text: /*
   *  net/dccp/output.c
   *
   *  An implementation of the DCCP protocol
   *  Arnaldo Carvalho de Melo <acme@conectiva.com.br>
   *
   *	This program is free software; you can redistribute it and/or
   *	modify it under the terms of the GNU General Public License
   *	as published by the Free Software Foundation; either version
   *	2 of the License, or (at your option) any later version.
   */
  
  #include <linux/dccp.h>
  #include <linux/kernel.h>
  #include <linux/skbuff.h>
  #include <linux/slab.h>
  
  #include <net/inet_sock.h>
  #include <net/sock.h>
  
  #include "ackvec.h"
  #include "ccid.h"
  #include "dccp.h"
  
  static inline void dccp_event_ack_sent(struct sock *sk)
  {
  	inet_csk_clear_xmit_timer(sk, ICSK_TIME_DACK);
  }
  
  /* enqueue @skb on sk_send_head for retransmission, return clone to send now */
  static struct sk_buff *dccp_skb_entail(struct sock *sk, struct sk_buff *skb)
  {
  	skb_set_owner_w(skb, sk);
  	WARN_ON(sk->sk_send_head);
  	sk->sk_send_head = skb;
  	return skb_clone(sk->sk_send_head, gfp_any());
  }
  
  /*
   * All SKB's seen here are completely headerless. It is our
   * job to build the DCCP header, and pass the packet down to
   * IP so it can do the same plus pass the packet off to the
   * device.
   */
  static int dccp_transmit_skb(struct sock *sk, struct sk_buff *skb)
  {
  	if (likely(skb != NULL)) {
  		struct inet_sock *inet = inet_sk(sk);
  		const struct inet_connection_sock *icsk = inet_csk(sk);
  		struct dccp_sock *dp = dccp_sk(sk);
  		struct dccp_skb_cb *dcb = DCCP_SKB_CB(skb);
  		struct dccp_hdr *dh;
  		/* XXX For now we're using only 48 bits sequence numbers */
  		const u32 dccp_header_size = sizeof(*dh) +
  					     sizeof(struct dccp_hdr_ext) +
  					  dccp_packet_hdr_len(dcb->dccpd_type);
  		int err, set_ack = 1;
  		u64 ackno = dp->dccps_gsr;
  		/*
  		 * Increment GSS here already in case the option code needs it.
  		 * Update GSS for real only if option processing below succeeds.
  		 */
  		dcb->dccpd_seq = ADD48(dp->dccps_gss, 1);
  
  		switch (dcb->dccpd_type) {
  		case DCCP_PKT_DATA:
  			set_ack = 0;
  			/* fall through */
  		case DCCP_PKT_DATAACK:
  		case DCCP_PKT_RESET:
  			break;
  
  		case DCCP_PKT_REQUEST:
  			set_ack = 0;
  			/* Use ISS on the first (non-retransmitted) Request. */
  			if (icsk->icsk_retransmits == 0)
  				dcb->dccpd_seq = dp->dccps_iss;
  			/* fall through */
  
  		case DCCP_PKT_SYNC:
  		case DCCP_PKT_SYNCACK:
  			ackno = dcb->dccpd_ack_seq;
  			/* fall through */
  		default:
  			/*
  			 * Set owner/destructor: some skbs are allocated via
  			 * alloc_skb (e.g. when retransmission may happen).
  			 * Only Data, DataAck, and Reset packets should come
  			 * through here with skb->sk set.
  			 */
  			WARN_ON(skb->sk);
  			skb_set_owner_w(skb, sk);
  			break;
  		}
  
  		if (dccp_insert_options(sk, skb)) {
  			kfree_skb(skb);
  			return -EPROTO;
  		}
  
