dccp ccid-2: Ack Vector interface clean-up

This patch brings the Ack Vector interface up to date. Its main purpose is
to lay the basis for the subsequent patches of this set, which will use the
new data structure fields and routines.

There are no real algorithmic changes, rather an adaptation:

 (1) Replaced the static Ack Vector size (2) with a #define so that it can
     be adapted (with low loss / Ack Ratio, a value of 1 works, so 2 seems
     to be sufficient for the moment) and added a solution so that computing
     the ECN nonce will continue to work - even with larger Ack Vectors.

 (2) Replaced the #defines for Ack Vector states with a complete enum.

 (3) Replaced #defines to compute Ack Vector length and state with general
     purpose routines (inlines), and updated code to use these.

 (4) Added a `tail' field (conversion to circular buffer in subsequent patch).

 (5) Updated the (outdated) documentation for Ack Vector struct.

 (6) All sequence number containers now trimmed to 48 bits.

 (7) Removal of unused bits:
     * removed dccpav_ack_nonce from struct dccp_ackvec, since this is already
       redundantly stored in the `dccpavr_ack_nonce' (of Ack Vector record);
     * removed Elapsed Time for Ack Vectors (it was nowhere used);
     * replaced semantics of dccpavr_sent_len with dccpavr_ack_runlen, since
       the code needs to be able to remember the old run length; 
     * reduced the de-/allocation routines (redundant / duplicate tests).


Justification for removing Elapsed Time information [can be removed]:
---------------------------------------------------------------------
 1. The Elapsed Time information for Ack Vectors was nowhere used in the code.
 2. DCCP does not implement rate-based pacing of acknowledgments. The only
    recommendation for always including Elapsed Time is in section 11.3 of
    RFC 4340: "Receivers that rate-pace acknowledgements SHOULD [...]
    include Elapsed Time options". But such is not the case here.
 3. It does not really improve estimation accuracy. The Elapsed Time field only
    records the time between the arrival of the last acknowledgeable packet and
    the time the Ack Vector is sent out. Since Linux does not (yet) implement
    delayed Acks, the time difference will typically be small, since often the
    arrival of a data packet triggers sending feedback at the HC-receiver.


Justification for changes in de-/allocation routines [can be removed]:
----------------------------------------------------------------------
  * INIT_LIST_HEAD in dccp_ackvec_record_new was redundant, since the list
    pointers were later overwritten when the node was added via list_add();
  * dccp_ackvec_record_new() was called in a single place only;
  * calls to list_del_init() before calling dccp_ackvec_record_delete() were
    redundant, since subsequently the entire element was k-freed;
  * since all calls to dccp_ackvec_record_delete() were preceded to a call to
    list_del_init(), the WARN_ON test would never evaluate to true;
  * since all calls to dccp_ackvec_record_delete() were made from within
    list_for_each_entry_safe(), the test for avr == NULL was redundant;
  * list_empty() in ackvec_free was redundant, since the same condition is
    embedded in the loop condition of the subsequent list_for_each_entry_safe().

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>

1 files changed, 56 insertions, 47 deletions

diff --git a/net/dccp/ackvec.h b/net/dccp/ackvec.h
index 1c10814fdf72..df18f9030dbb 100644
--- a/net/dccp/ackvec.h
+++ b/net/dccp/ackvec.h
@@ -3,9 +3,9 @@
 /*
  *  net/dccp/ackvec.h
  *
- *  An implementation of the DCCP protocol
+ *  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@mandriva.com>
- *
  *      This program is free software; you can redistribute it and/or modify it
  *      under the terms of the GNU General Public License version 2 as
  *      published by the Free Software Foundation.
@@ -13,75 +13,84 @@
 
 #include <linux/dccp.h>
 #include <linux/compiler.h>
-#include <linux/ktime.h>
 #include <linux/list.h>
 #include <linux/types.h>
 
-/* We can spread an ack vector across multiple options */
-#define DCCP_MAX_ACKVEC_LEN (DCCP_SINGLE_OPT_MAXLEN * 2)
+/*
+ * Ack Vector buffer space is static, in multiples of %DCCP_SINGLE_OPT_MAXLEN,
+ * the maximum size of a single Ack Vector. Setting %DCCPAV_NUM_ACKVECS to 1
+ * will be sufficient for most cases of low Ack Ratios, using a value of 2 gives
+ * more headroom if Ack Ratio is higher or when the sender acknowledges slowly.
+ */
+#define DCCPAV_NUM_ACKVECS      2
+#define DCCPAV_MAX_ACKVEC_LEN   (DCCP_SINGLE_OPT_MAXLEN * DCCPAV_NUM_ACKVECS)
 
 /* Estimated minimum average Ack Vector length - used for updating MPS */
 #define DCCPAV_MIN_OPTLEN       16
 
