net/dccp/ackvec.c

/*
 *  net/dccp/ackvec.c
 *
 *  An implementation of Ack Vectors for the DCCP protocol
 *  Copyright (c) 2007 University of Aberdeen, Scotland, UK
 *  Copyright (c) 2005 Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
 *
 *      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; version 2 of the License;
 */

#include "ackvec.h"
#include "dccp.h"

#include <linux/init.h>
#include <linux/errno.h>
#include <linux/kernel.h>
#include <linux/skbuff.h>
#include <linux/slab.h>

#include <net/sock.h>

static struct kmem_cache *dccp_ackvec_slab;
static struct kmem_cache *dccp_ackvec_record_slab;

struct dccp_ackvec *dccp_ackvec_alloc(const gfp_t priority)
{
	struct dccp_ackvec *av = kmem_cache_zalloc(dccp_ackvec_slab, priority);

	if (av != NULL) {
		av->av_buf_head	= av->av_buf_tail = DCCPAV_MAX_ACKVEC_LEN - 1;
		INIT_LIST_HEAD(&av->av_records);
	}
	return av;
}

static void dccp_ackvec_purge_records(struct dccp_ackvec *av)
{
	struct dccp_ackvec_record *cur, *next;

	list_for_each_entry_safe(cur, next, &av->av_records, avr_node)
		kmem_cache_free(dccp_ackvec_record_slab, cur);
	INIT_LIST_HEAD(&av->av_records);
}

void dccp_ackvec_free(struct dccp_ackvec *av)
{
	if (likely(av != NULL)) {
		dccp_ackvec_purge_records(av);
		kmem_cache_free(dccp_ackvec_slab, av);
	}
}

/**
 * dccp_ackvec_update_records  -  Record information about sent Ack Vectors
 * @av:		Ack Vector records to update
 * @seqno:	Sequence number of the packet carrying the Ack Vector just sent
 * @nonce_sum:	The sum of all buffer nonces contained in the Ack Vector
 */
int dccp_ackvec_update_records(struct dccp_ackvec *av, u64 seqno, u8 nonce_sum)
{
	struct dccp_ackvec_record *avr;

	avr = kmem_cache_alloc(dccp_ackvec_record_slab, GFP_ATOMIC);
	if (avr == NULL)
		return -ENOBUFS;

	avr->avr_ack_seqno  = seqno;
	avr->avr_ack_ptr    = av->av_buf_head;
	avr->avr_ack_ackno  = av->av_buf_ackno;
	avr->avr_ack_nonce  = nonce_sum;
	avr->avr_ack_runlen = dccp_ackvec_runlen(av->av_buf + av->av_buf_head);
	/*
	 * When the buffer overflows, we keep no more than one record. This is
	 * the simplest way of disambiguating sender-Acks dating from before the
	 * overflow from sender-Acks which refer to after the overflow; a simple
	 * solution is preferable here since we are handling an exception.
	 */
	if (av->av_overflow)
		dccp_ackvec_purge_records(av);
	/*
	 * Since GSS is incremented for each packet, the list is automatically
	 * arranged in descending order of @ack_seqno.
	 */
	list_add(&avr->avr_node, &av->av_records);

	dccp_pr_debug("Added Vector, ack_seqno=%llu, ack_ackno=%llu (rl=%u)\n",
		      (unsigned long long)avr->avr_ack_seqno,
		      (unsigned long long)avr->avr_ack_ackno,
		      avr->avr_ack_runlen);
	return 0;
}

static struct dccp_ackvec_record *dccp_ackvec_lookup(struct list_head *av_list,
						     const u64 ackno)
{
	struct dccp_ackvec_record *avr;
	/*
	 * Exploit that records are inserted in descending order of sequence
	 * number, start with the oldest record first. If @ackno is `before'
	 * the earliest ack_ackno, the packet is too old to be considered.
	 */
	list_for_each_entry_reverse(avr, av_list, avr_node) {
		if (avr->avr_ack_seqno == ackno)
			return avr;
		if (before48(ackno, avr->avr_ack_seqno))
			break;
	}
	return NULL;
}

