Re: [Codel] [RFC PATCH] codel: ecn mark at target

[Eric Dumazet <eric.dumazet@gmail.com>]

On Mon, 2012-08-06 at 18:22 +0200, Richard Scheffenegger wrote:
> Well, as long as the window is large enough, the delayed ACKs shouldn't 
> matter, even if the ECE is delayed for 1 segment; the argument about delayed 
> ACKs when cwnd is 1 is also true for non-ECN flows; they would run better 
> when every segment is acked individually; but can the receiver tell, if the 
> sender is running at cwnd=1?
> 

It should just be safe instead than lazy.

Frankly what's the deal to send an ACK right now instead of in 40ms ?

> Perhaps, if it tracks the RTT of the flow (which has to work without TS, as 
> they are undefined for pure ACKs), and the number of segments seen during 
> one RTT...

Linux does have an heuristic to _not_ use delayed acks for the first
packets received (about 15 MSS), _because_ it assumes sender has a small
cwnd.

When receiving out of order packets, this logic is restarted, because we
want to send SACKS as soon as possible, not after a delayed ack.

Every time we suspect sender has a small cwnd, we enter the quickack
mode.

So when receiving CE segments, we should do the same. Thats a flaw in
current linux code to do nothing and expect more packets to come.

It just works because RTO triggers. A lot of bugs are hidden because of
various timers that take some emergency actions.

tcp_enter_quickack_mode() has all the needed tweaks.


diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 2fd2bc9..2e55337 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -237,6 +237,9 @@ static inline void TCP_ECN_check_ce(struct tcp_sock *tp, const struct sk_buff *s
 			tcp_enter_quickack_mode((struct sock *)tp);
 		break;
 	case INET_ECN_CE:
+		/* Better not delay acks, sender can have a very low cwnd */
+		tcp_enter_quickack_mode((struct sock *)tp);
+
 		tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
 		/* fallinto */
 	default: