Date: Wed, 23 Oct 2002 18:44:57 -0700 (PDT) From: Bill Baumann <bbaumann@isilon.com> To: freebsd-net@FreeBSD.ORG Subject: tcp_input's header prediction and a collapsing send window Message-ID: <Pine.BSF.4.21.0210221616110.17276-100000@isilon.com> In-Reply-To: <20021015115315.U7412-100000@mammoth.eat.frenchfries.net>
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I'm experiencing a bug where snd_wnd collapses. I see snd_wnd approach zero even though data is sent/received and ack'ed successfully. After taking a close look at tcp_input, I think I see a senario where this could happen. Say header prediction handles ~2 GB of data without problems, then a retransmission happens. snd_wnd starts collapsing as it should. The header prediction code is correctly skipped as the snd_wnd no long matches the advertised window. We recover from the retransmission, *BUT* the code that reopens window is skipped because of rolled over sequence numbers. In the ack processing code (step 6), the variable snd_wl1 tracks the newest sequence number that we've seen. It helps prevent snd_wnd from being reopened on re-transmitted data. If snd_wl1 is greater than received sequence #, we skip it. This is fine unless we're 2^31 bytes ahead and SEQ_LT says we're behind. Since snd_wl1 is only updated if the condition is true -- we're stuck. snd_wl1 is only updated with in SYN/FIN processing code and in step 6. So if we process 2GB in the header prediction code -- where the step 6 never executes, and then somehow reach step 6. snd_wnd collapses and tcp_output stops sending. I have a trace mechanism that dumps various tcp_input variables that corroborates this theory. I have lined this up with tcpdump. The trace shows snd_wnd collapsing and snd_wl1 > th_seq even as healthy traffic is transmitted and received. The outcome is a halted transmitter. Possible remedy: update snd_wl1 in the header prediction code. What do you all think? Is this real? Or am I missing something? Regards, Bill Baumann To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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