof(struct sctphdr, checksum);
        }

        return pkt_count;
}

/* All packets are sent to the network through this function from
 * sctp_outq_tail().
 *
 * The return value is always 0 for now.
 */
int sctp_packet_transmit(struct sctp_packet *packet, gfp_t gfp)
{
        struct sctp_transport *tp = packet->transport;
        struct sctp_association *asoc = tp->asoc;
        struct sctp_chunk *chunk, *tmp;
        int pkt_count, gso = 0;
        struct sk_buff *head;
        struct sctphdr *sh;
        struct sock *sk;

        pr_debug("%s: packet:%p\n", __func__, packet);
        if (list_empty(&packet->chunk_list))
                return 0;
        chunk = list_entry(packet->chunk_list.next, struct sctp_chunk, list);
        sk = chunk->skb->sk;

        if (packet->size > tp->pathmtu && !packet->ipfragok && !chunk->pmtu_probe) {
                if (tp->pl.state == SCTP_PL_ERROR) { /* do IP fragmentation if in Error state */
                        packet->ipfragok = 1;
                } else {
                        if (!sk_can_gso(sk)) { /* check gso */
                                pr_err_once("Trying to GSO but underlying device doesn't support it.");
                                goto out;
                        }
                        gso = 1;
                }
        }

        /* alloc head skb */
        head = alloc_skb((gso ? packet->overhead : packet->size) +
                         MAX_HEADER, gfp);
        if (!head)
                goto out;
        skb_reserve(head, packet->overhead + MAX_HEADER);
        skb_set_owner_w(head, sk);

        /* set sctp header */
        sh = skb_push(head, sizeof(struct sctphdr));
        skb_reset_transport_header(head);
        sh->source = htons(packet->source_port);
        sh->dest = htons(packet->destination_port);
        sh->vtag = htonl(packet->vtag);
        sh->checksum = 0;

        /* drop packet if no dst */
        if (!tp->dst) {
                IP_INC_STATS(sock_net(sk), IPSTATS_MIB_OUTNOROUTES);
                kfree_skb(head);
                goto out;
        }

        /* pack up chunks */
        pkt_count = sctp_packet_pack(packet, head, gso, gfp);
        if (!pkt_count) {
                kfree_skb(head);
                goto out;
        }
        pr_debug("***sctp_transmit_packet*** skb->len:%d\n", head->len);

        /* start autoclose timer */
        if (packet->has_data && sctp_state(asoc, ESTABLISHED) &&
            asoc->timeouts[SCTP_EVENT_TIMEOUT_AUTOCLOSE]) {
                struct timer_list *timer =
                        &asoc->timers[SCTP_EVENT_TIMEOUT_AUTOCLOSE];
                unsigned long timeout =
                        asoc->timeouts[SCTP_EVENT_TIMEOUT_AUTOCLOSE];

                if (!mod_timer(timer, jiffies + timeout))
                        sctp_association_hold(asoc);
        }

        /* sctp xmit */
        tp->af_specific->ecn_capable(sk);
        if (asoc) {
                asoc->stats.opackets += pkt_count;
                if (asoc->peer.last_sent_to != tp)
                        asoc->peer.last_sent_to = tp;
        }
        head->ignore_df = packet->ipfragok;
        if (tp->dst_pending_confirm)
                skb_set_dst_pending_confirm(head, 1);
        /* neighbour should be confirmed on successful transmission or
         * positive error
         */
        if (tp->af_specific->sctp_xmit(head, tp) >= 0 &&
            tp->dst_pending_confirm)
                tp->dst_pending_confirm = 0;

out:
        list_for_each_entry_safe(chunk, tmp, &packet->chunk_list, list) {
                list_del_init(&chunk->list);
                if (!sctp_chunk_is_data(chunk))
                        sctp_chunk_free(chunk);
        }
        sctp_packet_reset(packet);
        return 0;
}

/********************************************************************
 * 2nd Level Abstractions
 ********************************************************************/

/* This private function check to see if a chunk can be added */
static enum sctp_xmit sctp_packet_can_append_data(struct sctp_packet *packet,
                                                  struct sctp_chunk *chunk)
{
        size_t datasize, rwnd, inflight, flight_size;
        struct sctp_transport *transport = packet->transport;
        struct sctp_association *asoc = transport->asoc;
        struct sctp_outq *q = &asoc->outqueue;

        /* RFC 2960 6.1  Transmission of DATA Chunks
         *
         * A) At any given time, the data sender MUST NOT transmit new data to
         * any destination transport address if its peer's rwnd indicates
         * that the peer has no buffer space (i.e. rwnd is 0, see Section
         * 6.2.1).  However, regardless of the value of rwnd (including if it
         * is 0), the data sender can always have one DATA chunk in flight to
         * the receiver if allowed by cwnd (see rule B below).  This rule
         * allows the sender to probe for a change in rwnd that the sender
         * missed due to the SACK having been lost in transit from the data
         * receiver to the data sender.
         */

        rwnd = asoc->peer.rwnd;
        inflight = q->outstanding_bytes;
        flight_size = transport->flight_size;

        datasize = sctp_data_size(chunk);

        if (datasize > rwnd && inflight > 0)
                /* We have (at least) one data chunk in flight,
                 * so we can't fall back to rule 6.1 B).
                 */
                return SCTP_XMIT_RWND_FULL;