  
  		/* Build DCCP header and checksum it. */
  		dh = dccp_zeroed_hdr(skb, dccp_header_size);
  		dh->dccph_type	= dcb->dccpd_type;
  		dh->dccph_sport	= inet->inet_sport;
  		dh->dccph_dport	= inet->inet_dport;
  		dh->dccph_doff	= (dccp_header_size + dcb->dccpd_opt_len) / 4;
  		dh->dccph_ccval	= dcb->dccpd_ccval;
  		dh->dccph_cscov = dp->dccps_pcslen;
  		/* XXX For now we're using only 48 bits sequence numbers */
  		dh->dccph_x	= 1;
  
  		dccp_update_gss(sk, dcb->dccpd_seq);
  		dccp_hdr_set_seq(dh, dp->dccps_gss);
  		if (set_ack)
  			dccp_hdr_set_ack(dccp_hdr_ack_bits(skb), ackno);
  
  		switch (dcb->dccpd_type) {
  		case DCCP_PKT_REQUEST:
  			dccp_hdr_request(skb)->dccph_req_service =
  							dp->dccps_service;
  			/*
  			 * Limit Ack window to ISS <= P.ackno <= GSS, so that
  			 * only Responses to Requests we sent are considered.
  			 */
  			dp->dccps_awl = dp->dccps_iss;
  			break;
  		case DCCP_PKT_RESET:
  			dccp_hdr_reset(skb)->dccph_reset_code =
  							dcb->dccpd_reset_code;
  			break;
  		}
  
  		icsk->icsk_af_ops->send_check(sk, skb);
  
  		if (set_ack)
  			dccp_event_ack_sent(sk);
  
  		DCCP_INC_STATS(DCCP_MIB_OUTSEGS);
  
  		err = icsk->icsk_af_ops->queue_xmit(skb, &inet->cork.fl);
  		return net_xmit_eval(err);
  	}
  	return -ENOBUFS;
  }
  
  /**
   * dccp_determine_ccmps  -  Find out about CCID-specific packet-size limits
   * We only consider the HC-sender CCID for setting the CCMPS (RFC 4340, 14.),
   * since the RX CCID is restricted to feedback packets (Acks), which are small
   * in comparison with the data traffic. A value of 0 means "no current CCMPS".
   */
  static u32 dccp_determine_ccmps(const struct dccp_sock *dp)
  {
  	const struct ccid *tx_ccid = dp->dccps_hc_tx_ccid;
  
  	if (tx_ccid == NULL || tx_ccid->ccid_ops == NULL)
  		return 0;
  	return tx_ccid->ccid_ops->ccid_ccmps;
  }
  
  unsigned int dccp_sync_mss(struct sock *sk, u32 pmtu)
  {
  	struct inet_connection_sock *icsk = inet_csk(sk);
  	struct dccp_sock *dp = dccp_sk(sk);
  	u32 ccmps = dccp_determine_ccmps(dp);
  	u32 cur_mps = ccmps ? min(pmtu, ccmps) : pmtu;
  
  	/* Account for header lengths and IPv4/v6 option overhead */
  	cur_mps -= (icsk->icsk_af_ops->net_header_len + icsk->icsk_ext_hdr_len +
  		    sizeof(struct dccp_hdr) + sizeof(struct dccp_hdr_ext));
  
  	/*
  	 * Leave enough headroom for common DCCP header options.
  	 * This only considers options which may appear on DCCP-Data packets, as
  	 * per table 3 in RFC 4340, 5.8. When running out of space for other
  	 * options (eg. Ack Vector which can take up to 255 bytes), it is better
  	 * to schedule a separate Ack. Thus we leave headroom for the following:
  	 *  - 1 byte for Slow Receiver (11.6)
  	 *  - 6 bytes for Timestamp (13.1)
  	 *  - 10 bytes for Timestamp Echo (13.3)
  	 *  - 8 bytes for NDP count (7.7, when activated)
  	 *  - 6 bytes for Data Checksum (9.3)
  	 *  - %DCCPAV_MIN_OPTLEN bytes for Ack Vector size (11.4, when enabled)
  	 */
  	cur_mps -= roundup(1 + 6 + 10 + dp->dccps_send_ndp_count * 8 + 6 +
  			   (dp->dccps_hc_rx_ackvec ? DCCPAV_MIN_OPTLEN : 0), 4);
  