/*
 * Buffer index and length computation using modulo-buffersize arithmetic.
 * Note that, as pointers move from right to left, head is `before' tail.
 */
static inline u16 __ackvec_idx_add(const u16 a, const u16 b)
{
	return (a + b) % DCCPAV_MAX_ACKVEC_LEN;
}

static inline u16 __ackvec_idx_sub(const u16 a, const u16 b)
{
	return __ackvec_idx_add(a, DCCPAV_MAX_ACKVEC_LEN - b);
}

u16 dccp_ackvec_buflen(const struct dccp_ackvec *av)
{
	if (unlikely(av->av_overflow))
		return DCCPAV_MAX_ACKVEC_LEN;
	return __ackvec_idx_sub(av->av_buf_tail, av->av_buf_head);
}

/**
 * dccp_ackvec_update_old  -  Update previous state as per RFC 4340, 11.4.1
 * @av:		non-empty buffer to update
 * @distance:   negative or zero distance of @seqno from buf_ackno downward
 * @seqno:	the (old) sequence number whose record is to be updated
 * @state:	state in which packet carrying @seqno was received
 */
static void dccp_ackvec_update_old(struct dccp_ackvec *av, s64 distance,
				   u64 seqno, enum dccp_ackvec_states state)
{
	u16 ptr = av->av_buf_head;

	BUG_ON(distance > 0);
	if (unlikely(dccp_ackvec_is_empty(av)))
		return;

	do {
		u8 runlen = dccp_ackvec_runlen(av->av_buf + ptr);

		if (distance + runlen >= 0) {
			/*
			 * Only update the state if packet has not been received
			 * yet. This is OK as per the second table in RFC 4340,
			 * 11.4.1; i.e. here we are using the following table:
			 *                     RECEIVED
			 *                      0   1   3
			 *              S     +---+---+---+
			 *              T   0 | 0 | 0 | 0 |
			 *              O     +---+---+---+
			 *              R   1 | 1 | 1 | 1 |
			 *              E     +---+---+---+
			 *              D   3 | 0 | 1 | 3 |
			 *                    +---+---+---+
			 * The "Not Received" state was set by reserve_seats().
			 */
			if (av->av_buf[ptr] == DCCPAV_NOT_RECEIVED)
				av->av_buf[ptr] = state;
			else
				dccp_pr_debug("Not changing %llu state to %u\n",
					      (unsigned long long)seqno, state);
			break;
		}

		distance += runlen + 1;
		ptr	  = __ackvec_idx_add(ptr, 1);

	} while (ptr != av->av_buf_tail);
}

/* Mark @num entries after buf_head as "Not yet received". */
static void dccp_ackvec_reserve_seats(struct dccp_ackvec *av, u16 num)
{
	u16 start = __ackvec_idx_add(av->av_buf_head, 1),
	    len	  = DCCPAV_MAX_ACKVEC_LEN - start;

	/* check for buffer wrap-around */
	if (num > len) {
		memset(av->av_buf + start, DCCPAV_NOT_RECEIVED, len);
		start = 0;
		num  -= len;
	}
	if (num)
		memset(av->av_buf + start, DCCPAV_NOT_RECEIVED, num);
}

/**
 * dccp_ackvec_add_new  -  Record one or more new entries in Ack Vector buffer
 * @av:		 container of buffer to update (can be empty or non-empty)
 * @num_packets: number of packets to register (must be >= 1)
 * @seqno:	 sequence number of the first packet in @num_packets
 * @state:	 state in which packet carrying @seqno was received
 */
static void dccp_ackvec_add_new(struct dccp_ackvec *av, u32 num_packets,
				u64 seqno, enum dccp_ackvec_states state)
{
	u32 num_cells = num_packets;

	if (num_packets > DCCPAV_BURST_THRESH) {
		u32 lost_packets = num_packets - 1;

