netdev - Re: [PATCH net-next] tcp: dont drop MTU reduction indications

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Message-Id: <20120723.005903.2179205107462793469.davem@davemloft.net>
Date:	Mon, 23 Jul 2012 00:59:03 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org, nanditad@...gle.com, ncardwell@...gle.com,
	maze@...gle.com, tore@....no, therbert@...gle.com
Subject: Re: [PATCH net-next] tcp: dont drop MTU reduction indications

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Mon, 23 Jul 2012 09:48:52 +0200

> From: Eric Dumazet <edumazet@...gle.com>
> 
> ICMP messages generated in output path if frame length is bigger than
> mtu are actually lost because socket is owned by user (doing the xmit)
> 
> One example is the ipgre_tunnel_xmit() calling 
> icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED, htonl(mtu));
> 
> We had a similar case fixed in commit a34a101e1e6 (ipv6: disable GSO on
> sockets hitting dst_allfrag).
> 
> Problem of such fix is that it relied on retransmit timers, so short tcp
> sessions paid a too big latency increase price.
> 
> This patch uses the tcp_release_cb() infrastructure so that MTU
> reduction messages (ICMP messages) are not lost, and no extra delay
> is added in TCP transmits.
> 
> Reported-by: Maciej Żenczykowski <maze@...gle.com>
> Diagnosed-by: Neal Cardwell <ncardwell@...gle.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied, thanks Eric.
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