From b3d14bff12a38ad13a174eb0cc83d2ac7169eee4 Mon Sep 17 00:00:00 2001 From: Gerrit Renker Date: Wed, 10 Nov 2010 21:21:35 +0100 Subject: dccp ccid-2: Implementation of circular Ack Vector buffer with overflow handling This completes the implementation of a circular buffer for Ack Vectors, by extending the current (linear array-based) implementation. The changes are: (a) An `overflow' flag to deal with the case of overflow. As before, dynamic growth of the buffer will not be supported; but code will be added to deal robustly with overflowing Ack Vector buffers. (b) A `tail_seqno' field. When naively implementing the algorithm of Appendix A in RFC 4340, problems arise whenever subsequent Ack Vector records overlap, which can bring the entire run length calculation completely out of synch. (This is documented on http://www.erg.abdn.ac.uk/users/gerrit/dccp/notes/\ ack_vectors/tracking_tail_ackno/ .) (c) The buffer length is now computed dynamically (i.e. current fill level), as the span between head to tail. As a result, dccp_ackvec_pending() is now simpler - the #ifdef is no longer necessary since buf_empty is always true when IP_DCCP_ACKVEC is not configured. Signed-off-by: Gerrit Renker --- net/dccp/ackvec.c | 31 ++++++++++++++++++++++++++++++- 1 file changed, 30 insertions(+), 1 deletion(-) (limited to 'net/dccp/ackvec.c') diff --git a/net/dccp/ackvec.c b/net/dccp/ackvec.c index af976fca407a..abaf241c7353 100644 --- a/net/dccp/ackvec.c +++ b/net/dccp/ackvec.c @@ -29,7 +29,7 @@ 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 = DCCPAV_MAX_ACKVEC_LEN - 1; + av->av_buf_head = av->av_buf_tail = DCCPAV_MAX_ACKVEC_LEN - 1; INIT_LIST_HEAD(&av->av_records); } return av; @@ -71,6 +71,14 @@ int dccp_ackvec_update_records(struct dccp_ackvec *av, u64 seqno, u8 nonce_sum) 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. @@ -84,6 +92,27 @@ int dccp_ackvec_update_records(struct dccp_ackvec *av, u64 seqno, u8 nonce_sum) return 0; } +/* + * 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); +} + /* * 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; -- cgit v1.2.3-59-g8ed1b