  	/* And store cached results */
  	icsk->icsk_pmtu_cookie = pmtu;
  	dp->dccps_mss_cache = cur_mps;
  
  	return cur_mps;
  }
  
  EXPORT_SYMBOL_GPL(dccp_sync_mss);
  
  void dccp_write_space(struct sock *sk)
  {
  	struct socket_wq *wq;
  
  	rcu_read_lock();
  	wq = rcu_dereference(sk->sk_wq);
  	if (wq_has_sleeper(wq))
  		wake_up_interruptible(&wq->wait);
  	/* Should agree with poll, otherwise some programs break */
  	if (sock_writeable(sk))
  		sk_wake_async(sk, SOCK_WAKE_SPACE, POLL_OUT);
  
  	rcu_read_unlock();
  }
  
  /**
   * dccp_wait_for_ccid  -  Await CCID send permission
   * @sk:    socket to wait for
   * @delay: timeout in jiffies
   *
   * This is used by CCIDs which need to delay the send time in process context.
   */
  static int dccp_wait_for_ccid(struct sock *sk, unsigned long delay)
  {
  	DEFINE_WAIT(wait);
  	long remaining;
  
  	prepare_to_wait(sk_sleep(sk), &wait, TASK_INTERRUPTIBLE);
  	sk->sk_write_pending++;
  	release_sock(sk);
  
  	remaining = schedule_timeout(delay);
  
  	lock_sock(sk);
  	sk->sk_write_pending--;
  	finish_wait(sk_sleep(sk), &wait);
  
  	if (signal_pending(current) || sk->sk_err)
  		return -1;
  	return remaining;
  }
  
  /**
   * dccp_xmit_packet  -  Send data packet under control of CCID
   * Transmits next-queued payload and informs CCID to account for the packet.
   */
  static void dccp_xmit_packet(struct sock *sk)
  {
  	int err, len;
  	struct dccp_sock *dp = dccp_sk(sk);
  	struct sk_buff *skb = dccp_qpolicy_pop(sk);
  
  	if (unlikely(skb == NULL))
  		return;
  	len = skb->len;
  
  	if (sk->sk_state == DCCP_PARTOPEN) {
  		const u32 cur_mps = dp->dccps_mss_cache - DCCP_FEATNEG_OVERHEAD;
  		/*
  		 * See 8.1.5 - Handshake Completion.
  		 *
  		 * For robustness we resend Confirm options until the client has
  		 * entered OPEN. During the initial feature negotiation, the MPS
  		 * is smaller than usual, reduced by the Change/Confirm options.
  		 */
  		if (!list_empty(&dp->dccps_featneg) && len > cur_mps) {
  			DCCP_WARN("Payload too large (%d) for featneg.\n", len);
  			dccp_send_ack(sk);
  			dccp_feat_list_purge(&dp->dccps_featneg);
  		}
  
  		inet_csk_schedule_ack(sk);
  		inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
  					      inet_csk(sk)->icsk_rto,
  					      DCCP_RTO_MAX);
  		DCCP_SKB_CB(skb)->dccpd_type = DCCP_PKT_DATAACK;
  	} else if (dccp_ack_pending(sk)) {
  		DCCP_SKB_CB(skb)->dccpd_type = DCCP_PKT_DATAACK;
  	} else {
  		DCCP_SKB_CB(skb)->dccpd_type = DCCP_PKT_DATA;
  	}
  
  	err = dccp_transmit_skb(sk, skb);
  	if (err)
  		dccp_pr_debug("transmit_skb() returned err=%d\n", err);
  	/*
  	 * Register this one as sent even if an error occurred. To the remote
  	 * end a local packet drop is indistinguishable from network loss, i.e.
  	 * any local drop will eventually be reported via receiver feedback.
  	 */
  	ccid_hc_tx_packet_sent(dp->dccps_hc_tx_ccid, sk, len);
  
  	/*
  	 * If the CCID needs to transfer additional header options out-of-band
  	 * (e.g. Ack Vectors or feature-negotiation options), it activates this
  	 * flag to schedule a Sync. The Sync will automatically incorporate all
  	 * currently pending header options, thus clearing the backlog.
  	 */
  	if (dp->dccps_sync_scheduled)
  		dccp_send_sync(sk, dp->dccps_gsr, DCCP_PKT_SYNC);
  }
  