		DCCP_WARN("Warning: large burst loss (%u)\n", lost_packets);
		/*
		 * We received 1 packet and have a loss of size "num_packets-1"
		 * which we squeeze into num_cells-1 rather than reserving an
		 * entire byte for each lost packet.
		 * The reason is that the vector grows in O(burst_length); when
		 * it grows too large there will no room left for the payload.
		 * This is a trade-off: if a few packets out of the burst show
		 * up later, their state will not be changed; it is simply too
		 * costly to reshuffle/reallocate/copy the buffer each time.
		 * Should such problems persist, we will need to switch to a
		 * different underlying data structure.
		 */
		for (num_packets = num_cells = 1; lost_packets; ++num_cells) {
			u8 len = min(lost_packets, (u32)DCCPAV_MAX_RUNLEN);

			av->av_buf_head = __ackvec_idx_sub(av->av_buf_head, 1);
			av->av_buf[av->av_buf_head] = DCCPAV_NOT_RECEIVED | len;

			lost_packets -= len;
		}
	}

	if (num_cells + dccp_ackvec_buflen(av) >= DCCPAV_MAX_ACKVEC_LEN) {
		DCCP_CRIT("Ack Vector buffer overflow: dropping old entries\n");
		av->av_overflow = true;
	}

	av->av_buf_head = __ackvec_idx_sub(av->av_buf_head, num_packets);
	if (av->av_overflow)
		av->av_buf_tail = av->av_buf_head;

	av->av_buf[av->av_buf_head] = state;
	av->av_buf_ackno	    = seqno;

	if (num_packets > 1)
		dccp_ackvec_reserve_seats(av, num_packets - 1);
}

/**
 * dccp_ackvec_input  -  Register incoming packet in the buffer
 */
void dccp_ackvec_input(struct dccp_ackvec *av, struct sk_buff *skb)
{
	u64 seqno = DCCP_SKB_CB(skb)->dccpd_seq;
	enum dccp_ackvec_states state = DCCPAV_RECEIVED;

	if (dccp_ackvec_is_empty(av)) {
		dccp_ackvec_add_new(av, 1, seqno, state);
		av->av_tail_ackno = seqno;

	} else {
		s64 num_packets = dccp_delta_seqno(av->av_buf_ackno, seqno);
		u8 *current_head = av->av_buf + av->av_buf_head;

		if (num_packets == 1 &&
		    dccp_ackvec_state(current_head) == state &&
		    dccp_ackvec_runlen(current_head) < DCCPAV_MAX_RUNLEN) {

			*current_head   += 1;
			av->av_buf_ackno = seqno;

		} else if (num_packets > 0) {
			dccp_ackvec_add_new(av, num_packets, seqno, state);
		} else {
			dccp_ackvec_update_old(av, num_packets, seqno, state);
		}
	}
}

/*
 * If several packets are missing, the HC-Receiver may prefer to enter multiple
 * bytes with run length 0, rather than a single byte with a larger run length;
 * this simplifies table updates if one of the missing packets arrives.
 */
static inline int dccp_ackvec_set_buf_head_state(struct dccp_ackvec *av,
						 const unsigned int packets,
						 const unsigned char state)
{
	unsigned int gap;
	long new_head;

	if (av->av_vec_len + packets > DCCPAV_MAX_ACKVEC_LEN)
		return -ENOBUFS;

	gap	 = packets - 1;
	new_head = av->av_buf_head - packets;

	if (new_head < 0) {
		if (gap > 0) {
			memset(av->av_buf, DCCPAV_NOT_RECEIVED,
			       gap + new_head + 1);
			gap = -new_head;
		}
		new_head += DCCPAV_MAX_ACKVEC_LEN;
	}

	av->av_buf_head = new_head;

	if (gap > 0)
		memset(av->av_buf + av->av_buf_head + 1,
		       DCCPAV_NOT_RECEIVED, gap);

