net/dccp/ccids/lib/packet_history.c - arm/linux - Git at Google

 /*
  *  net/dccp/packet_history.c
  *
  *  Copyright (c) 2007   The University of Aberdeen, Scotland, UK
  *  Copyright (c) 2005-7 The University of Waikato, Hamilton, New Zealand.
  *
  *  An implementation of the DCCP protocol
  *
  *  This code has been developed by the University of Waikato WAND
  *  research group. For further information please see http://www.wand.net.nz/
  *  or e-mail Ian McDonald - ian.mcdonald@jandi.co.nz
  *
  *  This code also uses code from Lulea University, rereleased as GPL by its
  *  authors:
  *  Copyright (c) 2003 Nils-Erik Mattsson, Joacim Haggmark, Magnus Erixzon
  *
  *  Changes to meet Linux coding standards, to make it meet latest ccid3 draft
  *  and to make it work as a loadable module in the DCCP stack written by
  *  Arnaldo Carvalho de Melo <acme@conectiva.com.br>.
  *
  *  Copyright (c) 2005 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.
  *
  *  This program is distributed in the hope that it will be useful,
  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  *  GNU General Public License for more details.
  *
  *  You should have received a copy of the GNU General Public License
  *  along with this program; if not, write to the Free Software
  *  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  */

 #include <linux/string.h>
 #include <linux/slab.h>
 #include "packet_history.h"
 #include "../../dccp.h"

 /**
  *  tfrc_tx_hist_entry  -  Simple singly-linked TX history list
  *  @next:  next oldest entry (LIFO order)
  *  @seqno: sequence number of this entry
  *  @stamp: send time of packet with sequence number @seqno
  */
 struct tfrc_tx_hist_entry {
 	struct tfrc_tx_hist_entry *next;
 	u64			  seqno;
 	ktime_t			  stamp;
 };

 /*
  * Transmitter History Routines
  */
 static struct kmem_cache *tfrc_tx_hist_slab;

 int __init tfrc_tx_packet_history_init(void)
 {
 	tfrc_tx_hist_slab = kmem_cache_create("tfrc_tx_hist",
 					      sizeof(struct tfrc_tx_hist_entry),
 					      0, SLAB_HWCACHE_ALIGN, NULL);
 	return tfrc_tx_hist_slab == NULL ? -ENOBUFS : 0;
 }

 void tfrc_tx_packet_history_exit(void)
 {
 	if (tfrc_tx_hist_slab != NULL) {
 		kmem_cache_destroy(tfrc_tx_hist_slab);
 		tfrc_tx_hist_slab = NULL;
 	}
 }

 static struct tfrc_tx_hist_entry *
 	tfrc_tx_hist_find_entry(struct tfrc_tx_hist_entry *head, u64 seqno)
 {
 	while (head != NULL && head->seqno != seqno)
 		head = head->next;

 	return head;
 }

 int tfrc_tx_hist_add(struct tfrc_tx_hist_entry **headp, u64 seqno)
 {
 	struct tfrc_tx_hist_entry *entry = kmem_cache_alloc(tfrc_tx_hist_slab, gfp_any());

 	if (entry == NULL)
 		return -ENOBUFS;
 	entry->seqno = seqno;
 	entry->stamp = ktime_get_real();
 	entry->next  = *headp;
 	*headp	     = entry;
 	return 0;
 }
 EXPORT_SYMBOL_GPL(tfrc_tx_hist_add);

 void tfrc_tx_hist_purge(struct tfrc_tx_hist_entry **headp)
 {
 	struct tfrc_tx_hist_entry *head = *headp;

 	while (head != NULL) {
 		struct tfrc_tx_hist_entry *next = head->next;

 		kmem_cache_free(tfrc_tx_hist_slab, head);
 		head = next;
 	}

 	*headp = NULL;
 }
 EXPORT_SYMBOL_GPL(tfrc_tx_hist_purge);

 u32 tfrc_tx_hist_rtt(struct tfrc_tx_hist_entry *head, const u64 seqno,
 		     const ktime_t now)
 {
 	u32 rtt = 0;
 	struct tfrc_tx_hist_entry *packet = tfrc_tx_hist_find_entry(head, seqno);

 	if (packet != NULL) {
 		rtt = ktime_us_delta(now, packet->stamp);
 		/*
 		 * Garbage-collect older (irrelevant) entries:
 		 */
 		tfrc_tx_hist_purge(&packet->next);
 	}

 	return rtt;
 }
 EXPORT_SYMBOL_GPL(tfrc_tx_hist_rtt);