-#define DCCP_ACKVEC_STATE_RECEIVED      0
-#define DCCP_ACKVEC_STATE_ECN_MARKED    (1 << 6)
-#define DCCP_ACKVEC_STATE_NOT_RECEIVED  (3 << 6)
+enum dccp_ackvec_states {
+        DCCPAV_RECEIVED =       0x00,
+        DCCPAV_ECN_MARKED =     0x40,
+        DCCPAV_RESERVED =       0x80,
+        DCCPAV_NOT_RECEIVED =   0xC0
+};
+#define DCCPAV_MAX_RUNLEN       0x3F
 
-#define DCCP_ACKVEC_STATE_MASK          0xC0 /* 11000000 */
-#define DCCP_ACKVEC_LEN_MASK            0x3F /* 00111111 */
+static inline u8 dccp_ackvec_runlen(const u8 *cell)
+{
+        return *cell & DCCPAV_MAX_RUNLEN;
+}
 
-/** struct dccp_ackvec - ack vector
- *
- * This data structure is the one defined in RFC 4340, Appendix A.
- *
- * @av_buf_head - circular buffer head
- * @av_buf_tail - circular buffer tail
- * @av_buf_ackno - ack # of the most recent packet acknowledgeable in the
- *                     buffer (i.e. %av_buf_head)
- * @av_buf_nonce - the one-bit sum of the ECN Nonces on all packets acked
- *                     by the buffer with State 0
- *
- * Additionally, the HC-Receiver must keep some information about the
- * Ack Vectors it has recently sent. For each packet sent carrying an
- * Ack Vector, it remembers four variables:
+static inline u8 dccp_ackvec_state(const u8 *cell)
+{
+        return *cell & ~DCCPAV_MAX_RUNLEN;
+}
+
+/** struct dccp_ackvec - Ack Vector main data structure
  *
- * @av_records - list of dccp_ackvec_record
- * @av_ack_nonce - the one-bit sum of the ECN Nonces for all State 0.
+ * This implements a fixed-size circular buffer within an array and is largely
+ * based on Appendix A of RFC 4340.
  *
- * @av_time - the time in usecs
- * @av_buf - circular buffer of acknowledgeable packets
+ * @av_buf:        circular buffer storage area
+ * @av_buf_head:   head index; begin of live portion in @av_buf
+ * @av_buf_tail:   tail index; first index _after_ the live portion in @av_buf
+ * @av_buf_ackno:  highest seqno of acknowledgeable packet recorded in @av_buf
+ * @av_buf_nonce:  ECN nonce sums, each covering subsequent segments of up to
+ *                 %DCCP_SINGLE_OPT_MAXLEN cells in the live portion of @av_buf
+ * @av_records:    list of %dccp_ackvec_record (Ack Vectors sent previously)
+ * @av_veclen:     length of the live portion of @av_buf
  */
 struct dccp_ackvec {
-        u64                     av_buf_ackno;
-        struct list_head        av_records;
-        ktime_t                 av_time;
+        u8                      av_buf[DCCPAV_MAX_ACKVEC_LEN];
         u16                     av_buf_head;
+        u16                     av_buf_tail;
+        u64                     av_buf_ackno:48;
+        bool                    av_buf_nonce[DCCPAV_NUM_ACKVECS];
+        struct list_head        av_records;
         u16                     av_vec_len;
-        u8                      av_buf_nonce;
-        u8                      av_ack_nonce;
-        u8                      av_buf[DCCP_MAX_ACKVEC_LEN];
 };
 
-/** struct dccp_ackvec_record - ack vector record
+/** struct dccp_ackvec_record - Records information about sent Ack Vectors
  *
- * ACK vector record as defined in Appendix A of spec.
+ * These list entries define the additional information which the HC-Receiver
+ * keeps about recently-sent Ack Vectors; again refer to RFC 4340, Appendix A.
  *
- * The list is sorted by avr_ack_seqno
+ * @avr_node:       the list node in @av_records
+ * @avr_ack_seqno:  sequence number of the packet the Ack Vector was sent on
+ * @avr_ack_ackno:  the Ack number that this record/Ack Vector refers to
+ * @avr_ack_ptr:    pointer into @av_buf where this record starts
+ * @avr_ack_runlen: run length of @avr_ack_ptr at the time of sending
+ * @avr_ack_nonce:  the sum of @av_buf_nonce's at the time this record was sent
  *
- * @avr_node - node in av_records
- * @avr_ack_seqno - sequence number of the packet this record was sent on
- * @avr_ack_ackno - sequence number being acknowledged
- * @avr_ack_ptr - pointer into av_buf where this record starts
- * @avr_ack_nonce - av_ack_nonce at the time this record was sent
- * @avr_sent_len - lenght of the record in av_buf
+ * The list as a whole is sorted in descending order by @avr_ack_seqno.
  */
 struct dccp_ackvec_record {
         struct list_head avr_node;
-        u64              avr_ack_seqno;
-        u64              avr_ack_ackno;
+        u64              avr_ack_seqno:48;
+        u64              avr_ack_ackno:48;
         u16              avr_ack_ptr;
-        u16              avr_sent_len;
-        u8               avr_ack_nonce;
+        u8               avr_ack_runlen;
+        u8               avr_ack_nonce:1;
 };
 
 struct sock;