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dccp ccid-3: Always perform receiver RTT sampling
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This updates the CCID-3 receiver in part with regard to errata 610 and 611
(http://www.rfc-editor.org/errata_list.php), which change RFC 4342 to use the
Receive Rate as specified in rfc3448bis, requiring to constantly sample the
RTT (or use a sender RTT).

Doing this requires reusing the RX history structure after dealing with a loss.

The patch does not resolve how to compute X_recv if the interval is less
than 1 RTT. A FIXME has been added (and is resolved in subsequent patch).

Furthermore, since this is all TFRC-based functionality, the RTT estimation
is now also performed by the dccp_tfrc_lib module. This further simplifies
the CCID-3 code.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
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Gerrit Renker committed Sep 4, 2008
1 parent 2f3e3bb commit 2b81143
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Showing 4 changed files with 73 additions and 49 deletions.
43 changes: 14 additions & 29 deletions net/dccp/ccids/ccid3.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -556,8 +556,8 @@ static void ccid3_hc_rx_send_feedback(struct sock *sk,
* would bring X down to s/t_mbi. That is why we return
* X_recv according to rfc3448bis-06 for the moment.
*/
u32 rtt = hcrx->rtt ? : DCCP_FALLBACK_RTT,
s = tfrc_rx_hist_packet_size(&hcrx->hist);
u32 s = tfrc_rx_hist_packet_size(&hcrx->hist),
rtt = tfrc_rx_hist_rtt(&hcrx->hist);

hcrx->x_recv = scaled_div32(s, 2 * rtt);
break;
Expand All @@ -576,6 +576,11 @@ static void ccid3_hc_rx_send_feedback(struct sock *sk,
break;
/* fall through */
case CCID3_FBACK_PERIODIC:
/*
* FIXME: check if delta is less than or equal to 1 RTT using
* the receiver RTT sample. This is described in Errata 610/611
* of RFC 4342 which reference section 6.2 of RFC 3448.
*/
delta = ktime_us_delta(now, hcrx->tstamp_last_feedback);
if (delta <= 0)
DCCP_BUG("delta (%ld) <= 0", (long)delta);
Expand Down Expand Up @@ -633,8 +638,8 @@ static int ccid3_hc_rx_insert_options(struct sock *sk, struct sk_buff *skb)
static u32 ccid3_first_li(struct sock *sk)
{
struct ccid3_hc_rx_sock *hcrx = ccid3_hc_rx_sk(sk);
u32 x_recv, p, delta,
s = tfrc_rx_hist_packet_size(&hcrx->hist);
u32 s = tfrc_rx_hist_packet_size(&hcrx->hist),
rtt = tfrc_rx_hist_rtt(&hcrx->hist), x_recv, p, delta;
u64 fval;

/*
Expand All @@ -645,11 +650,6 @@ static u32 ccid3_first_li(struct sock *sk)
if (unlikely(hcrx->feedback == CCID3_FBACK_NONE))
return 5;

if (hcrx->rtt == 0) {
DCCP_WARN("No RTT estimate available, using fallback RTT\n");
hcrx->rtt = DCCP_FALLBACK_RTT;
}

delta = ktime_to_us(net_timedelta(hcrx->tstamp_last_feedback));
x_recv = scaled_div32(hcrx->hist.bytes_recvd, delta);
if (x_recv == 0) { /* would also trigger divide-by-zero */
Expand All @@ -661,7 +661,7 @@ static u32 ccid3_first_li(struct sock *sk)
x_recv = hcrx->x_recv;
}

fval = scaled_div32(scaled_div(s, hcrx->rtt), x_recv);
fval = scaled_div32(scaled_div(s, rtt), x_recv);
p = tfrc_calc_x_reverse_lookup(fval);

ccid3_pr_debug("%s(%p), receive rate=%u bytes/s, implied "
Expand Down Expand Up @@ -695,26 +695,11 @@ static void ccid3_hc_rx_packet_recv(struct sock *sk, struct sk_buff *skb)
if (tfrc_rx_hist_loss_pending(&hcrx->hist))
return; /* done receiving */

/*
* Handle data packets: RTT sampling and monitoring p
*/
if (unlikely(!is_data_packet))
goto update_records;

if (!tfrc_lh_is_initialised(&hcrx->li_hist)) {
const u32 sample = tfrc_rx_hist_sample_rtt(&hcrx->hist, skb);
/*
* Empty loss history: no loss so far, hence p stays 0.
* Sample RTT values, since an RTT estimate is required for the
* computation of p when the first loss occurs; RFC 3448, 6.3.1.
*/
if (sample != 0)
hcrx->rtt = tfrc_ewma(hcrx->rtt, sample, 9);
}
/*
* Check if the periodic once-per-RTT feedback is due; RFC 4342, 10.3
*/
if (SUB16(dccp_hdr(skb)->dccph_ccval, hcrx->last_counter) > 3)
if (is_data_packet &&
SUB16(dccp_hdr(skb)->dccph_ccval, hcrx->last_counter) > 3)
do_feedback = CCID3_FBACK_PERIODIC;

