Date: Thu, 9 Oct 2014 22:47:27 +0000 From: Madhusudhan Ravi <mravi@vmware.com> To: "freebsd-net@freebsd.org" <freebsd-net@freebsd.org> Subject: TCP receive window management Message-ID: <D05C5E8E.968D%mravi@vmware.com>
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I have a question regarding the freeBSD receive window advertisement and updating sender side flow control state based on this.
Is it expected that a retransmitted packet not carry an updated receive window?
The following are the state variables used for maintaining a peer's rcv window state on the sender side. snd_wnd, snd_wl1, snd_wl2. In tcp_input at the label step6: where these states are updated there are checks which make sure rcv window is not updated by old segments.
if ((thflags & TH_ACK) &&
(SEQ_LT(tp->snd_wl1, th->th_seq) ||
(tp->snd_wl1 == th->th_seq && (SEQ_LT(tp->snd_wl2, th->th_ack) ||
(tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd))))) {
Based on the above checks for the case of a fast retransmitted packet that does ack additional data ((SEQ_LT(tp->snd_wl2, th->th_ack) is TRUE) and has the latest rcv window. th_seq could be < snd_wl1 and in this case rcv window is not updated to the latest one. Is this intentional or is there something that I am missing.
Thanks
Madhu
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