  /**
   * dccp_flush_write_queue  -  Drain queue at end of connection
   * Since dccp_sendmsg queues packets without waiting for them to be sent, it may
   * happen that the TX queue is not empty at the end of a connection. We give the
   * HC-sender CCID a grace period of up to @time_budget jiffies. If this function
   * returns with a non-empty write queue, it will be purged later.
   */
  void dccp_flush_write_queue(struct sock *sk, long *time_budget)
  {
  	struct dccp_sock *dp = dccp_sk(sk);
  	struct sk_buff *skb;
  	long delay, rc;
  
  	while (*time_budget > 0 && (skb = skb_peek(&sk->sk_write_queue))) {
  		rc = ccid_hc_tx_send_packet(dp->dccps_hc_tx_ccid, sk, skb);
  
  		switch (ccid_packet_dequeue_eval(rc)) {
  		case CCID_PACKET_WILL_DEQUEUE_LATER:
  			/*
  			 * If the CCID determines when to send, the next sending
  			 * time is unknown or the CCID may not even send again
  			 * (e.g. remote host crashes or lost Ack packets).
  			 */
  			DCCP_WARN("CCID did not manage to send all packets\n");
  			return;
  		case CCID_PACKET_DELAY:
  			delay = msecs_to_jiffies(rc);
  			if (delay > *time_budget)
  				return;
  			rc = dccp_wait_for_ccid(sk, delay);
  			if (rc < 0)
  				return;
  			*time_budget -= (delay - rc);
  			/* check again if we can send now */
  			break;
  		case CCID_PACKET_SEND_AT_ONCE:
  			dccp_xmit_packet(sk);
  			break;
  		case CCID_PACKET_ERR:
  			skb_dequeue(&sk->sk_write_queue);
  			kfree_skb(skb);
  			dccp_pr_debug("packet discarded due to err=%ld\n", rc);
  		}
  	}
  }
  
  void dccp_write_xmit(struct sock *sk)
  {
  	struct dccp_sock *dp = dccp_sk(sk);
  	struct sk_buff *skb;
  
  	while ((skb = dccp_qpolicy_top(sk))) {
  		int rc = ccid_hc_tx_send_packet(dp->dccps_hc_tx_ccid, sk, skb);
  
  		switch (ccid_packet_dequeue_eval(rc)) {
  		case CCID_PACKET_WILL_DEQUEUE_LATER:
  			return;
  		case CCID_PACKET_DELAY:
  			sk_reset_timer(sk, &dp->dccps_xmit_timer,
  				       jiffies + msecs_to_jiffies(rc));
  			return;
  		case CCID_PACKET_SEND_AT_ONCE:
  			dccp_xmit_packet(sk);
  			break;
  		case CCID_PACKET_ERR:
  			dccp_qpolicy_drop(sk, skb);
  			dccp_pr_debug("packet discarded due to err=%d\n", rc);
  		}
  	}
  }
  
  /**
   * dccp_retransmit_skb  -  Retransmit Request, Close, or CloseReq packets
   * There are only four retransmittable packet types in DCCP:
   * - Request  in client-REQUEST  state (sec. 8.1.1),
   * - CloseReq in server-CLOSEREQ state (sec. 8.3),
   * - Close    in   node-CLOSING  state (sec. 8.3),
   * - Acks in client-PARTOPEN state (sec. 8.1.5, handled by dccp_delack_timer()).
   * This function expects sk->sk_send_head to contain the original skb.
   */
  int dccp_retransmit_skb(struct sock *sk)
  {
  	WARN_ON(sk->sk_send_head == NULL);
  
  	if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk) != 0)
  		return -EHOSTUNREACH; /* Routing failure or similar. */
  
  	/* this count is used to distinguish original and retransmitted skb */
  	inet_csk(sk)->icsk_retransmits++;
  
  	return dccp_transmit_skb(sk, skb_clone(sk->sk_send_head, GFP_ATOMIC));
  }
  
  struct sk_buff *dccp_make_response(struct sock *sk, struct dst_entry *dst,
  				   struct request_sock *req)
  {
  	struct dccp_hdr *dh;
  	struct dccp_request_sock *dreq;
  	const u32 dccp_header_size = sizeof(struct dccp_hdr) +
  				     sizeof(struct dccp_hdr_ext) +
  				     sizeof(struct dccp_hdr_response);
  	struct sk_buff *skb = sock_wmalloc(sk, sk->sk_prot->max_header, 1,
  					   GFP_ATOMIC);
  	if (skb == NULL)
  		return NULL;
  
