net-next-2.6 [PATCH 0/3] dccp: ccid-3 fixes and simplifications
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A message that is not in the archive (sent off-list or not yet received): dccp_ccid3_simplifications
Dave,
please find attached a set of 3 CCID-3 patches from the test tree. All have been in the test tree a long time and compile independently.
Patch #1: Fixes a bug and simplifies the packet scheduling algorithm. Patch #2: Replaces buggy RTT sampling history lookup. Patch #3: Simplifies the CCID-3 tx-option parsing routine.
This set is also available based on a fresh (today's) copy of net-next-2.6,
git://eden-feed.erg.abdn.ac.uk/net-next-2.6 [subtree 'dccp']
PATCH 1/3 dccp ccid-3: A lower bound for the inter-packet scheduling algorithm
This fixes a subtle bug in the calculation of the inter-packet gap and shows that t_delta, as it is currently used, is not needed.
The algorithm from RFC 5348, 8.3 below continually computes a send time t_nom, which is initialised with the current time t_now; t_gran = 1E6 / HZ specifies the scheduling granularity, s the packet size, and X the sending rate:
t_distance = t_nom - t_now; // in microseconds
t_delta = min(t_ipi, t_gran) / 2; // `delta' parameter in microseconds
if (t_distance >= t_delta) {
reschedule after (t_distance / 1000) milliseconds;
} else {
t_ipi = s / X; // inter-packet interval in usec
t_nom += t_ipi; // compute the next send time
send packet now;}
Problem:
--------
Rescheduling requires a conversion into milliseconds (sk_reset_timer()). The highest jiffy resolution with HZ00 is 1 millisecond, so using a higher granularity does not make much sense here.
As a consequence, values of t_distance < 1000 are truncated to 0. This issue has so far been resolved by using instead
if (t_distance >= t_delta + 1000)
reschedule after (t_distance / 1000) milliseconds;
This is unnecessarily large, a lower bound is t_delta' = max(t_delta, 1000). And it implies a further simplification:
a) when HZ >= 500, then t_delta <= t_gran/2 = 10^6/(2*HZ) <= 1000, so that
t_delta' = MAX(1000, t_delta) = 1000 (constant value);
b) when HZ < 500, then t_delta = 1/2*MIN(rtt, t_ipi, t_gran) <= t_gran/2,
so that 1000 <= t_delta' <= t_gran/2.
The maximum error of using a constant t_delta in (b) is less than half a jiffy.
Fix:
----
The patch replaces t_delta with a constant, whose value depends on CONFIG_HZ, changing the above algorithm to:
if (t_distance >= t_delta')
reschedule after (t_distance / 1000) milliseconds;
where t_delta' = 10^6/(2*HZ) if HZ < 500, and t_delta' = 1000 otherwise.
--- net/dccp/ccids/ccid3.h | 18 +++++++++++++----- net/dccp/ccids/ccid3.c | 19 ++++++++----------- 2 files changed, 21 insertions(+), 16 deletions(-)
net/dccp/ccids/ccid3.h +13 -5
@@ -45,12 +45,22 @@/* Two seconds as per RFC 5348, 4.2 */ #define TFRC_INITIAL_TIMEOUT (2 * USEC_PER_SEC) -/* In usecs - half the scheduling granularity as per RFC3448 4.6 */ -#define TFRC_OPSYS_HALF_TIME_GRAN (USEC_PER_SEC / (2 * HZ)) -/* Parameter t_mbi from [RFC 3448, 4.3]: backoff interval in seconds */ #define TFRC_T_MBI 64 +/* +* The t_delta parameter (RFC 5348, 8.3): delays of less than %USEC_PER_MSEC are +* rounded down to 0, since sk_reset_timer() here uses millisecond granularity. +* Hence we can use a constant t_delta = %USEC_PER_MSEC when HZ >= 500. A coarse +* resolution of HZ < 500 means that the error is below one timer tick (t_gran) +* when using the constant t_delta = t_gran / 2 = %USEC_PER_SEC / (2 * HZ). +*/ +#if (HZ >=500 ) +# define TFRC_T_DELTA USEC_PER_MSEC +#else +# define TFRC_T_DELTA (USEC_PER_SEC / (2 * HZ)) +#endif +enum ccid3_options {TFRC_OPT_LOSS_EVENT_RATE =192 ,TFRC_OPT_LOSS_INTERVALS =193 ,
