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CS 498 Lecture 17 TCP Implementation in Linux

CS 498 Lecture 17 TCP Implementation in Linux. Jennifer Hou Department of Computer Science University of Illinois at Urbana-Champaign Reading: Chapter 24, The Linux Networking Architecture: Design and Implementation of Network Protocols in the Linux Kernel. Outline.

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CS 498 Lecture 17 TCP Implementation in Linux

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  1. CS 498 Lecture 17 TCP Implementation in Linux Jennifer Hou Department of Computer Science University of Illinois at Urbana-Champaign Reading: Chapter 24, The Linux Networking Architecture: Design and Implementation of Network Protocols in the Linux Kernel

  2. Outline • Paths of Incoming and outgoing segments • Connection management (this lecture) • Flow control and congestion control

  3. tcp_rcv_established TCP_ESTABLISHED TCP Implementation in Linux sk->data_ready send TCP tcp_ack_snd_check tcp_data _queue tcp_sendmsg Fast Path Retrans.Timer tcp_send_(delayed)_ack tcp_send_skb tcp_data tcp_data_snd_check Section 24.3 SlowPath tcp_write_timer PureACK tcp_rcv_state_process tcp_ack tcp_re -transmit_skb tcp_v4_do_rcv tcp_write_xmit __tcp_v4_lookup() tcp_transmit_skb tcp_v4_rcv ip_input.c ip_output.c ip_local_deliver ip_queue_xmit

  4. Initial State CLOSED Application.: passive opening send: --- timeoutsend: RST Application.: active openingsend: SYN LISTEN Receives: SYN; send: SYN, ACK Application.: send datasend: SYN Passive Opening Receive RST Receive: SYN send: SYN, ACK Simultaneous opening SYN_RECV SYN_SENT Application: close or timeout Receive: SYN, ACKsend: ACK Receive ACKsend: --- Application: closesen: FIN Passive Close ESTABLISHED Receive FINsend: ACK Data Transmission Application: closesend: FIN CLOSE_WAIT Simultaneous close Receive FINsend ACK Application: c;pse send: FIN FIN_WAIT_1 CLOSING Receive: ACKsend: --- Receive: ACKsend: --- Receive ACKsend: --- LAST_ACK Receive: FIN, ACKsend: ACK FIN_WAIT_2 TIME_WAIT Receive: FINsend: ACK MSL: max. segment life Aktive close 2 MSL timeout

  5. tcp_rcv_state_process() • Handles mainly state transitions and connection management. • For example, if the packet received contains an ACK flag, • If state=SYN_RECV, then stateESTABLISHED, and the acknowledgement is processed. • If state=FIN_WAIT_1, then state FIN-WAIT2 and the TIMEWAIT timer is set. • If state=CLOSING, then state TIMEWAIT • If state=LAST_ACK, stateCLOSED and the socket is reset.

  6. Transition CLOSED SYN_SENT • connect()  tcp_v4_connect() tcp_connect() • tcp_connect() changes the state to SYN_SENT by invoking tcp_set_state(sk, TCP_SYN_SENT).

  7. Transition LISTENSYN_RECV • The LISTEN state is set when the server application invokes listen(). • When a SYN is received, • tcp_rcv_state_process()tcp_v4_hnd_req() tcp_check_req() tcp_v4_syn_recv_sock() tcp_create_openreq_child(). • In tcp_create_openreq_child(), the state is set to TCP_SYN_RECV. • tcpaf_specificconn_request() (pointed to tcp_v4_conn_request()) is invoked to specify the initial SN. • tcp_v4_send_synack() sends a reply with the ACK and SYN flags set.

  8. Transition SYN_SENT ESTABLISHED • tcp_rcv_state_process() tcp_rcv_synsent_state_process() • if (thack) { …… if (!thsyn) goto discard; ……. tcp_set_state(sk, TCP_ESTABLISHED); ……. tcp_schedule_ack(tp); ……. }

  9. Transition SYN_SENT SYN_RECEIVED • This takes place in the case of simultaneous opening. • tcp_rcv_state_process() tcp_rcv_synsent_state_process() • if (thsyn) { tcp_set_state(sk, TCP_SYN_RECV); …… tcp_set_synack(sk); …… }

  10. Transition SYN_RECV ESTABLISHED • tcp_rcv_state_process() processes this case. • If (thack) { switch(skstate) { case TCP_SYN_RECV: …. tcp_set_state(sk, TCP_ESTALISHED); } } Now the connection is established and the two peers can exchange data

  11. Initial State CLOSED Application.: passive opening send: --- timeoutsend: RST Application.: active openingsend: SYN LISTEN Receives: SYN; send: SYN, ACK Application.: send datasend: SYN Passive Opening Receive RST Receive: SYN send: SYN, ACK Simultaneous opening SYN_RECV SYN_SENT Application: close or timeout Receive: SYN, ACKsend: ACK Receive ACKsend: --- Application: closesen: FIN Passive Close ESTABLISHED Receive FINsend: ACK Data Transmission Application: closesend: FIN CLOSE_WAIT Simultaneous close Receive FINsend ACK Application: c;pse send: FIN FIN_WAIT_1 CLOSING Receive: ACKsend: --- Receive: ACKsend: --- Receive ACKsend: --- LAST_ACK Receive: FIN, ACKsend: ACK FIN_WAIT_2 TIME_WAIT Receive: FINsend: ACK MSL: max. segment life Aktive close 2 MSL timeout

  12. Transition ESTABLISHED FIN_WAIT_1 • close() tcp_close() tcp_close_state(). • In tcp_close_state(), the state is changed from ESTABLISHED to FIN_WAIT_1.

  13. Transition FIN_WAIT_1 FIN_WAIT_2 • In tcp_rcv_state_process() • if (thack) { switch(skstate) { …… case TCP_FIN_WAIT1: ………………… tcp_set_state(sk,TCP_FIN_WAIT2); …… } }

  14. Transition FIN_WAIT2TIME_WAIT • In tcp_fin(), switch(skstate) { ….. case TCP_FIN_WAIT2: tcp_send_ack(sk); tcp_time_wait(sk, TCP_TIME_WAIT, 0); break; ….. } tcp_v4_do_rcv() tcp_rcv_state_process() tcp_data_queue() tcp_fin()

  15. Transition ESTABLISHED CLOSE_WAIT tcp_v4_do_rcv() tcp_rcv_established() tcp_data() tcp_ack_snd_check() tcp_data_queue() tcp_fin()

  16. Transition ESTABLISHED CLOSE_WAIT • In tcp_fin(), switch(skstate) { case TCP_SYN_RECV: case TCP_ESTABLISHED: tcp_set_state(sk,TCP_CLOSE_WAIT); if (thrst) skshutdown= SHUTDOWN_MASK; break; • In tcp_ack_snd_check(), a packet is sent with the ACK flag set.

  17. Transition CLOSE_WAIT LAST_ACK • TCP on the other hand has closed the connection. Now we are waiting for our TCP instance to close the connection. • tcp_close() tcp_close_state() • In tcp_close_state(), tcp_set_state(sk, LAST_ACK) is invoked.

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