svn commit: r333346 - head/sys/netinet

[Rodney W. Grimes]

> Author: mmacy
> Date: Tue May  8 02:22:34 2018
> New Revision: 333346
> URL: https://svnweb.freebsd.org/changeset/base/333346
> 
> Log:
>   Fix spurious retransmit recovery on low latency networks

Your commit message quality is defanitly much much better!!  Just one little nit...

>   
>   TCP's smoothed RTT (SRTT) can be much larger than an actual observed RTT. This can be either because of hz restricting the calculable RTT to 10ms in VMs or 1ms using the default 1000hz or simply because SRTT recently incorporated a larger value.

Can you wrap lines at 80 characters please.
Thanks,

>   If an ACK arrives before the calculated badrxtwin (now + SRTT):
>   tp->t_badrxtwin = ticks + (tp->t_srtt >> (TCP_RTT_SHIFT + 1));
>   
>   We'll erroneously reset snd_una to snd_max. If multiple segments were dropped and this happens repeatedly the transmit rate will be limited to 1MSS per RTO until we've retransmitted all drops.
>   
>   Reported by:	rstone
>   Reviewed by:	hiren, transport
>   Approved by:	sbruno
>   MFC after:	1 month
>   Differential Revision:	https://reviews.freebsd.org/D8556
> 
> Modified:
>   head/sys/netinet/tcp_input.c
>   head/sys/netinet/tcp_output.c
>   head/sys/netinet/tcp_timer.c
> 
> Modified: head/sys/netinet/tcp_input.c
> ==============================================================================
> --- head/sys/netinet/tcp_input.c	Tue May  8 01:39:45 2018	(r333345)
> +++ head/sys/netinet/tcp_input.c	Tue May  8 02:22:34 2018	(r333346)
> @@ -1682,6 +1682,9 @@ tcp_do_segment(struct mbuf *m, struct tcphdr *th, stru
>  		to.to_tsecr -= tp->ts_offset;
>  		if (TSTMP_GT(to.to_tsecr, tcp_ts_getticks()))
>  			to.to_tsecr = 0;
> +		else if (tp->t_flags & TF_PREVVALID &&
> +			 tp->t_badrxtwin != 0 && SEQ_LT(to.to_tsecr, tp->t_badrxtwin))
> +			cc_cong_signal(tp, th, CC_RTO_ERR);
>  	}
>  	/*
>  	 * Process options only when we get SYN/ACK back. The SYN case
> @@ -1794,9 +1797,10 @@ tcp_do_segment(struct mbuf *m, struct tcphdr *th, stru
>  				TCPSTAT_INC(tcps_predack);
>  
>  				/*
> -				 * "bad retransmit" recovery.
> +				 * "bad retransmit" recovery without timestamps.
>  				 */
> -				if (tp->t_rxtshift == 1 &&
> +				if ((to.to_flags & TOF_TS) == 0 &&
> +				    tp->t_rxtshift == 1 &&
>  				    tp->t_flags & TF_PREVVALID &&
>  				    (int)(ticks - tp->t_badrxtwin) < 0) {
>  					cc_cong_signal(tp, th, CC_RTO_ERR);
> @@ -2787,8 +2791,10 @@ process_ACK:
>  		 * original cwnd and ssthresh, and proceed to transmit where
>  		 * we left off.
>  		 */
> -		if (tp->t_rxtshift == 1 && tp->t_flags & TF_PREVVALID &&
> -		    (int)(ticks - tp->t_badrxtwin) < 0)
> +		if (tp->t_rxtshift == 1 &&
> +		    tp->t_flags & TF_PREVVALID &&
> +		    tp->t_badrxtwin &&
> +		    SEQ_LT(to.to_tsecr, tp->t_badrxtwin))
>  			cc_cong_signal(tp, th, CC_RTO_ERR);
>  
>  		/*
> 
> Modified: head/sys/netinet/tcp_output.c
> ==============================================================================
> --- head/sys/netinet/tcp_output.c	Tue May  8 01:39:45 2018	(r333345)
> +++ head/sys/netinet/tcp_output.c	Tue May  8 02:22:34 2018	(r333346)
> @@ -206,7 +206,7 @@ tcp_output(struct tcpcb *tp)
>  #if defined(IPSEC) || defined(IPSEC_SUPPORT)
>  	unsigned ipsec_optlen = 0;
>  #endif
> -	int idle, sendalot;
> +	int idle, sendalot, curticks;
>  	int sack_rxmit, sack_bytes_rxmt;
>  	struct sackhole *p;
>  	int tso, mtu;
> @@ -808,9 +808,12 @@ send:
>  		/* Timestamps. */
>  		if ((tp->t_flags & TF_RCVD_TSTMP) ||
>  		    ((flags & TH_SYN) && (tp->t_flags & TF_REQ_TSTMP))) {
> -			to.to_tsval = tcp_ts_getticks() + tp->ts_offset;
> +			curticks = tcp_ts_getticks();
> +			to.to_tsval = curticks + tp->ts_offset;
>  			to.to_tsecr = tp->ts_recent;
>  			to.to_flags |= TOF_TS;
> +			if (tp->t_rxtshift == 1)
> +				tp->t_badrxtwin = curticks;
>  		}
>  
>  		/* Set receive buffer autosizing timestamp. */
> 
> Modified: head/sys/netinet/tcp_timer.c
> ==============================================================================
> --- head/sys/netinet/tcp_timer.c	Tue May  8 01:39:45 2018	(r333345)
> +++ head/sys/netinet/tcp_timer.c	Tue May  8 02:22:34 2018	(r333346)
> @@ -693,7 +693,12 @@ tcp_timer_rexmt(void * xtp)
>  			tp->t_flags |= TF_WASCRECOVERY;
>  		else
>  			tp->t_flags &= ~TF_WASCRECOVERY;
> -		tp->t_badrxtwin = ticks + (tp->t_srtt >> (TCP_RTT_SHIFT + 1));
> +		if ((tp->t_flags & TF_RCVD_TSTMP) == 0)
> +			tp->t_badrxtwin = ticks + (tp->t_srtt >> (TCP_RTT_SHIFT + 1));
> +		/* In the event that we've negotiated timestamps
> +		 * badrxtwin will be set to the value that we set
> +		 * the retransmitted packet's to_tsval to by tcp_output
> +		 */
>  		tp->t_flags |= TF_PREVVALID;
>  	} else
>  		tp->t_flags &= ~TF_PREVVALID;
> 
> 

-- 
Rod Grimes                                                 rgrimes at freebsd.org