net/dccp/ccids/lib/packet_history.c - third_party/kernel - Git at Google

 // SPDX-License-Identifier: GPL-2.0-or-later
 /*
  *  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>
  */

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

 /*
  * 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;
 	}
 }

 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;
 }

 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;
 }

 /*
  *	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;
 	}
 }

 static inline void tfrc_rx_hist_entry_from_skb(struct tfrc_rx_hist_entry *entry,
 					       const struct sk_buff *skb,
 					       const u64 ndp)
 {
 	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();
 }

 void tfrc_rx_hist_add_packet(struct tfrc_rx_hist *h,
 			     const struct sk_buff *skb,
 			     const u64 ndp)
 {
 	struct tfrc_rx_hist_entry *entry = tfrc_rx_hist_last_rcv(h);

 	tfrc_rx_hist_entry_from_skb(entry, skb, ndp);
 }

 /* 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;
 }

 static void tfrc_rx_hist_swap(struct tfrc_rx_hist *h, const u8 a, const u8 b)
 {
 	const u8 idx_a = tfrc_rx_hist_index(h, a),
 		 idx_b = tfrc_rx_hist_index(h, b);

 	swap(h->ring[idx_a], h->ring[idx_b]);
 }

 /*
  * Private helper functions for loss detection.
  *
  * In the descriptions, `Si' refers to the sequence number of entry number i,
  * whose NDP count is `Ni' (lower case is used for variables).
  * Note: All __xxx_loss functions expect that a test against duplicates has been
  *       performed already: the seqno of the skb must not be less than the seqno
  *       of loss_prev; and it must not equal that of any valid history entry.
  */
 static void __do_track_loss(struct tfrc_rx_hist *h, struct sk_buff *skb, u64 n1)
 {
 	u64 s0 = tfrc_rx_hist_loss_prev(h)->tfrchrx_seqno,
 	    s1 = DCCP_SKB_CB(skb)->dccpd_seq;

 	if (!dccp_loss_free(s0, s1, n1)) {	/* gap between S0 and S1 */
 		h->loss_count = 1;
 		tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_entry(h, 1), skb, n1);
 	}
 }

 static void __one_after_loss(struct tfrc_rx_hist *h, struct sk_buff *skb, u32 n2)
 {
 	u64 s0 = tfrc_rx_hist_loss_prev(h)->tfrchrx_seqno,
 	    s1 = tfrc_rx_hist_entry(h, 1)->tfrchrx_seqno,
 	    s2 = DCCP_SKB_CB(skb)->dccpd_seq;

 	if (likely(dccp_delta_seqno(s1, s2) > 0)) {	/* S1  <  S2 */
 		h->loss_count = 2;
 		tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_entry(h, 2), skb, n2);
 		return;
 	}

 	/* S0  <  S2  <  S1 */

 	if (dccp_loss_free(s0, s2, n2)) {
 		u64 n1 = tfrc_rx_hist_entry(h, 1)->tfrchrx_ndp;

 		if (dccp_loss_free(s2, s1, n1)) {
 			/* hole is filled: S0, S2, and S1 are consecutive */
 			h->loss_count = 0;
 			h->loss_start = tfrc_rx_hist_index(h, 1);
 		} else
 			/* gap between S2 and S1: just update loss_prev */
 			tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_loss_prev(h), skb, n2);

 	} else {	/* gap between S0 and S2 */
 		/*
 		 * Reorder history to insert S2 between S0 and S1
 		 */
 		tfrc_rx_hist_swap(h, 0, 3);
 		h->loss_start = tfrc_rx_hist_index(h, 3);
 		tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_entry(h, 1), skb, n2);
 		h->loss_count = 2;
 	}
 }

 /* return 1 if a new loss event has been identified */
 static int __two_after_loss(struct tfrc_rx_hist *h, struct sk_buff *skb, u32 n3)
 {
 	u64 s0 = tfrc_rx_hist_loss_prev(h)->tfrchrx_seqno,
 	    s1 = tfrc_rx_hist_entry(h, 1)->tfrchrx_seqno,
 	    s2 = tfrc_rx_hist_entry(h, 2)->tfrchrx_seqno,
 	    s3 = DCCP_SKB_CB(skb)->dccpd_seq;

 	if (likely(dccp_delta_seqno(s2, s3) > 0)) {	/* S2  <  S3 */
 		h->loss_count = 3;
 		tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_entry(h, 3), skb, n3);
 		return 1;
 	}

 	/* S3  <  S2 */

 	if (dccp_delta_seqno(s1, s3) > 0) {		/* S1  <  S3  <  S2 */
 		/*
 		 * Reorder history to insert S3 between S1 and S2
 		 */
 		tfrc_rx_hist_swap(h, 2, 3);
 		tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_entry(h, 2), skb, n3);
 		h->loss_count = 3;
 		return 1;
 	}

