dccp ccid-3: Bug-Fix - Zero RTT is possible
In commit $(825de27d9e40b3117b29a79d412b7a4b78c5d815) (from 27th May, commit
message `dccp ccid-3: Fix "t_ipi explosion" bug'), the CCID-3 window counter
computation was fixed to cope with RTTs < 4 microseconds.
Such RTTs can be found e.g. when running CCID-3 over loopback. The fix removed
a check against RTT < 4, but introduced a divide-by-zero bug.
All steady-state RTTs in DCCP are filtered using dccp_sample_rtt(), which
ensures non-zero samples. However, a zero RTT is possible on initialisation,
when there is no RTT sample from the Request/Response exchange.
The fix is to use the fallback-RTT from RFC 4340, 3.4.
This is also better than just fixing update_win_count() since it allows other
parts of the code to always assume that the RTT is non-zero during the time
that the CCID is used.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c
index f813077..0474f4c 100644
--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -329,8 +329,14 @@
hctx->ccid3hctx_x = rfc3390_initial_rate(sk);
hctx->ccid3hctx_t_ld = now;
} else {
- /* Sender does not have RTT sample: X_pps = 1 pkt/sec */
- hctx->ccid3hctx_x = hctx->ccid3hctx_s;
+ /*
+ * Sender does not have RTT sample:
+ * - set fallback RTT (RFC 4340, 3.4) since a RTT value
+ * is needed in several parts (e.g. window counter);
+ * - set sending rate X_pps = 1pps as per RFC 3448, 4.2.
+ */
+ hctx->ccid3hctx_rtt = DCCP_FALLBACK_RTT;
+ hctx->ccid3hctx_x = hctx->ccid3hctx_s;
hctx->ccid3hctx_x <<= 6;
}
ccid3_update_send_interval(hctx);