 /*
  * 	Receiver History Routines
  */
 static struct kmem_cache *tfrc_rx_hist_slab;

 int __init tfrc_rx_packet_history_init(void)
 {
 	tfrc_rx_hist_slab = kmem_cache_create("tfrc_rxh_cache",
 					      sizeof(struct tfrc_rx_hist_entry),
 					      0, SLAB_HWCACHE_ALIGN, NULL);
 	return tfrc_rx_hist_slab == NULL ? -ENOBUFS : 0;
 }

 void tfrc_rx_packet_history_exit(void)
 {
 	if (tfrc_rx_hist_slab != NULL) {
 		kmem_cache_destroy(tfrc_rx_hist_slab);
 		tfrc_rx_hist_slab = NULL;
 	}
 }

 /**
  * tfrc_rx_hist_index - index to reach n-th entry after loss_start
  */
 static inline u8 tfrc_rx_hist_index(const struct tfrc_rx_hist *h, const u8 n)
 {
 	return (h->loss_start + n) & TFRC_NDUPACK;
 }

 /**
  * tfrc_rx_hist_last_rcv - entry with highest-received-seqno so far
  */
 static inline struct tfrc_rx_hist_entry *
 			tfrc_rx_hist_last_rcv(const struct tfrc_rx_hist *h)
 {
 	return h->ring[tfrc_rx_hist_index(h, h->loss_count)];
 }

 void tfrc_rx_hist_add_packet(struct tfrc_rx_hist *h,
 			     const struct sk_buff *skb,
 			     const u32 ndp)
 {
 	struct tfrc_rx_hist_entry *entry = tfrc_rx_hist_last_rcv(h);
 	const struct dccp_hdr *dh = dccp_hdr(skb);

 	entry->tfrchrx_seqno = DCCP_SKB_CB(skb)->dccpd_seq;
 	entry->tfrchrx_ccval = dh->dccph_ccval;
 	entry->tfrchrx_type  = dh->dccph_type;
 	entry->tfrchrx_ndp   = ndp;
 	entry->tfrchrx_tstamp = ktime_get_real();
 }
 EXPORT_SYMBOL_GPL(tfrc_rx_hist_add_packet);

 /**
  * tfrc_rx_hist_entry - return the n-th history entry after loss_start
  */
 static inline struct tfrc_rx_hist_entry *
 		tfrc_rx_hist_entry(const struct tfrc_rx_hist *h, const u8 n)
 {
 	return h->ring[tfrc_rx_hist_index(h, n)];
 }

 /**
  * tfrc_rx_hist_loss_prev - entry with highest-received-seqno before loss was detected
  */
 static inline struct tfrc_rx_hist_entry *
 			tfrc_rx_hist_loss_prev(const struct tfrc_rx_hist *h)
 {
 	return h->ring[h->loss_start];
 }

 /* has the packet contained in skb been seen before? */
 int tfrc_rx_hist_duplicate(struct tfrc_rx_hist *h, struct sk_buff *skb)
 {
 	const u64 seq = DCCP_SKB_CB(skb)->dccpd_seq;
 	int i;

 	if (dccp_delta_seqno(tfrc_rx_hist_loss_prev(h)->tfrchrx_seqno, seq) <= 0)
 		return 1;

 	for (i = 1; i <= h->loss_count; i++)
 		if (tfrc_rx_hist_entry(h, i)->tfrchrx_seqno == seq)
 			return 1;

 	return 0;
 }
 EXPORT_SYMBOL_GPL(tfrc_rx_hist_duplicate);

 /* initialise loss detection and disable RTT sampling */
 static inline void tfrc_rx_hist_loss_indicated(struct tfrc_rx_hist *h)
 {
 	h->loss_count = 1;
 }

 /* indicate whether previously a packet was detected missing */
 static inline int tfrc_rx_hist_loss_pending(const struct tfrc_rx_hist *h)
 {
 	return h->loss_count;
 }

