[PATCH 1/7] dccp ccid-3: Bug-Fix - Zero RTT is possible

[Gerrit Renker <gerrit@xxxxxxxxxxxxxx>]

Commit 825de27d9e40b3117b29a79d412b7a4b78c5d815 fixed the CCID-3 window counter
computation for RTTs < 4 microseconds (as happens on loopback).

Since it uses the term "1/RTT", it needs to be protected against divide-by-zero,
done in established state using dccp_sample_rtt().

But there was an oversight, as a zero RTT can happen on sender 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 a (fallback) RTT value is available.

Signed-off-by: Gerrit Renker <gerrit@xxxxxxxxxxxxxx>
---
 net/dccp/ccids/ccid3.c |   10 ++++++++--
 1 files changed, 8 insertions(+), 2 deletions(-)

--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -329,8 +329,14 @@ 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 */
-			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);
--
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