 	/* S0  <  S3  <  S1 */

 	if (dccp_loss_free(s0, s3, n3)) {
 		u64 n1 = tfrc_rx_hist_entry(h, 1)->tfrchrx_ndp;

 		if (dccp_loss_free(s3, s1, n1)) {
 			/* hole between S0 and S1 filled by S3 */
 			u64 n2 = tfrc_rx_hist_entry(h, 2)->tfrchrx_ndp;

 			if (dccp_loss_free(s1, s2, n2)) {
 				/* entire hole filled by S0, S3, S1, S2 */
 				h->loss_start = tfrc_rx_hist_index(h, 2);
 				h->loss_count = 0;
 			} else {
 				/* gap remains between S1 and S2 */
 				h->loss_start = tfrc_rx_hist_index(h, 1);
 				h->loss_count = 1;
 			}

 		} else /* gap exists between S3 and S1, loss_count stays at 2 */
 			tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_loss_prev(h), skb, n3);

 		return 0;
 	}

 	/*
 	 * The remaining case:  S0  <  S3  <  S1  <  S2;  gap between S0 and S3
 	 * Reorder history to insert S3 between S0 and S1.
 	 */
 	tfrc_rx_hist_swap(h, 0, 3);
 	h->loss_start = tfrc_rx_hist_index(h, 3);
 	tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_entry(h, 1), skb, n3);
 	h->loss_count = 3;

 	return 1;
 }

 /* recycle RX history records to continue loss detection if necessary */
 static void __three_after_loss(struct tfrc_rx_hist *h)
 {
 	/*
 	 * At this stage we know already that there is a gap between S0 and S1
 	 * (since S0 was the highest sequence number received before detecting
 	 * the loss). To recycle the loss record, it is	thus only necessary to
 	 * check for other possible gaps between S1/S2 and between S2/S3.
 	 */
 	u64 s1 = tfrc_rx_hist_entry(h, 1)->tfrchrx_seqno,
 	    s2 = tfrc_rx_hist_entry(h, 2)->tfrchrx_seqno,
 	    s3 = tfrc_rx_hist_entry(h, 3)->tfrchrx_seqno;
 	u64 n2 = tfrc_rx_hist_entry(h, 2)->tfrchrx_ndp,
 	    n3 = tfrc_rx_hist_entry(h, 3)->tfrchrx_ndp;

 	if (dccp_loss_free(s1, s2, n2)) {

 		if (dccp_loss_free(s2, s3, n3)) {
 			/* no gap between S2 and S3: entire hole is filled */
 			h->loss_start = tfrc_rx_hist_index(h, 3);
 			h->loss_count = 0;
 		} else {
 			/* gap between S2 and S3 */
 			h->loss_start = tfrc_rx_hist_index(h, 2);
 			h->loss_count = 1;
 		}

 	} else {	/* gap between S1 and S2 */
 		h->loss_start = tfrc_rx_hist_index(h, 1);
 		h->loss_count = 2;
 	}
 }

 /**
  *  tfrc_rx_handle_loss  -  Loss detection and further processing
  *  @h:		    The non-empty RX history object
  *  @lh:	    Loss Intervals database to update
  *  @skb:	    Currently received packet
  *  @ndp:	    The NDP count belonging to @skb
  *  @calc_first_li: Caller-dependent computation of first loss interval in @lh
  *  @sk:	    Used by @calc_first_li (see tfrc_lh_interval_add)
  *
  *  Chooses action according to pending loss, updates LI database when a new
  *  loss was detected, and does required post-processing. Returns 1 when caller
  *  should send feedback, 0 otherwise.
  *  Since it also takes care of reordering during loss detection and updates the
  *  records accordingly, the caller should not perform any more RX history
  *  operations when loss_count is greater than 0 after calling this function.
  */
 int tfrc_rx_handle_loss(struct tfrc_rx_hist *h,
 			struct tfrc_loss_hist *lh,
 			struct sk_buff *skb, const u64 ndp,
 			u32 (*calc_first_li)(struct sock *), struct sock *sk)
 {
 	int is_new_loss = 0;

 	if (h->loss_count == 0) {
 		__do_track_loss(h, skb, ndp);
 	} else if (h->loss_count == 1) {
 		__one_after_loss(h, skb, ndp);
 	} else if (h->loss_count != 2) {
 		DCCP_BUG("invalid loss_count %d", h->loss_count);
 	} else if (__two_after_loss(h, skb, ndp)) {
 		/*
 		 * Update Loss Interval database and recycle RX records
 		 */
 		is_new_loss = tfrc_lh_interval_add(lh, h, calc_first_li, sk);
 		__three_after_loss(h);
 	}
 	return is_new_loss;
 }

