Problem:
Large backlogs of packets will accumulate in the CCID3 TX module when
(i) the application idles and/or
(ii) the application emits at a rate slower than the allowed rate X/s and/or
(iii) due to scheduling inaccuracy (resolution only up to HZ).
The consequence is that a huge burst of packets can be sent immediately, which
violates the allowed sending rate and can (worst case) choke the network.
Furthermore (iii) is especially likely when using high-speed (Gbit) links.
Fix:
Avoid any backlog of sending time which is greater than one whole t_ipi. This
is a conservative tight bound which will ensure safe operation with regard to
the allowed fair-share rate in a wide range of possible hardware configurations.
The value is derived below with a simple model, showing that this choice is safe
from an operational point of view.
Signed-off-by: Gerrit Renker <gerrit@xxxxxxxxxxxxxx>
Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxxxxxxxxxx>
---
net/dccp/ccids/ccid3.c | 12 ++++++++++--
1 files changed, 10 insertions(+), 2 deletions(-)
diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c
index d7d9ce7..c623761 100644
--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -362,7 +362,15 @@ static int ccid3_hc_tx_send_packet(struct sock *sk, struct sk_buff *skb)
case TFRC_SSTATE_NO_FBACK:
case TFRC_SSTATE_FBACK:
delay = timeval_delta(&hctx->ccid3hctx_t_nom, &now);
- ccid3_pr_debug("delay=%ld\n", (long)delay);
+ /*
+ * Lagging behind for more than a full t_ipi: when this occurs,
+ * a send credit accrues which causes packet storms, violating
+ * even the average allowed sending rate. This case happens if
+ * the application idles for some time, or if it emits packets
+ * at a rate smaller than X/s. Avoid such accumulation.
+ */
+ if (delay + (suseconds_t)hctx->ccid3hctx_t_ipi < 0)
+ hctx->ccid3hctx_t_nom = now;
/*
* Scheduling of packet transmissions [RFC 3448, 4.6]
*
@@ -371,7 +379,7 @@ static int ccid3_hc_tx_send_packet(struct sock *sk, struct sk_buff *skb)
* else
* // send the packet in (t_nom - t_now) milliseconds.
*/
- if (delay - (suseconds_t)hctx->ccid3hctx_delta >= 0)
+ else if (delay - (suseconds_t)hctx->ccid3hctx_delta >= 0)
return delay / 1000L;
ccid3_hc_tx_update_win_count(hctx, &now);
--
1.5.0.6
-
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