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Commit 38024086 authored by Gerrit Renker's avatar Gerrit Renker
Browse files

dccp ccid-2: Update code for the Ack Vector input/registration routine 



This patch updates the code which registers new packets as received, using the
new circular buffer interface. It contributes a new algorithm which
 * supports both tail/head pointers and buffer wrap-around and
 * deals with overflow (head/tail move in lock-step).

Signed-off-by: default avatarGerrit Renker <gerrit@erg.abdn.ac.uk>
parent 5753fdfe

net/dccp/ackvec.c

+150 −0
Original line number Diff line number Diff line
@@ -131,6 +131,156 @@ u16 dccp_ackvec_buflen(const struct dccp_ackvec *av) 
	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;

net/dccp/ackvec.h

+4 −0
Original line number Diff line number Diff line
@@ -29,6 +29,9 @@ 
/* Estimated minimum average Ack Vector length - used for updating MPS */

#define DCCPAV_MIN_OPTLEN	16



/* Threshold for coping with large bursts of losses */

#define DCCPAV_BURST_THRESH	(DCCPAV_MAX_ACKVEC_LEN / 8)



enum dccp_ackvec_states {

	DCCPAV_RECEIVED =	0x00,

	DCCPAV_ECN_MARKED =	0x40,
@@ -116,6 +119,7 @@ extern int dccp_ackvec_parse(struct sock *sk, const struct sk_buff *skb, 
			     u64 *ackno, const u8 opt,

			     const u8 *value, const u8 len);



extern void dccp_ackvec_input(struct dccp_ackvec *av, struct sk_buff *skb);

extern int  dccp_ackvec_update_records(struct dccp_ackvec *av, u64 seq, u8 sum);

extern void dccp_ackvec_clear_state(struct dccp_ackvec *av, const u64 ackno);

extern u16  dccp_ackvec_buflen(const struct dccp_ackvec *av);