 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;
 }

 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;
 		}
 }

 /**
  * 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;
 }
	// SPDX-License-Identifier: GPL-2.0-or-later
	/*
	* 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>
	*/

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

	/*
	* 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;
	}
	}

	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;
	}

	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;
	}

	/*
	* 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;
	}
	}

	static inline void tfrc_rx_hist_entry_from_skb(struct tfrc_rx_hist_entry *entry,
	const struct sk_buff *skb,
	const u64 ndp)
	{
	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();
	}

	void tfrc_rx_hist_add_packet(struct tfrc_rx_hist *h,
	const struct sk_buff *skb,
	const u64 ndp)
	{
	struct tfrc_rx_hist_entry *entry = tfrc_rx_hist_last_rcv(h);

	tfrc_rx_hist_entry_from_skb(entry, skb, ndp);
	}

	/* 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;
	}

	static void tfrc_rx_hist_swap(struct tfrc_rx_hist *h, const u8 a, const u8 b)
	{
	const u8 idx_a = tfrc_rx_hist_index(h, a),
	idx_b = tfrc_rx_hist_index(h, b);

	swap(h->ring[idx_a], h->ring[idx_b]);
	}

	/*
	* Private helper functions for loss detection.
	*
	* In the descriptions, `Si' refers to the sequence number of entry number i,
	* whose NDP count is `Ni' (lower case is used for variables).
	* Note: All __xxx_loss functions expect that a test against duplicates has been
	* performed already: the seqno of the skb must not be less than the seqno
	* of loss_prev; and it must not equal that of any valid history entry.
	*/
	static void __do_track_loss(struct tfrc_rx_hist h, struct sk_buff skb, u64 n1)
	{
	u64 s0 = tfrc_rx_hist_loss_prev(h)->tfrchrx_seqno,
	s1 = DCCP_SKB_CB(skb)->dccpd_seq;

	if (!dccp_loss_free(s0, s1, n1)) { /* gap between S0 and S1 */
	h->loss_count = 1;
	tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_entry(h, 1), skb, n1);
	}
	}

	static void __one_after_loss(struct tfrc_rx_hist h, struct sk_buff skb, u32 n2)
	{
	u64 s0 = tfrc_rx_hist_loss_prev(h)->tfrchrx_seqno,
	s1 = tfrc_rx_hist_entry(h, 1)->tfrchrx_seqno,
	s2 = DCCP_SKB_CB(skb)->dccpd_seq;

	if (likely(dccp_delta_seqno(s1, s2) > 0)) { /* S1 < S2 */
	h->loss_count = 2;
	tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_entry(h, 2), skb, n2);
	return;
	}

	/* S0 < S2 < S1 */

	if (dccp_loss_free(s0, s2, n2)) {
	u64 n1 = tfrc_rx_hist_entry(h, 1)->tfrchrx_ndp;

	if (dccp_loss_free(s2, s1, n1)) {
	/* hole is filled: S0, S2, and S1 are consecutive */
	h->loss_count = 0;
	h->loss_start = tfrc_rx_hist_index(h, 1);
	} else
	/* gap between S2 and S1: just update loss_prev */
	tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_loss_prev(h), skb, n2);

	} else { /* gap between S0 and S2 */
	/*
	* Reorder history to insert S2 between S0 and S1
	*/
	tfrc_rx_hist_swap(h, 0, 3);
	h->loss_start = tfrc_rx_hist_index(h, 3);
	tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_entry(h, 1), skb, n2);
	h->loss_count = 2;
	}
	}

	/* return 1 if a new loss event has been identified */
	static int __two_after_loss(struct tfrc_rx_hist h, struct sk_buff skb, u32 n3)
	{
	u64 s0 = tfrc_rx_hist_loss_prev(h)->tfrchrx_seqno,
	s1 = tfrc_rx_hist_entry(h, 1)->tfrchrx_seqno,
	s2 = tfrc_rx_hist_entry(h, 2)->tfrchrx_seqno,
	s3 = DCCP_SKB_CB(skb)->dccpd_seq;

	if (likely(dccp_delta_seqno(s2, s3) > 0)) { /* S2 < S3 */
	h->loss_count = 3;
	tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_entry(h, 3), skb, n3);
	return 1;
	}

	/* S3 < S2 */

	if (dccp_delta_seqno(s1, s3) > 0) { /* S1 < S3 < S2 */
	/*
	* Reorder history to insert S3 between S1 and S2
	*/
	tfrc_rx_hist_swap(h, 2, 3);
	tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_entry(h, 2), skb, n3);
	h->loss_count = 3;
	return 1;
	}

