What is the reason and how to avoid the [FIN, ACK] , [RST] and [RST, ACK]
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Here is a rough explanation of the concepts.
[ACK] is the acknowledgement that the previously sent data packet was received.
[FIN] is sent by a host when it wants to terminate the connection; the TCP protocol requires both endpoints to send the termination request (i.e. FIN).
So, suppose
- host A sends a data packet to host B
- and then host B wants to close the connection.
-
Host B (depending on timing) can respond with
[FIN,ACK]indicating that it received the sent packet and wants to close the session. -
Host A should then respond with a
[FIN,ACK]indicating that it received the termination request (theACKpart) and that it too will close the connection (theFINpart).
However, if host A wants to close the session after sending the packet, it would only send a [FIN] packet (nothing to acknowledge) but host B would respond with [FIN,ACK] (acknowledges the request and responds with FIN).
Finally, some TCP stacks perform half-duplex termination, meaning that they can send [RST] instead of the usual [FIN,ACK]. This happens when the host actively closes the session without processing all the data that was sent to it. Linux is one operating system which does just this.
You can find a more detailed and comprehensive explanation here.
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Updated on November 04, 2020Comments
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Admin over 3 years
What is the reason and how to avoid the
[FIN, ACK],[RST]and[RST, ACK]?Is it due to some mismatch between the TCP parameters of the SO´s? What does it mean when the server replies
[FIN, ACK]in a TCP/IP connection?10.118.113.237is a Solaris box, while10.118.110.63is a Linux box.No. Time Source Destination Protocol Length Info 1 0.000000000 10.118.113.237 10.118.110.63 TCP 68 mmpft > 39679 [FIN, ACK] Seq=1 Ack=1 Win=49232 Len=0 TSval=62389927 TSecr=355193509 2 0.000015000 10.118.110.63 10.118.113.237 TCP 56 39679 > mmpft [RST] Seq=1 Win=0 Len=0 4 0.119357000 10.118.110.63 10.118.113.237 TCP 68 39707 > mmpft [ACK] Seq=1 Ack=93 Win=54 Len=0 TSval=355208473 TSecr=62389939 5 0.119475000 10.118.113.237 10.118.110.63 TCP 62 mmpft > 39707 [RST, ACK] Seq=93 Ack=1 Win=0 Len=0 6 0.321336000 10.118.110.63 10.118.113.237 TCP 76 55603 > mmpft [SYN] Seq=0 Win=5840 Len=0 MSS=1460 SACK_PERM=1 TSval=355208524 TSecr=0 WS=128 7 0.321835000 10.118.113.237 10.118.110.63 TCP 80 mmpft > 55603 [SYN, ACK] Seq=0 Ack=1 Win=49232 Len=0 TSval=62389959 TSecr=355208524 MSS=1460 WS=1 SACK_PERM=1 8 0.321854000 10.118.110.63 10.118.113.237 TCP 68 55603 > mmpft [ACK] Seq=1 Ack=1 Win=5888 Len=0 TSval=355208524 TSecr=62389959 9 0.322552000 10.118.110.63 10.118.113.237 DIAMETER 276 cmd=Capabilities-ExchangeRequest(257) flags=R--- appl=Diameter Common Messages(0) h2h=3f3197c e2e=e9200846 10 0.322891000 10.118.113.237 10.118.110.63 TCP 68 mmpft > 55603 [ACK] Seq=1 Ack=209 Win=49024 Len=0 TSval=62389959 TSecr=355208524 11 0.342554000 10.118.113.237 10.118.110.63 TCP 68 mmpft > 39707 [FIN, ACK] Seq=93 Ack=1 Win=49232 Len=0 TSval=62389961 TSecr=355200968 12 0.342567000 10.118.110.63 10.118.113.237 TCP 56 39707 > mmpft [RST] Seq=1 Win=0 Len=0 13 0.346940000 10.118.113.237 10.118.110.63 DIAMETER 312 cmd=Capabilities-ExchangeAnswer(257) flags=---- appl=Diameter Common Messages(0) h2h=3f3197c e2e=e9200846 14 0.347021000 10.118.110.63 10.118.113.237 TCP 68 55603 > mmpft [ACK] Seq=209 Ack=245 Win=6912 Len=0 TSval=355208530 TSecr=62389961 15 4.288733000 10.118.113.237 10.118.110.63 TCP 68 mmpft > 39652 [FIN, ACK] Seq=1 Ack=1 Win=49232 Len=0 TSval=62390356 TSecr=355186382 16 4.288757000 10.118.110.63 10.118.113.237 TCP 56 39652 > mmpft [RST] Seq=1 Win=0 Len=0 17 4.398722000 10.118.113.237 10.118.110.63 DIAMETER 160 [TCP Retransmission] cmd=Device-WatchdogRequest(280) flags=R--- appl=Diameter Common