  	/* Reserve space for headers. */
  	skb_reserve(skb, sk->sk_prot->max_header);
  
  	skb_dst_set(skb, dst_clone(dst));
  
  	dreq = dccp_rsk(req);
  	if (inet_rsk(req)->acked)	/* increase GSS upon retransmission */
  		dccp_inc_seqno(&dreq->dreq_gss);
  	DCCP_SKB_CB(skb)->dccpd_type = DCCP_PKT_RESPONSE;
  	DCCP_SKB_CB(skb)->dccpd_seq  = dreq->dreq_gss;
  
  	/* Resolve feature dependencies resulting from choice of CCID */
  	if (dccp_feat_server_ccid_dependencies(dreq))
  		goto response_failed;
  
  	if (dccp_insert_options_rsk(dreq, skb))
  		goto response_failed;
  
  	/* Build and checksum header */
  	dh = dccp_zeroed_hdr(skb, dccp_header_size);
  
  	dh->dccph_sport	= inet_rsk(req)->loc_port;
  	dh->dccph_dport	= inet_rsk(req)->rmt_port;
  	dh->dccph_doff	= (dccp_header_size +
  			   DCCP_SKB_CB(skb)->dccpd_opt_len) / 4;
  	dh->dccph_type	= DCCP_PKT_RESPONSE;
  	dh->dccph_x	= 1;
  	dccp_hdr_set_seq(dh, dreq->dreq_gss);
  	dccp_hdr_set_ack(dccp_hdr_ack_bits(skb), dreq->dreq_gsr);
  	dccp_hdr_response(skb)->dccph_resp_service = dreq->dreq_service;
  
  	dccp_csum_outgoing(skb);
  
  	/* We use `acked' to remember that a Response was already sent. */
  	inet_rsk(req)->acked = 1;
  	DCCP_INC_STATS(DCCP_MIB_OUTSEGS);
  	return skb;
  response_failed:
  	kfree_skb(skb);
  	return NULL;
  }
  
  EXPORT_SYMBOL_GPL(dccp_make_response);
  
  /* answer offending packet in @rcv_skb with Reset from control socket @ctl */
  struct sk_buff *dccp_ctl_make_reset(struct sock *sk, struct sk_buff *rcv_skb)
  {
  	struct dccp_hdr *rxdh = dccp_hdr(rcv_skb), *dh;
  	struct dccp_skb_cb *dcb = DCCP_SKB_CB(rcv_skb);
  	const u32 dccp_hdr_reset_len = sizeof(struct dccp_hdr) +
  				       sizeof(struct dccp_hdr_ext) +
  				       sizeof(struct dccp_hdr_reset);
  	struct dccp_hdr_reset *dhr;
  	struct sk_buff *skb;
  
  	skb = alloc_skb(sk->sk_prot->max_header, GFP_ATOMIC);
  	if (skb == NULL)
  		return NULL;
  
  	skb_reserve(skb, sk->sk_prot->max_header);
  
  	/* Swap the send and the receive. */
  	dh = dccp_zeroed_hdr(skb, dccp_hdr_reset_len);
  	dh->dccph_type	= DCCP_PKT_RESET;
  	dh->dccph_sport	= rxdh->dccph_dport;
  	dh->dccph_dport	= rxdh->dccph_sport;
  	dh->dccph_doff	= dccp_hdr_reset_len / 4;
  	dh->dccph_x	= 1;
  
  	dhr = dccp_hdr_reset(skb);
  	dhr->dccph_reset_code = dcb->dccpd_reset_code;
  
  	switch (dcb->dccpd_reset_code) {
  	case DCCP_RESET_CODE_PACKET_ERROR:
  		dhr->dccph_reset_data[0] = rxdh->dccph_type;
  		break;
  	case DCCP_RESET_CODE_OPTION_ERROR:	/* fall through */
  	case DCCP_RESET_CODE_MANDATORY_ERROR:
  		memcpy(dhr->dccph_reset_data, dcb->dccpd_reset_data, 3);
  		break;
  	}
  	/*
  	 * From RFC 4340, 8.3.1:
  	 *   If P.ackno exists, set R.seqno := P.ackno + 1.
  	 *   Else set R.seqno := 0.
  	 */
  	if (dcb->dccpd_ack_seq != DCCP_PKT_WITHOUT_ACK_SEQ)
  		dccp_hdr_set_seq(dh, ADD48(dcb->dccpd_ack_seq, 1));
  	dccp_hdr_set_ack(dccp_hdr_ack_bits(skb), dcb->dccpd_seq);
  
  	dccp_csum_outgoing(skb);
  	return skb;
  }
  
  EXPORT_SYMBOL_GPL(dccp_ctl_make_reset);
  