@@ -90,7 +100,6 @@ enum ccid3_hc_tx_states {* @tx_no_feedback_timer: Handle to no feedback timer * @tx_t_ld: Time last doubled during slow start * @tx_t_nom: Nominal send time of next packet -* @tx_delta: Send timer delta (RFC 3448, 4.6) in usecs * @tx_hist: Packet history * @tx_options_received: Parsed set of retrieved options */
@@ -109,7 +118,6 @@ struct ccid3_hc_tx_sock {struct timer_list tx_no_feedback_timer;ktime_t tx_t_ld;ktime_t tx_t_nom; -u32 tx_delta;struct tfrc_tx_hist_entry *tx_hist;struct ccid3_options_received tx_options_received; };
net/dccp/ccids/ccid3.c +8 -11
@@ -91,19 +91,16 @@ static inline u64 rfc3390_initial_rate(struct sock *sk)return scaled_div (w_init <<6 , hc->tx_rtt); } -/* -* Recalculate t_ipi and delta (should be called whenever X changes) +/** +* ccid3_update_send_interval - Calculate new t_ipi = s / X_inst +* This respects the granularity of X_inst (64 * bytes/second). */ static void ccid3_update_send_interval (struct ccid3_hc_tx_sock *hc) { -/* Calculate new t_ipi = s / X_inst (X_inst is in 64 * bytes/second) */ hc->tx_t_ipi =scaled_div32 (((u64 )hc->tx_s) <<6 , hc->tx_x); -/* Calculate new delta by delta = min(t_ipi / 2, t_gran / 2) */ - hc->tx_delta =min_t (u32, hc->tx_t_ipi /2 ,TFRC_OPSYS_HALF_TIME_GRAN ); - -ccid3_pr_debug ("t_ipi=%u, delta=%u, s=%u, X=%u\n" , hc->tx_t_ipi, - hc->tx_delta, hc->tx_s, (unsigned )(hc->tx_x >>6 )); +ccid3_pr_debug ("t_ipi=%u, s=%u, X=%u\n" , hc->tx_t_ipi, + hc->tx_s, (unsigned )(hc->tx_x >>6 )); }static u32 ccid3_hc_tx_idle_rtt (struct ccid3_hc_tx_sock *hc,ktime_t now)
@@ -332,15 +329,15 @@ static int ccid3_hc_tx_send_packet(struct sock *sk, struct sk_buff *skb) delay =ktime_us_delta (hc->tx_t_nom, now);ccid3_pr_debug ("delay=%ld\n" , (long )delay);/* -* Scheduling of packet transmissions [RFC 3448, 4.6] +* Scheduling of packet transmissions (RFC 5348, 8.3) * * if (t_now > t_nom - delta) * // send the packet now * else * // send the packet in (t_nom - t_now) milliseconds. */ -if (delay - (s64 )hc->tx_delta >=1000 ) -return (u32 )delay /1000L ; +if (delay >=TFRC_T_DELTA ) +return (u32 )delay /USEC_PER_MSEC ;ccid3_hc_tx_update_win_count (hc, now);break ;
PATCH 2/3 dccp ccid-3: remove buggy RTT-sampling history lookup
This removes the RTT-sampling function tfrc_tx_hist_rtt(), since
1. it suffered from complex passing of return values (the return value both
indicated successful lookup while the value doubled as RTT sample);
2. when for some odd reason the sample value equalled 0, this triggered a bug
warning about "bogus Ack", due to the ambiguity of the return value;
3. on a passive host which has not sent anything the TX history is empty and
thus will lead to unwanted "bogus Ack" warnings such as
ccid3_hc_tx_packet_recv: server(e7b7d518): DATAACK with bogus ACK-28197148
ccid3_hc_tx_packet_recv: server(e7b7d518): DATAACK with bogus ACK-26641606.The fix is to replace the implicit encoding by performing the steps manually.
Furthermore, the "bogus Ack" warning has been removed, since it can actually be triggered due to several reasons (network reordering, old packet, (3) above), hence it is not very useful.
