Commit 3294f202 authored Jun 11, 2008 by Gerrit Renker

dccp ccid-3: Bug-Fix - Zero RTT is possible

In commit $(825de27d) (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>

parent 513fd370

net/dccp/ccids/ccid3.c

+8 −2

Original line number	Diff line number	Diff line
		@@ -329,7 +329,13 @@ static int ccid3_hc_tx_send_packet(struct sock sk, struct sk_buff skb)
		hctx->ccid3hctx_x = rfc3390_initial_rate(sk);
		hctx->ccid3hctx_t_ld = now;
		} else {
		/* Sender does not have RTT sample: X_pps = 1 pkt/sec */
		/*
		* 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;
		}

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