update_records:
Expand Down Expand Up @@ -744,7 +729,7 @@ static void ccid3_hc_rx_exit(struct sock *sk)
static void ccid3_hc_rx_get_info(struct sock *sk, struct tcp_info *info)
{
info->tcpi_options |= TCPI_OPT_TIMESTAMPS;
info->tcpi_rcv_rtt = ccid3_hc_rx_sk(sk)->rtt;
info->tcpi_rcv_rtt = tfrc_rx_hist_rtt(&ccid3_hc_rx_sk(sk)->hist);
}

static int ccid3_hc_rx_getsockopt(struct sock *sk, const int optname, int len,
Expand All @@ -759,7 +744,7 @@ static int ccid3_hc_rx_getsockopt(struct sock *sk, const int optname, int len,
if (len < sizeof(rx_info))
return -EINVAL;
rx_info.tfrcrx_x_recv = hcrx->x_recv;
rx_info.tfrcrx_rtt = hcrx->rtt;
rx_info.tfrcrx_rtt = tfrc_rx_hist_rtt(&hcrx->hist);
rx_info.tfrcrx_p = tfrc_invert_loss_event_rate(hcrx->p_inverse);
len = sizeof(rx_info);
val = &rx_info;
Expand Down
2 changes: 0 additions & 2 deletions net/dccp/ccids/ccid3.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -127,7 +127,6 @@ enum ccid3_fback_type {
* @last_counter - Tracks window counter (RFC 4342, 8.1)
* @feedback - The type of the feedback last sent
* @x_recv - Receiver estimate of send rate (RFC 3448, sec. 4.3)
* @rtt - Receiver estimate of RTT
* @tstamp_last_feedback - Time at which last feedback was sent
* @hist - Packet history (loss detection + RTT sampling)
* @li_hist - Loss Interval database
Expand All @@ -137,7 +136,6 @@ struct ccid3_hc_rx_sock {
u8 last_counter:4;
enum ccid3_fback_type feedback:4;
u32 x_recv;
u32 rtt;
ktime_t tstamp_last_feedback;
struct tfrc_rx_hist hist;
struct tfrc_loss_hist li_hist;
Expand Down
60 changes: 44 additions & 16 deletions net/dccp/ccids/lib/packet_history.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -151,14 +151,31 @@ int tfrc_rx_hist_duplicate(struct tfrc_rx_hist *h, struct sk_buff *skb)
}
EXPORT_SYMBOL_GPL(tfrc_rx_hist_duplicate);


static void __tfrc_rx_hist_swap(struct tfrc_rx_hist *h, const u8 a, const u8 b)
{
struct tfrc_rx_hist_entry *tmp = h->ring[a];

h->ring[a] = h->ring[b];
h->ring[b] = tmp;
}

static void tfrc_rx_hist_swap(struct tfrc_rx_hist *h, const u8 a, const u8 b)
{
const u8 idx_a = tfrc_rx_hist_index(h, a),
idx_b = tfrc_rx_hist_index(h, b);
struct tfrc_rx_hist_entry *tmp = h->ring[idx_a];
__tfrc_rx_hist_swap(h, tfrc_rx_hist_index(h, a),
tfrc_rx_hist_index(h, b));
}

h->ring[idx_a] = h->ring[idx_b];
h->ring[idx_b] = tmp;
/**
* tfrc_rx_hist_resume_rtt_sampling - Prepare RX history for RTT sampling
* This is called after loss detection has finished, when the history entry
* with the index of `loss_count' holds the highest-received sequence number.
* RTT sampling requires this information at ring[0] (tfrc_rx_hist_sample_rtt).
*/
static inline void tfrc_rx_hist_resume_rtt_sampling(struct tfrc_rx_hist *h)
{
__tfrc_rx_hist_swap(h, 0, tfrc_rx_hist_index(h, h->loss_count));
h->loss_count = h->loss_start = 0;
}

/*
Expand Down Expand Up @@ -200,8 +217,7 @@ static void __one_after_loss(struct tfrc_rx_hist *h, struct sk_buff *skb, u32 n2

if (dccp_loss_free(s2, s1, n1)) {
/* hole is filled: S0, S2, and S1 are consecutive */
h->loss_count = 0;
h->loss_start = tfrc_rx_hist_index(h, 1);
tfrc_rx_hist_resume_rtt_sampling(h);
} else
/* gap between S2 and S1: just update loss_prev */
tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_loss_prev(h), skb, n2);
Expand Down Expand Up @@ -254,8 +270,7 @@ static int __two_after_loss(struct tfrc_rx_hist *h, struct sk_buff *skb, u32 n3)

if (dccp_loss_free(s1, s2, n2)) {
/* entire hole filled by S0, S3, S1, S2 */
h->loss_start = tfrc_rx_hist_index(h, 2);
h->loss_count = 0;
tfrc_rx_hist_resume_rtt_sampling(h);
} else {
/* gap remains between S1 and S2 */
h->loss_start = tfrc_rx_hist_index(h, 1);
Expand Down Expand Up @@ -299,8 +314,7 @@ static void __three_after_loss(struct tfrc_rx_hist *h)