Messages(0) h2h=f889ad2 e2e=5f8035e4 18 4.398734000 10.118.110.63 10.118.113.237 TCP 56 39707 > mmpft [RST] Seq=1 Win=0 Len=0 19 4.938748000 10.118.113.237 10.118.110.63 DIAMETER 160 cmd=Device-WatchdogRequest(280) flags=R--- appl=Diameter Common Messages(0) h2h=f889ad0 e2e=5f8035df 20 4.938770000 10.118.110.63 10.118.113.237 TCP 56 39598 > mmpft [RST] Seq=1 Win=0 Len=0 21 5.498759000 10.118.113.237 10.118.110.63 TCP 68 mmpft > 39544 [FIN, ACK] Seq=1 Ack=1 Win=49232 Len=0 TSval=62390477 TSecr=355156526 22 5.498783000 10.118.110.63 10.118.113.237 TCP 56 39544 > mmpft [RST] Seq=1 Win=0 Len=0 23 5.648780000 10.118.113.237 10.118.110.63 TCP 68 mmpft > 55774 [FIN, ACK] Seq=1 Ack=1 Win=49232 Len=0 TSval=62390492 TSecr=355111580 24 5.648804000 10.118.110.63 10.118.113.237 TCP 56 55774 > mmpft [RST] Seq=1 Win=0 Len=0 25 5.942885000 10.118.113.237 10.118.110.63 TCP 68 mmpft > 55828 [FIN, ACK] Seq=1 Ack=1 Win=49232 Len=0 TSval=62390521 TSecr=355126129 26 5.942901000 10.118.110.63 10.118.113.237 TCP 56 55828 > mmpft [RST] Seq=1 Win=0 Len=0 27 6.668742000 10.118.113.237 10.118.110.63 TCP 68 mmpft > 55666 [FIN, ACK] Seq=1 Ack=1 Win=49232 Len=0 TSval=62390594 TSecr=355081310 28 6.668760000 10.118.110.63 10.118.113.237 TCP 56 55666 > mmpft [RST] Seq=1 Win=0 Len=0 29 7.258815000 10.118.113.237 10.118.110.63 TCP 68 mmpft > 55720 [FIN, ACK] Seq=1 Ack=1 Win=49232 Len=0 TSval=62390653 TSecr=355096418 31 7.418827000 10.118.113.237 10.118.110.63 DIAMETER 160 cmd=Device-WatchdogRequest(280) flags=R--- appl=Diameter Common Messages(0) h2h=f889acd e2e=5f8035d9 32 7.418835000 10.118.110.63 10.118.113.237 TCP 56 39490 > mmpft [RST] Seq=1 Win=0 Len=0 33 12.948752000 10.118.113.237 10.118.110.63 DIAMETER 160 [TCP Retransmission] cmd=Device-WatchdogRequest(280) flags=R--- appl=Diameter Common Messages(0) h2h=f889ad2 e2e=5f8035e4 34 12.948776000 10.118.110.63 10.118.113.237 TCP 56 39707 > mmpft [RST] Seq=1 Win=0 Len=0 35 30.030087000 10.118.113.237 10.118.110.63 DIAMETER 160 [TCP Retransmission] cmd=Device-WatchdogRequest(280) flags=R--- appl=Diameter Common Messages(0) h2h=f889ad2 e2e=5f8035e4 36 30.030113000 10.118.110.63 10.118.113.237 TCP 56 39707 > mmpft [RST] Seq=1 Win=0 Len=0 38 30.369302000 10.118.110.63 10.118.113.237 TCP 68 55603 > mmpft [ACK] Seq=209 Ack=337 Win=6912 Len=0 TSval=355216035 TSecr=62392964 39 30.369413000 10.118.113.237 10.118.110.63 TCP 62 mmpft > 55603 [RST, ACK] Seq=337 Ack=209 Win=0 Len=0 40 30.571231000 10.118.110.63 10.118.113.237 TCP 76 55630 > mmpft [SYN] Seq=0 Win=5840 Len=0 MSS=1460 SACK_PERM=1 TSval=355216086 TSecr=0 WS=128 41 30.571603000 10.118.113.237 10.118.110.63 TCP 80 mmpft > 55630 [SYN, ACK] Seq=0 Ack=1 Win=49232 Len=0 TSval=62392984 TSecr=355216086 MSS=1460 WS=1 SACK_PERM=1 42 30.571620000 10.118.110.63 10.118.113.237 TCP 68 55630 > mmpft [ACK] Seq=1 Ack=1 Win=5888 Len=0 TSval=355216086 TSecr=62392984 43 30.572253000 10.118.110.63 10.118.113.237 DIAMETER 276 cmd=Capabilities-ExchangeRequest(257) flags=R--- appl=Diameter Common Messages(0) h2h=3f3197d e2e=e9200847 44 30.572638000 10.118.113.237 10.118.110.63 TCP 68 mmpft > 55630 [ACK] Seq=1 Ack=209 Win=49232 Len=0 TSval=62392984 TSecr=355216086 45 30.579815000 10.118.113.237 10.118.110.63 TCP 68 mmpft > 55603 [FIN, ACK] Seq=337 Ack=209 Win=49232 Len=0 TSval=62392985 TSecr=355208530 46 30.579826000 10.118.110.63 10.118.113.237 TCP 56 55603 > mmpft [RST] Seq=209 Win=0 Len=0 47 30.581517000 10.118.113.237 10.118.110.63 DIAMETER 312 cmd=Capabilities-ExchangeAnswer(257) flags=---- appl=Diameter Common Messages(0) h2h=3f3197d e2e=e9200847 48 30.581553000 10.118.110.63 10.118.113.237 TCP 68 55630 > mmpft [ACK] Seq=209 Ack=245 Win=6912 Len=0 TSval=355216088 TSecr=62392985 49 34.138766000 10.118.113.237 10.118.110.63 TCP 68 mmpft > 39679 [FIN, ACK] Seq=1 Ack=1 Win=49232 Len=0 TSval=62393341 TSecr=355193509 50 34.138790000 10.118.110.63 10.118.113.237 TCP 56 39679 > mmpft [RST] Seq=1 Win=0 Len=0