 /* any data packets missing between last reception and skb ? */
 int tfrc_rx_hist_new_loss_indicated(struct tfrc_rx_hist *h,
 				    const struct sk_buff *skb, u32 ndp)
 {
 	int delta = dccp_delta_seqno(tfrc_rx_hist_last_rcv(h)->tfrchrx_seqno,
 				     DCCP_SKB_CB(skb)->dccpd_seq);

 	if (delta > 1 && ndp < delta)
 		tfrc_rx_hist_loss_indicated(h);

 	return tfrc_rx_hist_loss_pending(h);
 }
 EXPORT_SYMBOL_GPL(tfrc_rx_hist_new_loss_indicated);

 int tfrc_rx_hist_alloc(struct tfrc_rx_hist *h)
 {
 	int i;

 	for (i = 0; i <= TFRC_NDUPACK; i++) {
 		h->ring[i] = kmem_cache_alloc(tfrc_rx_hist_slab, GFP_ATOMIC);
 		if (h->ring[i] == NULL)
 			goto out_free;
 	}

 	h->loss_count = h->loss_start = 0;
 	return 0;

 out_free:
 	while (i-- != 0) {
 		kmem_cache_free(tfrc_rx_hist_slab, h->ring[i]);
 		h->ring[i] = NULL;
 	}
 	return -ENOBUFS;
 }
 EXPORT_SYMBOL_GPL(tfrc_rx_hist_alloc);

 void tfrc_rx_hist_purge(struct tfrc_rx_hist *h)
 {
 	int i;

 	for (i = 0; i <= TFRC_NDUPACK; ++i)
 		if (h->ring[i] != NULL) {
 			kmem_cache_free(tfrc_rx_hist_slab, h->ring[i]);
 			h->ring[i] = NULL;
 		}
 }
 EXPORT_SYMBOL_GPL(tfrc_rx_hist_purge);

 /**
  * tfrc_rx_hist_rtt_last_s - reference entry to compute RTT samples against
  */
 static inline struct tfrc_rx_hist_entry *
 			tfrc_rx_hist_rtt_last_s(const struct tfrc_rx_hist *h)
 {
 	return h->ring[0];
 }

 /**
  * tfrc_rx_hist_rtt_prev_s: previously suitable (wrt rtt_last_s) RTT-sampling entry
  */
 static inline struct tfrc_rx_hist_entry *
 			tfrc_rx_hist_rtt_prev_s(const struct tfrc_rx_hist *h)
 {
 	return h->ring[h->rtt_sample_prev];
 }

 /**
  * tfrc_rx_hist_sample_rtt  -  Sample RTT from timestamp / CCVal
  * Based on ideas presented in RFC 4342, 8.1. Returns 0 if it was not able
  * to compute a sample with given data - calling function should check this.
  */
 u32 tfrc_rx_hist_sample_rtt(struct tfrc_rx_hist *h, const struct sk_buff *skb)
 {
 	u32 sample = 0,
 	    delta_v = SUB16(dccp_hdr(skb)->dccph_ccval,
 			    tfrc_rx_hist_rtt_last_s(h)->tfrchrx_ccval);

 	if (delta_v < 1 || delta_v > 4) {	/* unsuitable CCVal delta */
 		if (h->rtt_sample_prev == 2) {	/* previous candidate stored */
 			sample = SUB16(tfrc_rx_hist_rtt_prev_s(h)->tfrchrx_ccval,
 				       tfrc_rx_hist_rtt_last_s(h)->tfrchrx_ccval);
 			if (sample)
 				sample = 4 / sample *
 				         ktime_us_delta(tfrc_rx_hist_rtt_prev_s(h)->tfrchrx_tstamp,
 							tfrc_rx_hist_rtt_last_s(h)->tfrchrx_tstamp);
 			else    /*
 				 * FIXME: This condition is in principle not
 				 * possible but occurs when CCID is used for
 				 * two-way data traffic. I have tried to trace
 				 * it, but the cause does not seem to be here.
 				 */
 				DCCP_BUG("please report to dccp@vger.kernel.org"
 					 " => prev = %u, last = %u",
 					 tfrc_rx_hist_rtt_prev_s(h)->tfrchrx_ccval,
 					 tfrc_rx_hist_rtt_last_s(h)->tfrchrx_ccval);
 		} else if (delta_v < 1) {
 			h->rtt_sample_prev = 1;
 			goto keep_ref_for_next_time;
 		}