--- net/dccp/ccids/ccid3.c | 34 +++++++++++++++++------------- net/dccp/ccids/lib/packet_history.c | 39 ----------------------------------- net/dccp/ccids/lib/packet_history.h | 22 +++++++++++++++++-- 3 files changed, 38 insertions(+), 57 deletions(-)
net/dccp/ccids/ccid3.c +19 -15
@@ -370,6 +370,7 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb) {struct ccid3_hc_tx_sock *hc =ccid3_hc_tx_sk (sk);struct ccid3_options_received *opt_recv = &hc->tx_options_received; +struct tfrc_tx_hist_entry *acked;ktime_t now;unsigned long t_nfb;u32 pinv, r_sample;
@@ -383,17 +384,24 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb) hc->tx_state != TFRC_SSTATE_NO_FBACK)return ; - now =ktime_get_real (); - -/* Estimate RTT from history if ACK number is valid */ - r_sample =tfrc_tx_hist_rtt (hc->tx_hist, -DCCP_SKB_CB (skb)->dccpd_ack_seq, now); -if (r_sample =0 ) { -DCCP_WARN ("%s(%p): %s with bogus ACK-%llu\n" ,dccp_role (sk), sk, -dccp_packet_name (DCCP_SKB_CB (skb)->dccpd_type), - (unsigned long long )DCCP_SKB_CB (skb)->dccpd_ack_seq); +/* +* Locate the acknowledged packet in the TX history. +* +* Returning "entry not found" here can for instance happen when +* - the host has not sent out anything (e.g. a passive server), +* - the Ack is outdated (packet with higher Ack number was received), +* - it is a bogus Ack (for a packet not sent on this connection). +*/ + acked =tfrc_tx_hist_find_entry (hc->tx_hist,dccp_hdr_ack_seq (skb)); +if (acked =NULL )return ; - } +/* For the sake of RTT sampling, ignore/remove all older entries */ +tfrc_tx_hist_purge (&acked->next); + +/* Update the moving average for the RTT estimate (RFC 3448, 4.3) */ + now =ktime_get_real (); + r_sample =dccp_sample_rtt (sk,ktime_us_delta (now, acked->stamp)); + hc->tx_rtt =tfrc_ewma (hc->tx_rtt, r_sample,9 );/* Update receive rate in units of 64 * bytes/second */ hc->tx_x_recv = opt_recv->ccid3or_receive_rate;
@@ -405,11 +413,7 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb) hc->tx_p =0 ; else /* can not exceed 100% */ hc->tx_p = scaled_div(1, pinv); -/* -* Validate new RTT sample and update moving average -*/ - r_sample =dccp_sample_rtt (sk, r_sample); - hc->tx_rtt =tfrc_ewma (hc->tx_rtt, r_sample,9 ); +/* * Update allowed sending rate X as per draft rfc3448bis-00, 4.2/3 */
net/dccp/ccids/lib/packet_history.h +19 -3
@@ -40,12 +40,28 @@#include <linux/slab.h> #include "tfrc.h" -struct tfrc_tx_hist_entry ; +/** +* tfrc_tx_hist_entry - Simple singly-linked TX history list +* @next: next oldest entry (LIFO order) +* @seqno: sequence number of this entry +* @stamp: send time of packet with sequence number @seqno +*/ +struct tfrc_tx_hist_entry { +struct tfrc_tx_hist_entry *next; +u64 seqno; +ktime_t stamp; +}; + +static inline struct tfrc_tx_hist_entry * +tfrc_tx_hist_find_entry (struct tfrc_tx_hist_entry *head,u64 seqno) +{ +while (head !=NULL && head->seqno != seqno) + head = head->next; +return head; +}extern int tfrc_tx_hist_add (struct tfrc_tx_hist_entry **headp,u64 seqno);extern void tfrc_tx_hist_purge (struct tfrc_tx_hist_entry **headp); -extern u32 tfrc_tx_hist_rtt (struct tfrc_tx_hist_entry *head, -const u64 seqno,const ktime_t now);/* Subtraction a-b modulo-16, respects circular wrap-around */ #define SUB16 (a, b) (((a) + 16 - (b)) & 0xF)
net/dccp/ccids/lib/packet_history.c +0 -39