	av->av_buf[av->av_buf_head] = state;
	av->av_vec_len += packets;
	return 0;
}

/*
 * Implements the RFC 4340, Appendix A
 */
int dccp_ackvec_add(struct dccp_ackvec *av, const struct sock *sk,
		    const u64 ackno, const u8 state)
{
	u8 *cur_head = av->av_buf + av->av_buf_head,
	   *buf_end  = av->av_buf + DCCPAV_MAX_ACKVEC_LEN;
	/*
	 * Check at the right places if the buffer is full, if it is, tell the
	 * caller to start dropping packets till the HC-Sender acks our ACK
	 * vectors, when we will free up space in av_buf.
	 *
	 * We may well decide to do buffer compression, etc, but for now lets
	 * just drop.
	 *
	 * From Appendix A.1.1 (`New Packets'):
	 *
	 *	Of course, the circular buffer may overflow, either when the
	 *	HC-Sender is sending data at a very high rate, when the
	 *	HC-Receiver's acknowledgements are not reaching the HC-Sender,
	 *	or when the HC-Sender is forgetting to acknowledge those acks
	 *	(so the HC-Receiver is unable to clean up old state). In this
	 *	case, the HC-Receiver should either compress the buffer (by
	 *	increasing run lengths when possible), transfer its state to
	 *	a larger buffer, or, as a last resort, drop all received
	 *	packets, without processing them whatsoever, until its buffer
	 *	shrinks again.
	 */

	/* See if this is the first ackno being inserted */
	if (av->av_vec_len == 0) {
		*cur_head = state;
		av->av_vec_len = 1;
	} else if (after48(ackno, av->av_buf_ackno)) {
		const u64 delta = dccp_delta_seqno(av->av_buf_ackno, ackno);

		/*
		 * Look if the state of this packet is the same as the
		 * previous ackno and if so if we can bump the head len.
		 */
		if (delta == 1 && dccp_ackvec_state(cur_head) == state &&
		    dccp_ackvec_runlen(cur_head) < DCCPAV_MAX_RUNLEN)
			*cur_head += 1;
		else if (dccp_ackvec_set_buf_head_state(av, delta, state))
			return -ENOBUFS;
	} else {
		/*
		 * A.1.2.  Old Packets
		 *
		 *	When a packet with Sequence Number S <= buf_ackno
		 *	arrives, the HC-Receiver will scan the table for
		 *	the byte corresponding to S. (Indexing structures
		 *	could reduce the complexity of this scan.)
		 */
		u64 delta = dccp_delta_seqno(ackno, av->av_buf_ackno);

		while (1) {
			const u8 len = dccp_ackvec_runlen(cur_head);
			/*
			 * valid packets not yet in av_buf have a reserved
			 * entry, with a len equal to 0.
			 */
			if (*cur_head == DCCPAV_NOT_RECEIVED && delta == 0) {
				dccp_pr_debug("Found %llu reserved seat!\n",
					      (unsigned long long)ackno);
				*cur_head = state;
				goto out;
			}
			/* len == 0 means one packet */
			if (delta < len + 1)
				goto out_duplicate;

			delta -= len + 1;
			if (++cur_head == buf_end)
				cur_head = av->av_buf;
		}
	}

	av->av_buf_ackno = ackno;
out:
	return 0;

out_duplicate:
	/* Duplicate packet */
	dccp_pr_debug("Received a dup or already considered lost "
		      "packet: %llu\n", (unsigned long long)ackno);
	return -EILSEQ;
}

static void dccp_ackvec_throw_record(struct dccp_ackvec *av,
				     struct dccp_ackvec_record *avr)
{
	struct dccp_ackvec_record *next;

	/* sort out vector length */
	if (av->av_buf_head <= avr->avr_ack_ptr)
		av->av_vec_len = avr->avr_ack_ptr - av->av_buf_head;
	else
		av->av_vec_len = DCCPAV_MAX_ACKVEC_LEN - 1 -
				 av->av_buf_head + avr->avr_ack_ptr;