 	} else if (delta_v == 4) /* optimal match */
 		sample = ktime_to_us(net_timedelta(tfrc_rx_hist_rtt_last_s(h)->tfrchrx_tstamp));
 	else {			 /* suboptimal match */
 		h->rtt_sample_prev = 2;
 		goto keep_ref_for_next_time;
 	}

 	if (unlikely(sample > DCCP_SANE_RTT_MAX)) {
 		DCCP_WARN("RTT sample %u too large, using max\n", sample);
 		sample = DCCP_SANE_RTT_MAX;
 	}

 	h->rtt_sample_prev = 0;	       /* use current entry as next reference */
 keep_ref_for_next_time:

 	return sample;
 }
 EXPORT_SYMBOL_GPL(tfrc_rx_hist_sample_rtt);
	/*
	* net/dccp/packet_history.c
	*
	* Copyright (c) 2007 The University of Aberdeen, Scotland, UK
	* Copyright (c) 2005-7 The University of Waikato, Hamilton, New Zealand.
	*
	* An implementation of the DCCP protocol
	*
	* This code has been developed by the University of Waikato WAND
	* research group. For further information please see http://www.wand.net.nz/
	* or e-mail Ian McDonald - ian.mcdonald@jandi.co.nz
	*
	* This code also uses code from Lulea University, rereleased as GPL by its
	* authors:
	* Copyright (c) 2003 Nils-Erik Mattsson, Joacim Haggmark, Magnus Erixzon
	*
	* Changes to meet Linux coding standards, to make it meet latest ccid3 draft
	* and to make it work as a loadable module in the DCCP stack written by
	* Arnaldo Carvalho de Melo <acme@conectiva.com.br>.
	*
	* Copyright (c) 2005 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.
	*
	* This program is distributed in the hope that it will be useful,
	* but WITHOUT ANY WARRANTY; without even the implied warranty of
	* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
	* GNU General Public License for more details.
	*
	* You should have received a copy of the GNU General Public License
	* along with this program; if not, write to the Free Software
	* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
	*/

	#include <linux/string.h>
	#include <linux/slab.h>
	#include "packet_history.h"
	#include "../../dccp.h"

	/**
	* tfrc_tx_hist_entry - Simple singly-linked TX history list
	* @next: next oldest entry (LIFO order)
	* @seqno: sequence number of this entry
	* @stamp: send time of packet with sequence number @seqno
	*/
	struct tfrc_tx_hist_entry {
	struct tfrc_tx_hist_entry *next;
	u64 seqno;
	ktime_t stamp;
	};

	/*
	* Transmitter History Routines
	*/
	static struct kmem_cache *tfrc_tx_hist_slab;

	int __init tfrc_tx_packet_history_init(void)
	{
	tfrc_tx_hist_slab = kmem_cache_create("tfrc_tx_hist",
	sizeof(struct tfrc_tx_hist_entry),
	0, SLAB_HWCACHE_ALIGN, NULL);
	return tfrc_tx_hist_slab == NULL ? -ENOBUFS : 0;
	}

	void tfrc_tx_packet_history_exit(void)
	{
	if (tfrc_tx_hist_slab != NULL) {
	kmem_cache_destroy(tfrc_tx_hist_slab);
	tfrc_tx_hist_slab = NULL;
	}
	}

	static struct tfrc_tx_hist_entry *
	tfrc_tx_hist_find_entry(struct tfrc_tx_hist_entry *head, u64 seqno)
	{
	while (head != NULL && head->seqno != seqno)
	head = head->next;

	return head;
	}

	int tfrc_tx_hist_add(struct tfrc_tx_hist_entry **headp, u64 seqno)
	{
	struct tfrc_tx_hist_entry *entry = kmem_cache_alloc(tfrc_tx_hist_slab, gfp_any());

	if (entry == NULL)
	return -ENOBUFS;
	entry->seqno = seqno;
	entry->stamp = ktime_get_real();
	entry->next = *headp;
	*headp = entry;
	return 0;
	}
	EXPORT_SYMBOL_GPL(tfrc_tx_hist_add);

	void tfrc_tx_hist_purge(struct tfrc_tx_hist_entry **headp)
	{
	struct tfrc_tx_hist_entry head = headp;

	while (head != NULL) {
	struct tfrc_tx_hist_entry *next = head->next;

	kmem_cache_free(tfrc_tx_hist_slab, head);
	head = next;
	}

	*headp = NULL;
	}
	EXPORT_SYMBOL_GPL(tfrc_tx_hist_purge);

	u32 tfrc_tx_hist_rtt(struct tfrc_tx_hist_entry *head, const u64 seqno,
	const ktime_t now)
	{
	u32 rtt = 0;
	struct tfrc_tx_hist_entry *packet = tfrc_tx_hist_find_entry(head, seqno);

	if (packet != NULL) {
	rtt = ktime_us_delta(now, packet->stamp);
	/*
	* Garbage-collect older (irrelevant) entries:
	*/
	tfrc_tx_hist_purge(&packet->next);
	}

	return rtt;
	}
	EXPORT_SYMBOL_GPL(tfrc_tx_hist_rtt);