@@ -38,18 +38,6 @@#include "packet_history.h" #include "../../dccp.h" -/** -* tfrc_tx_hist_entry - Simple singly-linked TX history list -* @next: next oldest entry (LIFO order) -* @seqno: sequence number of this entry -* @stamp: send time of packet with sequence number @seqno -*/ -struct tfrc_tx_hist_entry { -struct tfrc_tx_hist_entry *next; -u64 seqno; -ktime_t stamp; -}; -/* * Transmitter History Routines */
@@ -71,15 +59,6 @@ void tfrc_tx_packet_history_exit(void) } } -static struct tfrc_tx_hist_entry * -tfrc_tx_hist_find_entry (struct tfrc_tx_hist_entry *head,u64 seqno) -{ -while (head !=NULL && head->seqno != seqno) - head = head->next; - -return head; -} -int tfrc_tx_hist_add (struct tfrc_tx_hist_entry **headp,u64 seqno) {struct tfrc_tx_hist_entry *entry =kmem_cache_alloc (tfrc_tx_hist_slab,gfp_any ());
@@ -107,24 +86,6 @@ void tfrc_tx_hist_purge(struct tfrc_tx_hist_entry **headp) *headp =NULL ; } -u32 tfrc_tx_hist_rtt (struct tfrc_tx_hist_entry *head,const u64 seqno, -const ktime_t now) -{ -u32 rtt =0 ; -struct tfrc_tx_hist_entry *packet =tfrc_tx_hist_find_entry (head, seqno); - -if (packet !=NULL ) { - rtt =ktime_us_delta (now, packet->stamp); -/* -* Garbage-collect older (irrelevant) entries: -*/ -tfrc_tx_hist_purge (&packet->next); - } - -return rtt; -} - -/* * Receiver History Routines */
PATCH 3/3 dccp ccid-3: Simplify and consolidate tx_parse_options
This simplifies and consolidates the TX option-parsing code:
1. The Loss Intervals option is not currently used, so dead code related to this option is removed. I am aware of no plans to support the option, but if someone wants to implement it (e.g. for inter-op tests), it is better to start afresh than having to also update currently unused code.
2. The Loss Event and Receive Rate options have a lot of code in common (both are 32 bit, both have same length etc.), so this is consolidated.
3. The test against GSR is not necessary, because
- on first loading CCID3, ccid_new() zeroes out all fields in the socket;
- ccid3_hc_tx_packet_recv() treats 0 and ~0U equivalently, due to
pinv = opt_recv->ccid3or_loss_event_rate;
if (pinv = ~0U || pinv = 0)
hctx->p = 0;
- as a result, the sequence number field is removed from opt_recv.
--- net/dccp/ccids/ccid3.h | 3 -- net/dccp/ccids/ccid3.c | 57 +++++++++++------------------------------------ 2 files changed, 14 insertions(+), 46 deletions(-)
net/dccp/ccids/ccid3.h +0 -3
@@ -68,9 +68,6 @@ enum ccid3_options { };struct ccid3_options_received { -u64 ccid3or_seqno:48 , - ccid3or_loss_intervals_idx:16 ; -u16 ccid3or_loss_intervals_len;u32 ccid3or_loss_event_rate;u32 ccid3or_receive_rate; };
net/dccp/ccids/ccid3.c +14 -43
74 lines not on this page · raw message
David Miller
maintainerFrom: Gerrit Renker <[email protected]>
Date: Wed, 15 Sep 2010 14:10:20 +0200
Quoted 10 lines
please find attached a set of 3 CCID-3 patches from the test tree. All have been in the test tree a long time and compile independently.
Patch #1: Fixes a bug and simplifies the packet scheduling algorithm. Patch #2: Replaces buggy RTT sampling history lookup. Patch #3: Simplifies the CCID-3 tx-option parsing routine.
This set is also available based on a fresh (today's) copy of net-next-2.6,
git://eden-feed.erg.abdn.ac.uk/net-next-2.6 [subtree 'dccp']
Looks good, pulled, thanks a lot Gerrit!