  /* send Reset on established socket, to close or abort the connection */
  int dccp_send_reset(struct sock *sk, enum dccp_reset_codes code)
  {
  	struct sk_buff *skb;
  	/*
  	 * FIXME: what if rebuild_header fails?
  	 * Should we be doing a rebuild_header here?
  	 */
  	int err = inet_csk(sk)->icsk_af_ops->rebuild_header(sk);
  
  	if (err != 0)
  		return err;
  
  	skb = sock_wmalloc(sk, sk->sk_prot->max_header, 1, GFP_ATOMIC);
  	if (skb == NULL)
  		return -ENOBUFS;
  
  	/* Reserve space for headers and prepare control bits. */
  	skb_reserve(skb, sk->sk_prot->max_header);
  	DCCP_SKB_CB(skb)->dccpd_type	   = DCCP_PKT_RESET;
  	DCCP_SKB_CB(skb)->dccpd_reset_code = code;
  
  	return dccp_transmit_skb(sk, skb);
  }
  
  /*
   * Do all connect socket setups that can be done AF independent.
   */
  int dccp_connect(struct sock *sk)
  {
  	struct sk_buff *skb;
  	struct dccp_sock *dp = dccp_sk(sk);
  	struct dst_entry *dst = __sk_dst_get(sk);
  	struct inet_connection_sock *icsk = inet_csk(sk);
  
  	sk->sk_err = 0;
  	sock_reset_flag(sk, SOCK_DONE);
  
  	dccp_sync_mss(sk, dst_mtu(dst));
  
  	/* do not connect if feature negotiation setup fails */
  	if (dccp_feat_finalise_settings(dccp_sk(sk)))
  		return -EPROTO;
  
  	/* Initialise GAR as per 8.5; AWL/AWH are set in dccp_transmit_skb() */
  	dp->dccps_gar = dp->dccps_iss;
  
  	skb = alloc_skb(sk->sk_prot->max_header, sk->sk_allocation);
  	if (unlikely(skb == NULL))
  		return -ENOBUFS;
  
  	/* Reserve space for headers. */
  	skb_reserve(skb, sk->sk_prot->max_header);
  
  	DCCP_SKB_CB(skb)->dccpd_type = DCCP_PKT_REQUEST;
  
  	dccp_transmit_skb(sk, dccp_skb_entail(sk, skb));
  	DCCP_INC_STATS(DCCP_MIB_ACTIVEOPENS);
  
  	/* Timer for repeating the REQUEST until an answer. */
  	icsk->icsk_retransmits = 0;
  	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
  				  icsk->icsk_rto, DCCP_RTO_MAX);
  	return 0;
  }
  
  EXPORT_SYMBOL_GPL(dccp_connect);
  
  void dccp_send_ack(struct sock *sk)
  {
  	/* If we have been reset, we may not send again. */
  	if (sk->sk_state != DCCP_CLOSED) {
  		struct sk_buff *skb = alloc_skb(sk->sk_prot->max_header,
  						GFP_ATOMIC);
  
  		if (skb == NULL) {
  			inet_csk_schedule_ack(sk);
  			inet_csk(sk)->icsk_ack.ato = TCP_ATO_MIN;
  			inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
  						  TCP_DELACK_MAX,
  						  DCCP_RTO_MAX);
  			return;
  		}
  
  		/* Reserve space for headers */
  		skb_reserve(skb, sk->sk_prot->max_header);
  		DCCP_SKB_CB(skb)->dccpd_type = DCCP_PKT_ACK;
  		dccp_transmit_skb(sk, skb);
  	}
  }
  
  EXPORT_SYMBOL_GPL(dccp_send_ack);
  
  #if 0
  /* FIXME: Is this still necessary (11.3) - currently nowhere used by DCCP. */
  void dccp_send_delayed_ack(struct sock *sk)
  {
  	struct inet_connection_sock *icsk = inet_csk(sk);
  	/*
  	 * FIXME: tune this timer. elapsed time fixes the skew, so no problem
  	 * with using 2s, and active senders also piggyback the ACK into a
  	 * DATAACK packet, so this is really for quiescent senders.
  	 */
  	unsigned long timeout = jiffies + 2 * HZ;
  