if (dccp_loss_free(s2, s3, n3)) {
/* no gap between S2 and S3: entire hole is filled */
h->loss_start = tfrc_rx_hist_index(h, 3);
h->loss_count = 0;
tfrc_rx_hist_resume_rtt_sampling(h);
} else {
/* gap between S2 and S3 */
h->loss_start = tfrc_rx_hist_index(h, 2);
Expand Down Expand Up @@ -340,6 +354,7 @@ int tfrc_rx_handle_loss(struct tfrc_rx_hist *h,

if (h->loss_count == 0) {
__do_track_loss(h, skb, ndp);
tfrc_rx_hist_sample_rtt(h, skb);
} else if (h->loss_count == 1) {
__one_after_loss(h, skb, ndp);
} else if (h->loss_count != 2) {
Expand Down Expand Up @@ -435,11 +450,24 @@ static inline struct tfrc_rx_hist_entry *
* Based on ideas presented in RFC 4342, 8.1. Returns 0 if it was not able
* to compute a sample with given data - calling function should check this.
*/
u32 tfrc_rx_hist_sample_rtt(struct tfrc_rx_hist *h, const struct sk_buff *skb)
void tfrc_rx_hist_sample_rtt(struct tfrc_rx_hist *h, const struct sk_buff *skb)
{
u32 sample = 0,
delta_v = SUB16(dccp_hdr(skb)->dccph_ccval,
tfrc_rx_hist_rtt_last_s(h)->tfrchrx_ccval);
u32 sample = 0, delta_v;

/*
* When not to sample:
* - on non-data packets
* (RFC 4342, 8.1: CCVal only fully defined for data packets);
* - when no data packets have been received yet
* (FIXME: using sampled packet size as indicator here);
* - as long as there are gaps in the sequence space (pending loss).
*/
if (!dccp_data_packet(skb) || h->packet_size == 0 ||
tfrc_rx_hist_loss_pending(h))
return;

delta_v = SUB16(dccp_hdr(skb)->dccph_ccval,
tfrc_rx_hist_rtt_last_s(h)->tfrchrx_ccval);

if (delta_v < 1 || delta_v > 4) { /* unsuitable CCVal delta */
if (h->rtt_sample_prev == 2) { /* previous candidate stored */
Expand Down Expand Up @@ -479,6 +507,6 @@ u32 tfrc_rx_hist_sample_rtt(struct tfrc_rx_hist *h, const struct sk_buff *skb)
h->rtt_sample_prev = 0; /* use current entry as next reference */
keep_ref_for_next_time:

return sample;
h->rtt_estimate = tfrc_ewma(h->rtt_estimate, sample, 9);
}
EXPORT_SYMBOL_GPL(tfrc_rx_hist_sample_rtt);
17 changes: 15 additions & 2 deletions net/dccp/ccids/lib/packet_history.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -91,14 +91,18 @@ struct tfrc_rx_hist_entry {
* @loss_count: Number of entries in circular history
* @loss_start: Movable index (for loss detection)
* @rtt_sample_prev: Used during RTT sampling, points to candidate entry
* @rtt_estimate: Receiver RTT estimate
* @packet_size: Packet size in bytes (as per RFC 3448, 3.1)
* @bytes_recvd: Number of bytes received since last sending feedback
*/
struct tfrc_rx_hist {
struct tfrc_rx_hist_entry *ring[TFRC_NDUPACK + 1];
u8 loss_count:2,
loss_start:2;
/* Receiver RTT sampling */
#define rtt_sample_prev loss_start
u32 rtt_estimate;
/* Receiver sampling of application payload lengths */
u32 packet_size,
bytes_recvd;
};
Expand Down Expand Up @@ -154,6 +158,15 @@ static inline u32 tfrc_rx_hist_packet_size(const struct tfrc_rx_hist *h)
return TCP_MIN_RCVMSS;
}
return h->packet_size;

}
static inline u32 tfrc_rx_hist_rtt(const struct tfrc_rx_hist *h)
{
if (h->rtt_estimate == 0) {
DCCP_WARN("No RTT estimate available, using fallback RTT\n");
return DCCP_FALLBACK_RTT;
}
return h->rtt_estimate;
}

extern void tfrc_rx_hist_add_packet(struct tfrc_rx_hist *h,
Expand All @@ -167,8 +180,8 @@ extern int tfrc_rx_handle_loss(struct tfrc_rx_hist *h,
struct sk_buff *skb, const u64 ndp,
u32 (*first_li)(struct sock *sk),
struct sock *sk);
extern u32 tfrc_rx_hist_sample_rtt(struct tfrc_rx_hist *h,
const struct sk_buff *skb);
extern void tfrc_rx_hist_sample_rtt(struct tfrc_rx_hist *h,
const struct sk_buff *skb);
extern int tfrc_rx_hist_init(struct tfrc_rx_hist *h, struct sock *sk);
extern void tfrc_rx_hist_purge(struct tfrc_rx_hist *h);

Expand Down

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