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: Gerrit Renker <gerrit@erg.abdn.ac.uk>

2 files changed, 154 insertions, 0 deletions

diff --git a/net/dccp/ackvec.c b/net/dccp/ackvec.c
index e9a0f66e4afe..f7e647e608bb 100644
--- a/net/dccp/ackvec.c
+++ b/net/dccp/ackvec.c
@@ -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;
diff --git a/net/dccp/ackvec.h b/net/dccp/ackvec.h
index 3f7008187b8e..21c4212d1be0 100644
--- a/net/dccp/ackvec.h
+++ b/net/dccp/ackvec.h
@@ -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);