Subject: Re: [net-next PATCH v2 2/3] net: TCP thin linear timeouts

From: Andreas Petlund <[email-masked]>
Date: Tue, 09 Feb 2010 17:40:20 +0100

On 02/08/2010 09:50 PM, Damian Lukowski wrote:
>>  out_reset_timer:
>> -	icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
>> +	/* If stream is thin, use linear timeouts. Since 'icsk_backoff' is
>> +	 * used to reset timer, set to 0. Recalculate 'icsk_rto' as this
>> +	 * might be increased if the stream oscillates between thin and thick,
>> +	 * thus the old value might already be too high compared to the value
>> +	 * set by 'tcp_set_rto' in tcp_input.c which resets the rto without
>> +	 * backoff. Limit to TCP_THIN_LT_RETRIES before initiating exponential
>> +	 * backoff behaviour to avoid continue hammering linear-timeout 
>> +	 * retransmissions into a black hole*/
>> +	if ((tp->thin_lt || sysctl_tcp_force_thin_linear_timeouts) &&
>> +	    tcp_stream_is_thin(sk) && sk->sk_state == TCP_ESTABLISHED &&
>> +	    icsk->icsk_retransmits <= TCP_THIN_LT_RETRIES) {
>> +		icsk->icsk_backoff = 0;
> 
> Hi,
> I think, this value should be at least 1, as icsk_backoff
> might be decreased to -1 and used for bit-shifting in tcp_v4_err().
> A lower boundary check might be even better.

Hi

Thanks for the feedback.

As far as I can see, the check a couple of lines above the decrementation
stops the icsk->icsk_backoff from being decremented if already zero.
Beyond that I cannot find any more places where this situation may arise.
Please correct me if I'm wrong and a boundary check is indeed warranted.

Excerpt from tcp_ipv4.c
------------------
                if (seq != tp->snd_una  || !icsk->icsk_retransmits ||
                    !icsk->icsk_backoff)
                        break;

                icsk->icsk_backoff--;
                inet_csk(sk)->icsk_rto = __tcp_set_rto(tp) <<
                                         icsk->icsk_backoff;
------------------

Best regards,
Andreas