	/* free records */
	list_for_each_entry_safe_from(avr, next, &av->av_records, avr_node) {
		list_del(&avr->avr_node);
		kmem_cache_free(dccp_ackvec_record_slab, avr);
	}
}

void dccp_ackvec_check_rcv_ackno(struct dccp_ackvec *av, struct sock *sk,
				 const u64 ackno)
{
	struct dccp_ackvec_record *avr;

	/*
	 * If we traverse backwards, it should be faster when we have large
	 * windows. We will be receiving ACKs for stuff we sent a while back
	 * -sorbo.
	 */
	list_for_each_entry_reverse(avr, &av->av_records, avr_node) {
		if (ackno == avr->avr_ack_seqno) {
			dccp_pr_debug("%s ACK packet 0, len=%d, ack_seqno=%llu, "
				      "ack_ackno=%llu, ACKED!\n",
				      dccp_role(sk), avr->avr_ack_runlen,
				      (unsigned long long)avr->avr_ack_seqno,
				      (unsigned long long)avr->avr_ack_ackno);
			dccp_ackvec_throw_record(av, avr);
			break;
		} else if (avr->avr_ack_seqno > ackno)
			break; /* old news */
	}
}

static void dccp_ackvec_check_rcv_ackvector(struct dccp_ackvec *av,
					    struct sock *sk, u64 *ackno,
					    const unsigned char len,
					    const unsigned char *vector)
{
	unsigned char i;
	struct dccp_ackvec_record *avr;

	/* Check if we actually sent an ACK vector */
	if (list_empty(&av->av_records))
		return;

	i = len;
	/*
	 * XXX
	 * I think it might be more efficient to work backwards. See comment on
	 * rcv_ackno. -sorbo.
	 */
	avr = list_entry(av->av_records.next, struct dccp_ackvec_record, avr_node);
	while (i--) {
		const u8 rl = dccp_ackvec_runlen(vector);
		u64 ackno_end_rl;

		dccp_set_seqno(&ackno_end_rl, *ackno - rl);

		/*
		 * If our AVR sequence number is greater than the ack, go
		 * forward in the AVR list until it is not so.
		 */
		list_for_each_entry_from(avr, &av->av_records, avr_node) {
			if (!after48(avr->avr_ack_seqno, *ackno))
				goto found;
		}
		/* End of the av_records list, not found, exit */
		break;
found:
		if (between48(avr->avr_ack_seqno, ackno_end_rl, *ackno)) {
			if (dccp_ackvec_state(vector) != DCCPAV_NOT_RECEIVED) {
				dccp_pr_debug("%s ACK vector 0, len=%d, "
					      "ack_seqno=%llu, ack_ackno=%llu, "
					      "ACKED!\n",
					      dccp_role(sk), len,
					      (unsigned long long)
					      avr->avr_ack_seqno,
					      (unsigned long long)
					      avr->avr_ack_ackno);
				dccp_ackvec_throw_record(av, avr);
				break;
			}
			/*
			 * If it wasn't received, continue scanning... we might
			 * find another one.
			 */
		}

		dccp_set_seqno(ackno, ackno_end_rl - 1);
		++vector;
	}
}

int dccp_ackvec_parse(struct sock *sk, const struct sk_buff *skb,
		      u64 *ackno, const u8 opt, const u8 *value, const u8 len)
{
	if (len > DCCP_SINGLE_OPT_MAXLEN)
		return -1;

	/* dccp_ackvector_print(DCCP_SKB_CB(skb)->dccpd_ack_seq, value, len); */
	dccp_ackvec_check_rcv_ackvector(dccp_sk(sk)->dccps_hc_rx_ackvec, sk,
					ackno, len, value);
	return 0;
}