	/*
	* Receiver History Routines
	*/
	static struct kmem_cache *tfrc_rx_hist_slab;

	int __init tfrc_rx_packet_history_init(void)
	{
	tfrc_rx_hist_slab = kmem_cache_create("tfrc_rxh_cache",
	sizeof(struct tfrc_rx_hist_entry),
	0, SLAB_HWCACHE_ALIGN, NULL);
	return tfrc_rx_hist_slab == NULL ? -ENOBUFS : 0;
	}

	void tfrc_rx_packet_history_exit(void)
	{
	if (tfrc_rx_hist_slab != NULL) {
	kmem_cache_destroy(tfrc_rx_hist_slab);
	tfrc_rx_hist_slab = NULL;
	}
	}

	/**
	* tfrc_rx_hist_index - index to reach n-th entry after loss_start
	*/
	static inline u8 tfrc_rx_hist_index(const struct tfrc_rx_hist *h, const u8 n)
	{
	return (h->loss_start + n) & TFRC_NDUPACK;
	}

	/**
	* tfrc_rx_hist_last_rcv - entry with highest-received-seqno so far
	*/
	static inline struct tfrc_rx_hist_entry *
	tfrc_rx_hist_last_rcv(const struct tfrc_rx_hist *h)
	{
	return h->ring[tfrc_rx_hist_index(h, h->loss_count)];
	}

	void tfrc_rx_hist_add_packet(struct tfrc_rx_hist *h,
	const struct sk_buff *skb,
	const u32 ndp)
	{
	struct tfrc_rx_hist_entry *entry = tfrc_rx_hist_last_rcv(h);
	const struct dccp_hdr *dh = dccp_hdr(skb);

	entry->tfrchrx_seqno = DCCP_SKB_CB(skb)->dccpd_seq;
	entry->tfrchrx_ccval = dh->dccph_ccval;
	entry->tfrchrx_type = dh->dccph_type;
	entry->tfrchrx_ndp = ndp;
	entry->tfrchrx_tstamp = ktime_get_real();
	}
	EXPORT_SYMBOL_GPL(tfrc_rx_hist_add_packet);

	/**
	* tfrc_rx_hist_entry - return the n-th history entry after loss_start
	*/
	static inline struct tfrc_rx_hist_entry *
	tfrc_rx_hist_entry(const struct tfrc_rx_hist *h, const u8 n)
	{
	return h->ring[tfrc_rx_hist_index(h, n)];
	}

	/**
	* tfrc_rx_hist_loss_prev - entry with highest-received-seqno before loss was detected
	*/
	static inline struct tfrc_rx_hist_entry *
	tfrc_rx_hist_loss_prev(const struct tfrc_rx_hist *h)
	{
	return h->ring[h->loss_start];
	}

	/* has the packet contained in skb been seen before? */
	int tfrc_rx_hist_duplicate(struct tfrc_rx_hist h, struct sk_buff skb)
	{
	const u64 seq = DCCP_SKB_CB(skb)->dccpd_seq;
	int i;

	if (dccp_delta_seqno(tfrc_rx_hist_loss_prev(h)->tfrchrx_seqno, seq) <= 0)
	return 1;

	for (i = 1; i <= h->loss_count; i++)
	if (tfrc_rx_hist_entry(h, i)->tfrchrx_seqno == seq)
	return 1;

	return 0;
	}
	EXPORT_SYMBOL_GPL(tfrc_rx_hist_duplicate);

	/* initialise loss detection and disable RTT sampling */
	static inline void tfrc_rx_hist_loss_indicated(struct tfrc_rx_hist *h)
	{
	h->loss_count = 1;
	}

	/* indicate whether previously a packet was detected missing */
	static inline int tfrc_rx_hist_loss_pending(const struct tfrc_rx_hist *h)
	{
	return h->loss_count;
	}

