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TCP: Connection and Timer Management

TCP: Connection and Timer Management. Reading: Section 24.3, 24.5. Initial state. CLOSED. Application.: passive Öpening Send: ---. Send timeout: RST. Application.: active opening send: SYN. LISTEN. Receive: SYN; send: SYN, ACK.

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TCP: Connection and Timer Management

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  1. TCP: Connection and Timer Management Reading: Section 24.3, 24.5 FSU CIS 5930 Internet Protocols

  2. Initial state CLOSED Application.: passive Öpening Send: --- Sendtimeout: RST Application.: active openingsend: SYN LISTEN Receive: SYN; send: SYN, ACK Application.: send dataSend: SYN Passive opening Receive: RST SYN_RECV SYN_SENT Receive: SYN send: SYN, ACK Simultanous opening Application: close ortimeout Receive: SYN, ACKSend: ACK Receive: ACKSend: --- Application: close Send: FIN Passive close ESTABLISHED Receive: FINSend: ACK Application: close Send: FIN Data transmission CLOSE_WAIT Simultanous close Application: closeSend: FIN Receive: FINSend: ACK CLOSING FIN_WAIT_1 Receive: ACKSend: --- LAST_ACK Receive: ACKSend: --- Receive: ACKSend: --- Receive: FIN, ACKSend: ACK FIN_WAIT_2 TIME_WAIT Receive: FINSend: ACK Active close 2 MSL timeout MSL: max. segment life FSU CIS 5930 Internet Protocols

  3. Road to establish a connection • LISTEN • Passive opening, waiting for connection request from others • SYN_SENT • After sending SYN, but before receiving ACK • SYN_RECV • Receiving SYN from others • ESTABLISHED • Finished three-way handshaking FSU CIS 5930 Internet Protocols

  4. Transition from CLOSED to SYN_SENT • User/application calls connect() socket API • sys_socketcall maps it to tcp_v4_connect() • tcp_v4_connect() invokes tcp_connect() • tcp_connect() sends SYN packet • Changes state from CLOSED to SYN_SENT FSU CIS 5930 Internet Protocols

  5. Transition from LISTEN to SYN_RECV • User/application calls listen() socket API • Changing state from CLOSED to LISTEN • Receiving SYN packet from another party • Changing state from LISTEN to SYN_RECV • Sending packet with SYN and ACK FSU CIS 5930 Internet Protocols

  6. Transition from SYN_SENT to ESTABLISHED • Currently in SYN_SENT state • Receiving packet with SYN and ACK • Sending ACK • Changing from SYN_SENT to ESTABLISHED state FSU CIS 5930 Internet Protocols

  7. Transition from SYN_SENT to SYN_RECV • Currently in SYN_SENT state • Receiving SYN packet (without ACK) • Sending SYN and ACK • Changing to SYN_RECV state • Simultaneous connection establishment FSU CIS 5930 Internet Protocols

  8. Transition from SYN_RECV to ESTABLISHED • Currently in SYN_RECV state • (we sent SYN/ACK) • Receiving ACK packet • Changing to ESTABLISHED state FSU CIS 5930 Internet Protocols

  9. tcp_rcv_state_process() • Big function to handle TCP state transitions • Specific behavior depending on current state and packet received if (th->ack) { switch (sk->state) { case TCP_SYN_RECV: … tcp_set_state(sk, TCP_ESTABLISHED); } } FSU CIS 5930 Internet Protocols

  10. Tearing down a connection • Two ways to terminate a connection • Graceful close: all data transmitted • Abort: data can get lost • Closing related state • FIN_WAIT_1: We close but not receive ACK • FIN_WAIT_2: We close and receive ACK • CLOSING: Both FINed, waiting for ACK • TIME_WAIT: graceful close (wait some time) • CLOSE_WAIT: They close and we ACK • LAST_ACK: They close, then we close, waiting for ACK • CLOSED: connection is now closed FSU CIS 5930 Internet Protocols

  11. Transition from ESTABLISHED to FIN_WAIT_1 • User/application calls close() socket API • sys_socketcall() maps it to sys_shutdown() • Which calls tcp_close() (in TCP case) • Sending FIN packet • Changing state from ESTABLISHED to FIN_WAIT_1 FSU CIS 5930 Internet Protocols

  12. Transition from ESTABLISHED to CLOSE_WAIT • Currently in ESTABLISHED state • Receiving FIN packet • Sending ACK to FIN • Changing state from ESTABLISHED to CLOSE_WAIT • tcp_fin() FSU CIS 5930 Internet Protocols

  13. Transition from CLOSE_WAIT to LAST_ACK • Currently in CLOSE_WAIT state • (receiving FIN from another party) • Finally we finish data transmission, we also close • We send FIN packet • Changing to LAST_ACK state to wait for the ACK packet FSU CIS 5930 Internet Protocols

  14. Transition from FIN_WAIT_1 to FIN_WAIT_2 • Currently we are in FIN_WAIT_1 • (We sent FIN) • Receiving ACK (to FIN) • Changing state from FIN_WAIT_1 to FIN_WAIT_2 • (we have not received FIN from another party) FSU CIS 5930 Internet Protocols

  15. Transition from FIN_WAIT_2 to TIME_WAIT • We are in FIN_WAIT_2 • (we sent FIN and ACKed) • Receiving FIN, sending ACK • Changing state from FIN_WAIT_2 to TIME_WAIT • For graceful close, wait for 2 MSL FSU CIS 5930 Internet Protocols

  16. Transition from FIN_WAIT_1 to TIME_WAIT • Currently we are in FIN_WAIT_1 • (we sent FIN) • Receiving ACK and FIN • Changing state from FIN_WAIT_1 to TIME_WAIT FSU CIS 5930 Internet Protocols

  17. Transition from FIN_WAIT_1 to CLOSING • Currently in FIN_WAIT_1 state • (we sent FIN but not ACKed) • Receiving FIN • Sending ACK • Changing from FIN_WAIT_1 to CLOSING state • Waiting for ACK FSU CIS 5930 Internet Protocols

  18. Transition from CLOSING to TIME_WAIT • Currently in CLOSING state • Both sides FINed • We Acked another party • We waiting for being ACKed by another party • Receiving ACK • Changing state from CLOSING to TIME_WAIT FSU CIS 5930 Internet Protocols

  19. Timer management • Seven different timers are maintained in TCP • SYNACK: waiting for ACK to our SYN • Retransmit: for data retransmission, exponential back-off • Delay ACK: hoping for piggy-back ACK • Keepalive: checking if a connection alive • Probe: testing if zero window size still applies • FIN_WAIT_2: switch to CLOSED if no FIN received • TWKill: how long to stay in TIME_WAIT FSU CIS 5930 Internet Protocols

  20. Timer data structure struct timer_list { struct list_head list; unsigned long expires; unsigned long data; void (*function)(unsigned long); volatile int running; } FSU CIS 5930 Internet Protocols

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