  	/* Use new timeout only if there wasn't a older one earlier. */
  	if (icsk->icsk_ack.pending & ICSK_ACK_TIMER) {
  		/* If delack timer was blocked or is about to expire,
  		 * send ACK now.
  		 *
  		 * FIXME: check the "about to expire" part
  		 */
  		if (icsk->icsk_ack.blocked) {
  			dccp_send_ack(sk);
  			return;
  		}
  
  		if (!time_before(timeout, icsk->icsk_ack.timeout))
  			timeout = icsk->icsk_ack.timeout;
  	}
  	icsk->icsk_ack.pending |= ICSK_ACK_SCHED | ICSK_ACK_TIMER;
  	icsk->icsk_ack.timeout = timeout;
  	sk_reset_timer(sk, &icsk->icsk_delack_timer, timeout);
  }
  #endif
  
  void dccp_send_sync(struct sock *sk, const u64 ackno,
  		    const enum dccp_pkt_type pkt_type)
  {
  	/*
  	 * We are not putting this on the write queue, so
  	 * dccp_transmit_skb() will set the ownership to this
  	 * sock.
  	 */
  	struct sk_buff *skb = alloc_skb(sk->sk_prot->max_header, GFP_ATOMIC);
  
  	if (skb == NULL) {
  		/* FIXME: how to make sure the sync is sent? */
  		DCCP_CRIT("could not send %s", dccp_packet_name(pkt_type));
  		return;
  	}
  
  	/* Reserve space for headers and prepare control bits. */
  	skb_reserve(skb, sk->sk_prot->max_header);
  	DCCP_SKB_CB(skb)->dccpd_type = pkt_type;
  	DCCP_SKB_CB(skb)->dccpd_ack_seq = ackno;
  
  	/*
  	 * Clear the flag in case the Sync was scheduled for out-of-band data,
  	 * such as carrying a long Ack Vector.
  	 */
  	dccp_sk(sk)->dccps_sync_scheduled = 0;
  
  	dccp_transmit_skb(sk, skb);
  }
  
  EXPORT_SYMBOL_GPL(dccp_send_sync);
  
  /*
   * Send a DCCP_PKT_CLOSE/CLOSEREQ. The caller locks the socket for us. This
   * cannot be allowed to fail queueing a DCCP_PKT_CLOSE/CLOSEREQ frame under
   * any circumstances.
   */
  void dccp_send_close(struct sock *sk, const int active)
  {
  	struct dccp_sock *dp = dccp_sk(sk);
  	struct sk_buff *skb;
  	const gfp_t prio = active ? GFP_KERNEL : GFP_ATOMIC;
  
  	skb = alloc_skb(sk->sk_prot->max_header, prio);
  	if (skb == NULL)
  		return;
  
  	/* Reserve space for headers and prepare control bits. */
  	skb_reserve(skb, sk->sk_prot->max_header);
  	if (dp->dccps_role == DCCP_ROLE_SERVER && !dp->dccps_server_timewait)
  		DCCP_SKB_CB(skb)->dccpd_type = DCCP_PKT_CLOSEREQ;
  	else
  		DCCP_SKB_CB(skb)->dccpd_type = DCCP_PKT_CLOSE;
  
  	if (active) {
  		skb = dccp_skb_entail(sk, skb);
  		/*
  		 * Retransmission timer for active-close: RFC 4340, 8.3 requires
  		 * to retransmit the Close/CloseReq until the CLOSING/CLOSEREQ
  		 * state can be left. The initial timeout is 2 RTTs.
  		 * Since RTT measurement is done by the CCIDs, there is no easy
  		 * way to get an RTT sample. The fallback RTT from RFC 4340, 3.4
  		 * is too low (200ms); we use a high value to avoid unnecessary
  		 * retransmissions when the link RTT is > 0.2 seconds.
  		 * FIXME: Let main module sample RTTs and use that instead.
  		 */
  		inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
  					  DCCP_TIMEOUT_INIT, DCCP_RTO_MAX);
  	}
  	dccp_transmit_skb(sk, skb);
  }
  