Le mardi 23 août 2011 à 23:31 +0300, Daniel Baluta a écrit :
Hello,
Please help me understanding the behavior of the following TCP conversation.
You can find bellow a snippet of the (FTP) conversation captured both on client (C) and server (S).
[client]$ tcpdump -n -r client-6-conv.cap [P1] 49.045690 IP 10.10.0.1.21 > 10.10.0.20.58277: Flags [P.], seq 197:220, ack 81, win 757, length 23 [P2] 49.046600 IP 10.10.0.20.58277 > 10.10.0.1.21: Flags [.], ack 220, win 738, length 0 [P3] 49.047462 IP 10.10.0.20.58277 > 10.10.0.1.21: Flags [P.], seq 81:87, ack 220, win 738, length 6 [P5] 49.048757 IP 10.10.0.1.21 > 10.10.0.20.58277: Flags [F.], seq 242, ack 87, win 757, length 0 [P6] 49.048794 IP 10.10.0.20.58277 > 10.10.0.1.21: Flags [.], ack 220, win 738, options [nop,nop,sack 1 {242:243}], length 0 [P4] 49.048801 IP 10.10.0.1.21 > 10.10.0.20.58277: Flags [P.], seq 220:242, ack 87, win 757, length 22 [P7] 49.048833 IP 10.10.0.20.58277 > 10.10.0.1.21: Flags [.], ack 243, win 715, length 0 [P8] 49.049566 IP 10.10.0.20.58277 > 10.10.0.1.21: Flags [F.], seq 87, ack 243, win 715, length 0 [P9] 49.050889 IP 10.10.0.1.21 > 10.10.0.20.58277: Flags [R], seq 1672731590, win 0, length 0
[server]$ tcpdump -n -r server-6-conv.cap [P1] 49.059740 IP 10.10.0.1.21 > 10.10.0.20.58277: Flags [P.], seq 197:220, ack 81, win 757, length 23 [P2] 49.061394 IP 10.10.0.20.58277 > 10.10.0.1.21: Flags [.], ack 220, win 738, length 0 [P3] 49.061760 IP 10.10.0.20.58277 > 10.10.0.1.21: Flags [P.], seq 81:87, ack 220, win 738, length 6 [P4] 49.062794 IP 10.10.0.1.21 > 10.10.0.20.58277: Flags [P.], seq 220:242, ack 87, win 757, length 22 [P5] 49.062843 IP 10.10.0.1.21 > 10.10.0.20.58277: Flags [F.], seq 242, ack 87, win 757, length 0 [P6] 49.063808 IP 10.10.0.20.58277 > 10.10.0.1.21: Flags [.], ack 220, win 738, options [nop,nop,sack 1 {242:243}], length 0 [P7] 49.063823 IP 10.10.0.20.58277 > 10.10.0.1.21: Flags [.], ack 243, win 715, length 0 [P8] 49.064271 IP 10.10.0.20.58277 > 10.10.0.1.21: Flags [F.], seq 87, ack 243, win 715, length 0 [P9] 49.064481 IP 10.10.0.1.21 > 10.10.0.20.58277: Flags [R], seq 1672731590, win 0, length 0
What happens is that servers sends packets P4 and P5, but client receives P5 before P4. Since SACK is enabled, client will send a SACK (P6) ack-ing P5.
Then client sees P4, and will send an ack (P7) for P4, then sends P8 with FIN flag set. What I don't understand, is why server responds with RST (P9) instead of ACK?
This is was obtained on 2.6.32.43. I have also attached full capture files.
I am reading TCP's RFC and kernel code, but so far I haven't reached a conclusion.
TCP in RFC 1122 section 4.2.2.13: "A host MAY implement a "half-duplex" TCP close sequence, so that an application that has called CLOSE cannot continue to read data from the connection. If such a host issues a CLOSE call while received data is still pending in TCP, or if new data is received after CLOSE is called, its TCP SHOULD send a RST to show that data was lost." This is why some apps first call shutdown(), then drain receive queue, then close()