	/* S0 < S3 < S1 */

	if (dccp_loss_free(s0, s3, n3)) {
	u64 n1 = tfrc_rx_hist_entry(h, 1)->tfrchrx_ndp;

	if (dccp_loss_free(s3, s1, n1)) {
	/* hole between S0 and S1 filled by S3 */
	u64 n2 = tfrc_rx_hist_entry(h, 2)->tfrchrx_ndp;

	if (dccp_loss_free(s1, s2, n2)) {
	/* entire hole filled by S0, S3, S1, S2 */
	h->loss_start = tfrc_rx_hist_index(h, 2);
	h->loss_count = 0;
	} else {
	/* gap remains between S1 and S2 */
	h->loss_start = tfrc_rx_hist_index(h, 1);
	h->loss_count = 1;
	}

	} else /* gap exists between S3 and S1, loss_count stays at 2 */
	tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_loss_prev(h), skb, n3);

	return 0;
	}

	/*
	* The remaining case: S0 < S3 < S1 < S2; gap between S0 and S3
	* Reorder history to insert S3 between S0 and S1.
	*/
	tfrc_rx_hist_swap(h, 0, 3);
	h->loss_start = tfrc_rx_hist_index(h, 3);
	tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_entry(h, 1), skb, n3);
	h->loss_count = 3;

	return 1;
	}

	/* recycle RX history records to continue loss detection if necessary */
	static void __three_after_loss(struct tfrc_rx_hist *h)
	{
	/*
	* At this stage we know already that there is a gap between S0 and S1
	* (since S0 was the highest sequence number received before detecting
	* the loss). To recycle the loss record, it is thus only necessary to
	* check for other possible gaps between S1/S2 and between S2/S3.
	*/
	u64 s1 = tfrc_rx_hist_entry(h, 1)->tfrchrx_seqno,
	s2 = tfrc_rx_hist_entry(h, 2)->tfrchrx_seqno,
	s3 = tfrc_rx_hist_entry(h, 3)->tfrchrx_seqno;
	u64 n2 = tfrc_rx_hist_entry(h, 2)->tfrchrx_ndp,
	n3 = tfrc_rx_hist_entry(h, 3)->tfrchrx_ndp;

	if (dccp_loss_free(s1, s2, n2)) {

	if (dccp_loss_free(s2, s3, n3)) {
	/* no gap between S2 and S3: entire hole is filled */
	h->loss_start = tfrc_rx_hist_index(h, 3);
	h->loss_count = 0;
	} else {
	/* gap between S2 and S3 */
	h->loss_start = tfrc_rx_hist_index(h, 2);
	h->loss_count = 1;
	}

	} else { /* gap between S1 and S2 */
	h->loss_start = tfrc_rx_hist_index(h, 1);
	h->loss_count = 2;
	}
	}

	/**
	* tfrc_rx_handle_loss - Loss detection and further processing
	* @h: The non-empty RX history object
	* @lh: Loss Intervals database to update
	* @skb: Currently received packet
	* @ndp: The NDP count belonging to @skb
	* @calc_first_li: Caller-dependent computation of first loss interval in @lh
	* @sk: Used by @calc_first_li (see tfrc_lh_interval_add)
	*
	* Chooses action according to pending loss, updates LI database when a new
	* loss was detected, and does required post-processing. Returns 1 when caller
	* should send feedback, 0 otherwise.
	* Since it also takes care of reordering during loss detection and updates the
	* records accordingly, the caller should not perform any more RX history
	* operations when loss_count is greater than 0 after calling this function.
	*/
	int tfrc_rx_handle_loss(struct tfrc_rx_hist *h,
	struct tfrc_loss_hist *lh,
	struct sk_buff *skb, const u64 ndp,
	u32 (calc_first_li)(struct sock ), struct sock *sk)
	{
	int is_new_loss = 0;

	if (h->loss_count == 0) {
	__do_track_loss(h, skb, ndp);
	} else if (h->loss_count == 1) {
	__one_after_loss(h, skb, ndp);
	} else if (h->loss_count != 2) {
	DCCP_BUG("invalid loss_count %d", h->loss_count);
	} else if (__two_after_loss(h, skb, ndp)) {
	/*
	* Update Loss Interval database and recycle RX records
	*/
	is_new_loss = tfrc_lh_interval_add(lh, h, calc_first_li, sk);
	__three_after_loss(h);
	}
	return is_new_loss;
	}

	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;
	}

	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;
	}
	}

	/**
	* 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;
	}