        /* RFC 2960 6.1  Transmission of DATA Chunks
         *
         * B) At any given time, the sender MUST NOT transmit new data
         * to a given transport address if it has cwnd or more bytes
         * of data outstanding to that transport address.
         */
        /* RFC 7.2.4 & the Implementers Guide 2.8.
         *
         * 3) ...
         *    When a Fast Retransmit is being performed the sender SHOULD
         *    ignore the value of cwnd and SHOULD NOT delay retransmission.
         */
        if (chunk->fast_retransmit != SCTP_NEED_FRTX &&
            flight_size >= transport->cwnd)
                return SCTP_XMIT_RWND_FULL;

        /* Nagle's algorithm to solve small-packet problem:
         * Inhibit the sending of new chunks when new outgoing data arrives
         * if any previously transmitted data on the connection remains
         * unacknowledged.
         */

        if ((sctp_sk(asoc->base.sk)->nodelay || inflight == 0) &&
            !asoc->force_delay)
                /* Nothing unacked */
                return SCTP_XMIT_OK;

        if (!sctp_packet_empty(packet))
                /* Append to packet */
                return SCTP_XMIT_OK;

        if (!sctp_state(asoc, ESTABLISHED))
                return SCTP_XMIT_OK;

        /* Check whether this chunk and all the rest of pending data will fit
         * or delay in hopes of bundling a full sized packet.
         */
        if (chunk->skb->len + q->out_qlen > transport->pathmtu -
            packet->overhead - sctp_datachk_len(&chunk->asoc->stream) - 4)
                /* Enough data queued to fill a packet */
                return SCTP_XMIT_OK;

        /* Don't delay large message writes that may have been fragmented */
        if (!chunk->msg->can_delay)
                return SCTP_XMIT_OK;

        /* Defer until all data acked or packet full */
        return SCTP_XMIT_DELAY;
}

/* This private function does management things when adding DATA chunk */
static void sctp_packet_append_data(struct sctp_packet *packet,
                                struct sctp_chunk *chunk)
{
        struct sctp_transport *transport = packet->transport;
        size_t datasize = sctp_data_size(chunk);
        struct sctp_association *asoc = transport->asoc;
        u32 rwnd = asoc->peer.rwnd;

        /* Keep track of how many bytes are in flight over this transport. */
        transport->flight_size += datasize;

        /* Keep track of how many bytes are in flight to the receiver. */
        asoc->outqueue.outstanding_bytes += datasize;

        /* Update our view of the receiver's rwnd. */
        if (datasize < rwnd)
                rwnd -= datasize;
        else
                rwnd = 0;

        asoc->peer.rwnd = rwnd;
        sctp_chunk_assign_tsn(chunk);
        asoc->stream.si->assign_number(chunk);
}

static enum sctp_xmit sctp_packet_will_fit(struct sctp_packet *packet,
                                           struct sctp_chunk *chunk,
                                           u16 chunk_len)
{
        enum sctp_xmit retval = SCTP_XMIT_OK;
        size_t psize, pmtu, maxsize;

        /* Don't bundle in this packet if this chunk's auth key doesn't
         * match other chunks already enqueued on this packet. Also,
         * don't bundle the chunk with auth key if other chunks in this
         * packet don't have auth key.
         */
        if ((packet->auth && chunk->shkey != packet->auth->shkey) ||
            (!packet->auth && chunk->shkey &&
             chunk->chunk_hdr->type != SCTP_CID_AUTH))
                return SCTP_XMIT_PMTU_FULL;

        psize = packet->size;
        if (packet->transport->asoc)
                pmtu = packet->transport->asoc->pathmtu;
        else
                pmtu = packet->transport->pathmtu;

        /* Decide if we need to fragment or resubmit later. */
        if (psize + chunk_len > pmtu) {
                /* It's OK to fragment at IP level if any one of the following
                 * is true:
                 *      1. The packet is empty (meaning this chunk is greater
                 *         the MTU)
                 *      2. The packet doesn't have any data in it yet and data
                 *         requires authentication.
                 */
                if (sctp_packet_empty(packet) ||
                    (!packet->has_data && chunk->auth)) {
                        /* We no longer do re-fragmentation.
                         * Just fragment at the IP layer, if we
                         * actually hit this condition
                         */
                        packet->ipfragok = 1;
                        goto out;
                }

                /* Similarly, if this chunk was built before a PMTU
                 * reduction, we have to fragment it at IP level now. So
                 * if the packet already contains something, we need to
                 * flush.
                 */
                maxsize = pmtu - packet->overhead;
                if (packet->auth)
                        maxsize -= SCTP_PAD4(packet->auth->skb->len);
                if (chunk_len > maxsize)
                        retval = SCTP_XMIT_PMTU_FULL;

                /* It is also okay to fragment if the chunk we are
                 * adding is a control chunk, but only if current packet
                 * is not a GSO one otherwise it causes fragmentation of
                 * a large frame. So in this case we allow the
                 * fragmentation by forcing it to be in a new packet.
                 */
                if (!sctp_chunk_is_data(chunk) && packet->has_data)
                        retval = SCTP_XMIT_PMTU_FULL;

                if (psize + chunk_len > packet->max_size)
                        /* Hit GSO/PMTU limit, gotta flush */
                        retval = SCTP_XMIT_PMTU_FULL;

                if (!packet->transport->burst_limited &&
                    psize + chunk_len > (packet->transport->cwnd >> 1))
                        /* Do not allow a single GSO packet to use more
                         * than half of cwnd.
                         */
                        retval = SCTP_XMIT_PMTU_FULL;

                if (packet->transport->burst_limited &&
                    psize + chunk_len > (packet->transport->burst_limited >> 1))
                        /* Do not allow a single GSO packet to use more
                         * than half of original cwnd.
                         */
                        retval = SCTP_XMIT_PMTU_FULL;
                /* Otherwise it will fit in the GSO packet */
        }

out:
        return retval;
}