	/* any data packets missing between last reception and skb ? */
	int tfrc_rx_hist_new_loss_indicated(struct tfrc_rx_hist *h,
	const struct sk_buff *skb, u32 ndp)
	{
	int delta = dccp_delta_seqno(tfrc_rx_hist_last_rcv(h)->tfrchrx_seqno,
	DCCP_SKB_CB(skb)->dccpd_seq);

	if (delta > 1 && ndp < delta)
	tfrc_rx_hist_loss_indicated(h);

	return tfrc_rx_hist_loss_pending(h);
	}
	EXPORT_SYMBOL_GPL(tfrc_rx_hist_new_loss_indicated);

	int tfrc_rx_hist_alloc(struct tfrc_rx_hist *h)
	{
	int i;

	for (i = 0; i <= TFRC_NDUPACK; i++) {
	h->ring[i] = kmem_cache_alloc(tfrc_rx_hist_slab, GFP_ATOMIC);
	if (h->ring[i] == NULL)
	goto out_free;
	}

	h->loss_count = h->loss_start = 0;
	return 0;

	out_free:
	while (i-- != 0) {
	kmem_cache_free(tfrc_rx_hist_slab, h->ring[i]);
	h->ring[i] = NULL;
	}
	return -ENOBUFS;
	}
	EXPORT_SYMBOL_GPL(tfrc_rx_hist_alloc);

	void tfrc_rx_hist_purge(struct tfrc_rx_hist *h)
	{
	int i;

	for (i = 0; i <= TFRC_NDUPACK; ++i)
	if (h->ring[i] != NULL) {
	kmem_cache_free(tfrc_rx_hist_slab, h->ring[i]);
	h->ring[i] = NULL;
	}
	}
	EXPORT_SYMBOL_GPL(tfrc_rx_hist_purge);

	/**
	* tfrc_rx_hist_rtt_last_s - reference entry to compute RTT samples against
	*/
	static inline struct tfrc_rx_hist_entry *
	tfrc_rx_hist_rtt_last_s(const struct tfrc_rx_hist *h)
	{
	return h->ring[0];
	}

	/**
	* tfrc_rx_hist_rtt_prev_s: previously suitable (wrt rtt_last_s) RTT-sampling entry
	*/
	static inline struct tfrc_rx_hist_entry *
	tfrc_rx_hist_rtt_prev_s(const struct tfrc_rx_hist *h)
	{
	return h->ring[h->rtt_sample_prev];
	}

	/**
	* tfrc_rx_hist_sample_rtt - Sample RTT from timestamp / CCVal
	* Based on ideas presented in RFC 4342, 8.1. Returns 0 if it was not able
	* to compute a sample with given data - calling function should check this.
	*/
	u32 tfrc_rx_hist_sample_rtt(struct tfrc_rx_hist h, const struct sk_buff skb)
	{
	u32 sample = 0,
	delta_v = SUB16(dccp_hdr(skb)->dccph_ccval,
	tfrc_rx_hist_rtt_last_s(h)->tfrchrx_ccval);

	if (delta_v < 1 \|\| delta_v > 4) { /* unsuitable CCVal delta */
	if (h->rtt_sample_prev == 2) { /* previous candidate stored */
	sample = SUB16(tfrc_rx_hist_rtt_prev_s(h)->tfrchrx_ccval,
	tfrc_rx_hist_rtt_last_s(h)->tfrchrx_ccval);
	if (sample)
	sample = 4 / sample *
	ktime_us_delta(tfrc_rx_hist_rtt_prev_s(h)->tfrchrx_tstamp,
	tfrc_rx_hist_rtt_last_s(h)->tfrchrx_tstamp);
	else /*
	* FIXME: This condition is in principle not
	* possible but occurs when CCID is used for
	* two-way data traffic. I have tried to trace
	* it, but the cause does not seem to be here.
	*/
	DCCP_BUG("please report to dccp@vger.kernel.org"
	" => prev = %u, last = %u",
	tfrc_rx_hist_rtt_prev_s(h)->tfrchrx_ccval,
	tfrc_rx_hist_rtt_last_s(h)->tfrchrx_ccval);
	} else if (delta_v < 1) {
	h->rtt_sample_prev = 1;
	goto keep_ref_for_next_time;
	}

	} else if (delta_v == 4) /* optimal match */
	sample = ktime_to_us(net_timedelta(tfrc_rx_hist_rtt_last_s(h)->tfrchrx_tstamp));
	else { /* suboptimal match */
	h->rtt_sample_prev = 2;
	goto keep_ref_for_next_time;
	}

	if (unlikely(sample > DCCP_SANE_RTT_MAX)) {
	DCCP_WARN("RTT sample %u too large, using max\n", sample);
	sample = DCCP_SANE_RTT_MAX;
	}

	h->rtt_sample_prev = 0; /* use current entry as next reference */
	keep_ref_for_next_time:

	return sample;
	}
	EXPORT_SYMBOL_GPL(tfrc_rx_hist_sample_rtt);