Guido found another bug. There is a situation with
timestamped TCP packets where FreeBSD will send DATA+FIN and A W2K box will ack just the DATA portion. If this occurs after FreeBSD has done a (NewReno) fast-retransmit and is recovering it (dupacks > threshold) it triggers a case in tcp_newreno_partial_ack() (tcp_newreno() in stable) where tcp_output() is called with the expectation that the retransmit timer will be reloaded. But tcp_output() falls through and returns without doing anything, causing the persist timer to be loaded instead. This causes the connection to hang until W2K gives up. This occurs because in the case where only the FIN must be acked, the 'len' calculation in tcp_output() will be 0, a lot of checks will be skipped, and the FIN check will also be skipped because it is designed to handle FIN retransmits, not forced transmits from tcp_newreno(). The solution is to simply set TF_ACKNOW before calling tcp_output() to absolute guarentee that it will run the send code and reset the retransmit timer. TF_ACKNOW is already used for this purpose in other cases. For some unknown reason this patch also seems to greatly reduce the number of duplicate acks received when Guido runs his tests over a lossy network. It is quite possible that there are other tcp_newreno{_partial_ack()} cases which were not generating the expected output which this patch also fixes. X-MFC after: Will be MFC'd after the freeze is over
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2020-12-20 02:59:44 +00:00
svn path=/head/; revision=104226
@ -2777,6 +2777,7 @@ tcp_newreno_partial_ack(tp, th)
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* (tp->snd_una has not yet been updated when this function is called.)
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*/
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tp->snd_cwnd = tp->t_maxseg + (th->th_ack - tp->snd_una);
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tp->t_flags |= TF_ACKNOW;
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(void) tcp_output(tp);
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tp->snd_cwnd = ocwnd;
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if (SEQ_GT(onxt, tp->snd_nxt))
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@ -2777,6 +2777,7 @@ tcp_newreno_partial_ack(tp, th)
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* (tp->snd_una has not yet been updated when this function is called.)
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*/
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tp->snd_cwnd = tp->t_maxseg + (th->th_ack - tp->snd_una);
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tp->t_flags |= TF_ACKNOW;
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(void) tcp_output(tp);
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tp->snd_cwnd = ocwnd;
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if (SEQ_GT(onxt, tp->snd_nxt))
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