/**
 * dccp_ackvec_clear_state  -  Perform house-keeping / garbage-collection
 * This routine is called when the peer acknowledges the receipt of Ack Vectors
 * up to and including @ackno. While based on on section A.3 of RFC 4340, here
 * are additional precautions to prevent corrupted buffer state. In particular,
 * we use tail_ackno to identify outdated records; it always marks the earliest
 * packet of group (2) in 11.4.2.
 */
void dccp_ackvec_clear_state(struct dccp_ackvec *av, const u64 ackno)
 {
	struct dccp_ackvec_record *avr, *next;
	u8 runlen_now, eff_runlen;
	s64 delta;

	avr = dccp_ackvec_lookup(&av->av_records, ackno);
	if (avr == NULL)
		return;
	/*
	 * Deal with outdated acknowledgments: this arises when e.g. there are
	 * several old records and the acks from the peer come in slowly. In
	 * that case we may still have records that pre-date tail_ackno.
	 */
	delta = dccp_delta_seqno(av->av_tail_ackno, avr->avr_ack_ackno);
	if (delta < 0)
		goto free_records;
	/*
	 * Deal with overlapping Ack Vectors: don't subtract more than the
	 * number of packets between tail_ackno and ack_ackno.
	 */
	eff_runlen = delta < avr->avr_ack_runlen ? delta : avr->avr_ack_runlen;

	runlen_now = dccp_ackvec_runlen(av->av_buf + avr->avr_ack_ptr);
	/*
	 * The run length of Ack Vector cells does not decrease over time. If
	 * the run length is the same as at the time the Ack Vector was sent, we
	 * free the ack_ptr cell. That cell can however not be freed if the run
	 * length has increased: in this case we need to move the tail pointer
	 * backwards (towards higher indices), to its next-oldest neighbour.
	 */
	if (runlen_now > eff_runlen) {

		av->av_buf[avr->avr_ack_ptr] -= eff_runlen + 1;
		av->av_buf_tail = __ackvec_idx_add(avr->avr_ack_ptr, 1);

		/* This move may not have cleared the overflow flag. */
		if (av->av_overflow)
			av->av_overflow = (av->av_buf_head == av->av_buf_tail);
	} else {
		av->av_buf_tail	= avr->avr_ack_ptr;
		/*
		 * We have made sure that avr points to a valid cell within the
		 * buffer. This cell is either older than head, or equals head
		 * (empty buffer): in both cases we no longer have any overflow.
		 */
		av->av_overflow	= 0;
	}

	/*
	 * The peer has acknowledged up to and including ack_ackno. Hence the
	 * first packet in group (2) of 11.4.2 is the successor of ack_ackno.
	 */
	av->av_tail_ackno = ADD48(avr->avr_ack_ackno, 1);

free_records:
	list_for_each_entry_safe_from(avr, next, &av->av_records, avr_node) {
		list_del(&avr->avr_node);
		kmem_cache_free(dccp_ackvec_record_slab, avr);
	}
}

int __init dccp_ackvec_init(void)
{
	dccp_ackvec_slab = kmem_cache_create("dccp_ackvec",
					     sizeof(struct dccp_ackvec), 0,
					     SLAB_HWCACHE_ALIGN, NULL);
	if (dccp_ackvec_slab == NULL)
		goto out_err;

	dccp_ackvec_record_slab = kmem_cache_create("dccp_ackvec_record",
					     sizeof(struct dccp_ackvec_record),
					     0, SLAB_HWCACHE_ALIGN, NULL);
	if (dccp_ackvec_record_slab == NULL)
		goto out_destroy_slab;

	return 0;

out_destroy_slab:
	kmem_cache_destroy(dccp_ackvec_slab);
	dccp_ackvec_slab = NULL;
out_err:
	DCCP_CRIT("Unable to create Ack Vector slab cache");
	return -ENOBUFS;
}

void dccp_ackvec_exit(void)
{
	if (dccp_ackvec_slab != NULL) {
		kmem_cache_destroy(dccp_ackvec_slab);
		dccp_ackvec_slab = NULL;
	}
	if (dccp_ackvec_record_slab != NULL) {
		kmem_cache_destroy(dccp_ackvec_record_slab);
		dccp_ackvec_record_slab = NULL;
	}
}